A Medley of Potpourri: Aug 14, 2022

Sunday, August 14, 2022

Residual-current device

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Residual-current_device

Typical GFCI receptacle found in North America

A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground. It is to protect equipment and to reduce the risk of serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human receives a brief shock before the electrical circuit is isolated, falls after receiving a shock, or if the person touches both conductors at the same time.

If the RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO. An earth leakage circuit breaker may be a RCD, although an older type of voltage-operated earth leakage circuit breaker (ELCB) also exists.

These electrical wiring devices are designed to quickly and automatically isolate a circuit when it detects that the electric current is unbalanced between the supply and return conductors of a circuit. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. Alternating 60 Hz current above 20 mA (0.020 amperes) through the human body is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. RCDs are designed to disconnect the conducting wires ("trip") quickly enough to potentially prevent serious injury to humans, and to prevent damage to electrical devices.

RCDs are testable and resettable devices—a test button safely creates a small leakage condition, and another button resets the conductors after a fault condition has been cleared. Some RCDs disconnect both the energized and return conductors upon a fault (double pole), while a single pole RCD only disconnects the energized conductor. If the fault has left the return wire "floating" or not at its expected ground potential for any reason, then a single-pole RCD will leave this conductor still connected to the circuit when it detects the fault.

A two-pole, or double-pole, residual-current device. The test button and connect/disconnect switch are colored blue. A fault will trigger the switch to its down (off) position, which in this device would disconnect both conductors.
Log-log graph of the effect of alternating current I of duration T passing from left hand to feet as defined in IEC 60479-1.
1: Electromagnet with help electronics
2: Current transformer secondary winding
3: Transformer core
4: Test push-button
L: Line conductor
N: Neutral conductor

Purpose and operation

RCDs are designed to disconnect the circuit if there is a leakage current. In their first implementation in the 1950s, power companies used them to prevent electricity theft where consumers grounded returning circuits rather than connecting them to neutral to inhibit electrical meters from registering their power consumption.

The most common modern application is as a safety device to detect small leakage currents (typically 5–30 mA) and disconnecting quickly enough (<30 milliseconds) to prevent device damage or electrocution. They are an essential part of the automatic disconnection of supply (ADS), i.e. to switch off when a fault develops, rather than rely on human intervention, one of the essential tenets of modern electrical practice.

To reduce the risk of electrocution, RCDs should operate within 25–40 milliseconds with any leakage currents (through a person) of greater than 30 mA, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would probably not increase the total current enough for a fuse or overload circuit breaker to isolate the circuit, and not fast enough to save a life.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between current flowing through the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground or to another circuit), and the device will open its contacts. Operation does not require a fault current to return through the earth wire in the installation; the trip will operate just as well if the return path is through plumbing, contact with the ground or any other current path. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete.

For an RCD used with three-phase power, all three live conductors and the neutral (if fitted) must pass through the current transformer.

Application

Electrical plugs with incorporated RCD are sometimes installed on appliances that might be considered to pose a particular safety hazard, for example long extension leads, which might be used outdoors, or garden equipment or hair dryers, which may be used near a bath or sink. Occasionally an in-line RCD may be used to serve a similar function to one in a plug. By putting the RCD in the extension lead, protection is provided at whatever outlet is used even if the building has old wiring, such as knob and tube, or wiring that does not contain a grounding conductor. The in-line RCD can also have a lower tripping threshold than the building to further improve safety for a specific electrical device.

In North America, GFI receptacles can be used in cases where there is no grounding conductor, but they must be labeled as "no equipment ground". This is referenced in the National Electric Code section 406 (D) 2, however codes change and someone should always consult a licensed professional and their local building and safety departments. The code is An ungrounded GFI receptacle will trip using the built-in "test" button, but will not trip using a GFI test plug, because the plug tests by passing a small current from line to the non-existent ground. It is worth noting that despite this, only one GFCI receptacle at the beginning of each circuit is necessary to protect downstream receptacles. There does not appear to be a risk of using multiple GFI receptacles on the same circuit, though it is considered redundant.

In Europe, RCDs can fit on the same DIN rail as the miniature circuit breakers; much like in miniature circuit breakers, the busbar arrangements in consumer units and distribution boards provides protection for anything downstream.

RCBO

A pure RCD will detect imbalance in the currents of the supply and return conductors of a circuit. But it cannot protect against overload or short circuit like a fuse or a miniature circuit breaker (MCB) does (except for the special case of a short circuit from live to ground, not live to neutral).

However, a RCD and a MCB often come integrated in the same device, thus being able to detect both supply imbalance and overload current. Such a device is called

RCBO (for residual-current circuit breaker with overcurrent protection) in Europe and Australia;
GFCI breaker (for ground fault circuit interrupter) in USA and Canada;

Typical design

Internal mechanism of RCD

Opened 3-phase residual-current device

The diagram depicts the internal mechanism of a residual-current device (RCD). The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13 A and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation. Some early RCDs were entirely electromechanical and relied on finely balanced sprung over-centre mechanisms driven directly from the current transformer. As these are hard to manufacture to the required accuracy and prone to drift in sensitivity both from pivot wear and lubricant dry-out, the electronically-amplified type with a more robust solenoid part as illustrated are now dominant.

In the internal mechanism of an RCD, the incoming supply and the neutral conductors are connected to the terminals at (1), and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted. When the reset button (3) is pressed, the contacts ((4) and another, hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.

The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out.

Any fault to earth (for example caused by a person touching a live component in the attached appliance) causes some of the current to take a different return path, which means that there is an imbalance (difference) in the current in the two conductors (single-phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor).

This difference causes a current in the sense coil (6), which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5), and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance. A power failure will also remove power from the solenoid and cause the contacts to open, causing the safe trip-on-power-failure behaviour mentioned above.

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense coil. If the RCD does not trip when this button is pressed, then the device must be replaced.

RCD with additional overcurrent protection circuitry (RCBO or GFCI breaker)

An example of a rail-mounted RCBO

Residual-current and over-current protection may be combined in one device for installation into the service panel; this device is known as a GFCI (Ground-Fault Circuit Interrupter) breaker in the US and Canada, and as a RCBO (residual-current circuit breaker with over-current protection) in Europe and Australia. They are effectively a combination of a RCD and a MCB. In the US, GFCI breakers are more expensive than GFCI outlets.

As well as requiring both live and neutral inputs and outputs (or, full three-phase), many GFCI/RCBO devices require a functional earth (FE) connection. This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but live and earth remain.

For reasons of space, many devices, especially in DIN rail format, use flying leads rather than screw terminals, especially for the neutral input and FE connections. Additionally, because of the small form factor, the output cables of some models (Eaton/MEM) are used to form the primary winding of the RCD part, and the outgoing circuit cables must be led through a specially dimensioned terminal tunnel with the current transformer part around it. This can lead to incorrect failed trip results when testing with meter probes from the screw heads of the terminals, rather than from the final circuit wiring.

Having one RCD feeding another is generally unnecessary, provided they have been wired properly. One exception is the case of a TT earthing system, where the earth loop impedance may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100 mA (or greater) trip current time-delayed RCD is installed, covering the whole installation, and then more sensitive RCDs should be installed downstream of it for sockets and other circuits that are considered high-risk.

RCD with additional arc fault protection circuitry

In addition to ground fault circuit interrupters (GFCIs), arc-fault circuit interrupters (AFCI) are equally important as they offer added protection from potentially hazardous arc-faults resulting from damage in branch circuit wiring as well as extensions to branches such as appliances and cord sets. By detecting hazardous arc-faults and responding by interrupting power, AFCIs helps reduce the likelihood of the home's electrical system being an ignition source of a fire. Dual function AFCI/GFCI devices offer both electrical fire prevention and shock prevention in one device making them a solution for many rooms in the home, especially when replacing an existing standard receptacle or existing ungrounded receptacle.

Common features and variations

Differences in disconnection actions

Major differences exist regarding the manner in which an RCD-unit will act to disconnect the power to a circuit or appliance.

There are four situations in which different types of RCD-units are used:

At the consumer power distribution level, usually in conjunction with an RCBO resettable circuit breaker;
Built into a wall socket;
Plugged into a wall socket, which may be part of a power-extension cable; and
Built into the cord of a portable appliance, such as those intended to be used in outdoor or wet areas.

The first three of those situations, relate largely to usage as part of a power-distribution system and are almost always of the 'passive' or 'latched' variety, whereas the fourth relates solely to specific appliances and are always of the 'active' or 'non-latching' variety. 'Active' means prevention of any 're-activation' of the power supply after any inadvertent form of power outage, as soon as the mains supply becomes re-established; 'latch' relates to a 'switch' inside the unit housing the RCD that remains as set following any form of power outage, but has to be reset manually after the detection of an error-condition.

In the fourth situation, it would be deemed to be highly undesirable, and probably very unsafe, for a connected appliance to automatically resume operation after a power disconnection, without having the operator in attendance – as such manual reactivation of the RCD is necessary.

The difference between the modes of operation of the essentially two different types of RCD functionality is that the operation for power distribution purposes requires the internal latch to remain set within the RCD after any form of power disconnection caused by either the user turning the power off, or after any power outage; such arrangements are particularly applicable for connections to refrigerators and freezers.

Situation two is mostly installed just as described above, but some wall socket RCDs are available to fit the fourth situation, often by operating a switch on the fascia panel.

RCDs for the first and third situation are most commonly rated at 30 mA and 40 ms. For the fourth situation, there is generally a greater choice of ratings available – generally all lower than the other forms, but lower values often result in more nuisance tripping. Sometimes users apply protection in addition to one of the other forms, when they wish to override those with a lower rating. It may be wise to have a selection of type 4 RCDs available, because connections made under damp conditions or using lengthy power cables are more prone to trip-out when any of the lower ratings of RCD are used; ratings as low as 10 mA are available.

Number of poles and pole terminology

The number of poles represents the number of conductors that are interrupted when a fault condition occurs. RCDs used on single-phase AC supplies (two current paths), such as domestic power, are usually one- or two-pole designs, also known as single- and double-pole. A single-pole RCD interrupts only the energized conductor, while a double-pole RCD interrupts both the energized and return conductors. (In a single-pole RCD, the return conductor is usually anticipated to be at ground potential at all times and therefore safe on its own).

RCDs with three or more poles can be used on three-phase AC supplies (three current paths) or to disconnect an earth conductor as well, with four-pole RCDs used to interrupt three-phase and neutral supplies. Specially designed RCDs can also be used with both AC and DC power distribution systems.

The following terms are sometimes used to describe the manner in which conductors are connected and disconnected by an RCD:

Single-pole or one-pole – the RCD will disconnect the energized wire only.
Double-pole or two-pole – the RCD will disconnect both the energized and return wires.
1+N and 1P+N – non-standard terms used in the context of RCBOs, at times used differently by different manufacturers. Typically these terms may signify that the return (neutral) conductor is an isolating pole only, without a protective element (an unprotected but switched neutral), that the RCBO provides a conducting path and connectors for the return (neutral) conductor but this path remains uninterrupted when a fault occurs (sometimes known as "solid neutral"), or that both conductors are disconnected for some faults (such as RCD detected leakage) but only one conductor is disconnected for other faults (such as overload).

Sensitivity

RCD sensitivity is expressed as the rated residual operating current, noted I_Δn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their I_Δn value:

high sensitivity (HS): 5** – 10 – 30 mA (for direct-contact or life injury protection),
medium sensitivity (MS): 100 – 300 – 500 – 1000 mA (for fire protection),
low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine).

The 5 mA sensitivity is typical for GFCI outlets.

Break time (response speed)

There are two groups of devices. G (general use) instantaneous RCDs have no intentional time delay. They must never trip at one-half of the nominal current rating, but must trip within 200 milliseconds for rated current, and within 40 milliseconds at five times rated current. S (selective) or T (time-delayed) RCDs have a short time delay. They are typically used at the origin of an installation for fire protection to discriminate with G devices at the loads, and in circuits containing surge suppressors. They must not trip at one-half of rated current. They provide at least 130 milliseconds delay of tripping at rated current, 60 milliseconds at twice rated, and 50 milliseconds at five times rated. The maximum break time is 500 ms at rated current, 200 ms at twice rated, and 150 ms at five times rated.

Programmable earth fault relays are available to allow co-ordinated installations to minimise outage. For example, a power distribution system might have a 300 mA, 300 ms device at the service entry of a building, feeding several 100 mA S type at each sub-board, and 30 mA G type for each final circuit. In this way, a failure of a device to detect the fault will eventually be cleared by a higher-level device, at the cost of interrupting more circuits.

Type, or mode (types of current leakage issue detected)

IEC Standard 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the waveforms and frequency of the fault current.

Type AC RCDs trip on sinusoidal residual current.
Type A RCDs, in addition to Type AC, also respond to pulsating or continuous direct current of either polarity.
Type B RCDs, in addition to Type A, also respond to steady DC, and higher frequency current, or for combinations of alternating and direct current as may be found from single-phase or multi-phase rectifying circuits, like all the switching power supplies used at home, or for example washing machines etc., equipped with DC motors.

The BEAMA RCD Handbook - Guide to the Selection and Application of RCDs summaries this as follows:

Type AC RCDs trip on alternating sinusoidal residual current, suddenly applied or smoothly increasing.
Type A RCDs trip on alternating sinusoidal residual current and on residual pulsating direct current, suddenly applied or smoothly increasing.
Type F RCDs trip in the same conditions as Type A and in addition:
- for composite residual currents, whether suddenly applied or slowly rising intended for circuit supplied between phase and neutral or phase and earthed middle conductor;
- for residual pulsating direct currents superimposed on smooth direct current.
Type B RCDs trip in the same conditions as Type F and in addition:
- for residual sinusoidal alternating currents up to 1 kHz;
- for residual alternating currents superimposed on a smooth direct current;
- for residual pulsating direct currents superimposed on a smooth direct current;
- for residual pulsating rectified direct current which results from two or more phases;
- for residual smooth direct currents whether suddenly applied or slowly increased independent of polarity.

and notes that these designations have been introduced because some designs of type A and AC RCD can be disabled if a DC current is present that saturates the core of the detector.

Surge current resistance

The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics. The IEC 61008 and IEC 61009 standards require that RCDs withstand a 200 A "ring wave" impulse. The standards also require RCDs classified as "selective" to withstand a 3000 A impulse surge current of specified waveform.

Testing of correct operation

Test button

RCDs can be tested with a built-in test button to confirm functionality on a regular basis. RCDs may not operate correctly if wired improperly, so they are generally tested by the installer to verify correct operation. Use of a multifunction tester in the EU or a solenoid voltmeter in the USA. This introduces a controlled fault current from live to earth and measures the RCD operating time. This tests if the device is operational and can test the wiring to the RCD. Such a test may be performed on installation of the device and at any "downstream" outlet. (Upstream outlets are not protected.) To avoid needless tripping, only one RCD should be installed on any single circuit (excluding corded RCDs, such as bathroom small appliances).

Limitations

A residual-current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detect overload conditions, phase-to-neutral short circuits or phase-to-phase short circuits (see three-phase electric power). Over-current protection (fuses or circuit breakers) must be provided. Circuit breakers that combine the functions of an RCD with overcurrent protection respond to both types of fault. These are known as RCBOs and are available in 2-, 3- and 4-pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but use a common interrupting mechanism.

An RCD helps to protect against electric shock when current flows through a person from a phase (live / line / hot) to earth. It cannot protect against electric shock when current flows through a person from phase to neutral or from phase to phase, for example where a finger touches both live and neutral contacts in a light fitting; a device cannot differentiate between current flow through an intended load from flow through a person, though the RCD may still trip if the person is in contact with the ground (earth), as some current may still pass through the persons finger and body to earth.

Whole installations on a single RCD, common in older installations in the UK, are prone to "nuisance" trips that can cause secondary safety problems with loss of lighting and defrosting of food. Frequently the trips are caused by deteriorating insulation on heater elements, such as water heaters and cooker elements or rings. Although regarded as a nuisance, the fault is with the deteriorated element and not the RCD: replacement of the offending element will resolve the problem, but replacing the RCD will not.

In the case of RCDs that need a power supply, a dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding live wire remains uninterrupted. The tripping circuit needs power to work and does not trip when the power supply fails. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with the energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the live wire is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.

Related to this, a single-pole RCD/RCBO interrupts the energized conductor only, while a double-pole device interrupts both the energized and return conductors. Usually this is a standard and safe practice, since the return conductor is held at ground potential anyway. However, because of its design, a single-pole RCD will not isolate or disconnect all relevant wires in certain uncommon situations, for example where the return conductor is not being held, as expected, at ground potential, or where current leakage occurs between the return and earth conductors. In these cases, a double-pole RCD will offer protection, since the return conductor would also be disconnected.

History and nomenclature

The world's first high-sensitivity earth leakage protection system (i.e. a system capable of protecting people from the hazards of direct contact between a live conductor and earth), was a second-harmonic magnetic amplifier core-balance system, known as the magamp, developed in South Africa by Henri Rubin. Electrical hazards were of great concern in South African gold mines, and Rubin, an engineer at the company C.J. Fuchs Electrical Industries of Alberton Johannesburg, initially developed a cold-cathode system in 1955 which operated at 525 V and had a tripping sensitivity of 250 mA. Prior to this, core balance earth leakage protection systems operated at sensitivities of about 10 A.

The cold cathode system was installed in a number of gold mines and worked reliably. However, Rubin began working on a completely novel system with greatly improved sensitivity, and by early 1956, he had produced a prototype second-harmonic magnetic amplifier-type core balance system (South African Patent No. 2268/56 and Australian Patent No. 218360). The prototype magamp was rated at 220 V, 60 A and had an internally adjustable tripping sensitivity of 12.5–17.5 mA. Very rapid tripping times were achieved through a novel design, and this combined with the high sensitivity was well within the safe current-time envelope for ventricular fibrillation determined by Charles Dalziel of the University of California, Berkeley, USA, who had estimated electrical shock hazards in humans. This system, with its associated circuit breaker, included overcurrent and short-circuit protection. In addition, the original prototype was able to trip at a lower sensitivity in the presence of an interrupted neutral, thus protecting against an important cause of electrical fire.

Following the accidental electrocution of a woman in a domestic accident at the Stilfontein gold mining village near Johannesburg, a few hundred F.W.J. 20 mA magamp earth leakage protection units were installed in the homes of the mining village during 1957 and 1958. F.W.J. Electrical Industries, which later changed its name to FW Electrical Industries, continued to manufacture 20 mA single phase and three phase magamp units.

At the time that he worked on the magamp, Rubin also considered using transistors in this application, but concluded that the early transistors then available were too unreliable. However, with the advent of improved transistors, the company that he worked for and other companies later produced transistorized versions of earth leakage protection.

In 1961, Dalziel, working with Rucker Manufacturing Co., developed a transistorized device for earth leakage protection which became known as a Ground Fault Circuit Interrupter (GFCI), sometimes colloquially shortened to Ground Fault Interrupter (GFI). This name for high-sensitivity earth leakage protection is still in common use in the U.S.A.

In the early 1970s most North American GFCI devices were of the circuit breaker type. GFCIs built into the outlet receptacle became commonplace beginning in the 1980s. The circuit breaker type, installed into a distribution panel, suffered from accidental trips mainly caused by poor or inconsistent insulation on the wiring. False trips were frequent when insulation problems were compounded by long circuit lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current imbalance. The migration to outlet receptacle based protection in North American installations reduced the accidental trips and provided obvious verification that wet areas were under electrical code-required protection. European installations continue to use primarily RCDs installed at the distribution board, which provides protection in case of damage to fixed wiring; In Europe socket-based RCDs are primarily used for retrofitting.

Regulation and adoption

Regulations differ widely from country to country. A single RCD installed for an entire electrical installation provides protection against shock hazards to all circuits, however, any fault may cut all power to the premises. A solution is to create groups of circuits each with a RCD, or to use RCBO for each individual circuit.

Australia

In Australia, residual current devices have been mandatory on power circuits since 1991 and on light circuits since 2000. A minimum of two RCDs is required per domestic installation. All socket outlets and lighting circuits are to be distributed over circuit RCDs. A maximum of three subcircuits only, may be connected to a single RCD.

Austria

Austria regulated residual current devices in the ÖVE E8001-1/A1:2013-11-01 norm (most recent revision). It has been required in private housing since 1980. The maximum activation time must not exceed 0.4 seconds. It needs to be installed on all circuits with power plugs with a maximum leakage current of 30 mA and a maximum rated current of 16 A.

Additional requirements are placed on circuits in wet areas, construction sites and commercial buildings.

Belgium

Belgian domestic installations are required to be equipped with a 300 mA residual current device that protects all circuits. Furthermore, at least one 30 mA residual current device is required that protects all circuits in "wet rooms" (e.g. bathroom, kitchen) as well as circuits that power certain "wet" appliances (washing machine, tumble dryer, dishwasher). Electrical underfloor heating is required to be protected by a 100 mA RCD. These RCDs must be of type A.

Brazil

Since NBR 5410 (1997) residual current devices and grounding are required for new construction or repair in wet areas, outdoor areas, interior outlets used for external appliances, or in areas where water is more probable like bathrooms and kitchens.

Denmark

Denmark requires 30 mA RCDs on all circuits that are rated for less than 20 A (circuits at greater rating are mostly used for distribution). RCDs became mandatory in 1975 for new buildings, and then for all buildings in 2008.

France

According to the NF C15-100 regulation (1911 -> 2002), a general RCD not exceeding 100 to 300 mA at the origin of the installation is mandatory. Moreover, in rooms where there is water, high power or sensitive equipment (bathrooms, kitchens, IT...), each socket outlet must be protected by an RCD not exceeding 30 mA. The type of RCD required (A, AC, F) depends upon the type of the equipment that will be connected and the maximum power of the socket outlet. Minimal distances between electrical devices and water or the floor are described and mandatory.

Germany

Since 1 May 1984, RCDs are mandatory for all rooms with a bath tub or a shower. Since June 2007 Germany requires the use of RCDs with a trip current of no more than 30 mA on sockets rated up to 32 A which are for general use. (DIN VDE 0100-410 Nr. 411.3.3). It isn't allowed to use type "AC" RCDs since 1987, to be used to protect humans against electrical shocks. It must be Type "A" or type "B".

India

According to Regulation 36 of the Electricity Regulations 1990

a) For a place of public entertainment, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 10 mA.

b) For a place where the floor is likely to be wet or where the wall or enclosure is of low electrical resistance, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 10 mA.

c) For an installation where hand-held equipment, apparatus or appliance is likely to be used, protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 30 mA.

d) For an installation other than the installation in (a), (b) and (c), protection against earth leakage current must be provided by a residual current device of sensitivity not exceeding 100 mA.

Italy

The Italian law (n. 46 March 1990) prescribes RCDs with no more than 30 mA residual current (informally called "salvavita"—life saver, after early BTicino models, or differential circuit breaker for the mode of operation) for all domestic installations to protect all the lines. The law was recently updated to mandate at least two separate RCDs for separate domestic circuits. Short-circuit and overload protection has been compulsory since 1968.

Malaysia

In the latest guidelines for electrical wiring in residential buildings (2008) handbook, the overall residential wiring need to be protected by a residual current device of sensitivity not exceeding 100 mA. Additionally, all power sockets need to be protected by a residual current device of sensitivity not exceeding 30 mA and all equipment in wet places (water heater, water pump) need to be protected by a residual current device of sensitivity not exceeding 10 mA.

New Zealand

From January 2003, all new circuits originating at the switchboard supplying lighting or socket outlets (power points) in domestic buildings must have RCD protection. Residential facilities (such as boarding houses, hospitals, hotels and motels) will also require RCD protection for all new circuits originating at the switchboard supplying socket outlets. These RCDs will normally be located at the switchboard. They will provide protection for all electrical wiring and appliances plugged into the new circuits.

North America

A Leviton GFCI "Decora" socket in a North American kitchen. Local electrical code requires tamper-resistant socket in homes, and requires a GFCI for socket within 1 metre of a sink. The T-slot indicates this device is rated 20 A and can take either a NEMA 5-15 or a NEMA 5-20 plug, though the latter type is rare on household appliances.

In North America socket-outlets located in places where an easy path to ground exists—such as wet areas and rooms with uncovered concrete floors—must be protected by a GFCI. The US National Electrical Code has required devices in certain locations to be protected by GFCIs since the 1960s. Beginning with underwater swimming pool lights (1968) successive editions of the code have expanded the areas where GFCIs are required to include: construction sites (1974), bathrooms and outdoor areas (1975), garages (1978), areas near hot tubs or spas (1981), hotel bathrooms (1984), kitchen counter sockets (1987), crawl spaces and unfinished basements (1990), near wet bar sinks (1993), near laundry sinks (2005) and in laundry rooms (2014).

GFCIs are commonly available as an integral part of a socket or a circuit breaker installed in the distribution panelboard. GFCI sockets invariably have rectangular faces and accept so-called Decora face plates, and can be mixed with regular outlets or switches in a multi-gang box with standard cover plates. In both Canada and the US older two-wire, ungrounded NEMA 1 sockets may be replaced with NEMA 5 sockets protected by a GFCI (integral with the socket or with the corresponding circuit breaker) in lieu of rewiring the entire circuit with a grounding conductor. In such cases the sockets must be labeled "no equipment ground" and "GFCI protected"; GFCI manufacturers typically provide tags for the appropriate installation description.

GFCIs approved for protection against electric shock trip at 5 mA within 25 ms. A GFCI device which protects equipment (not people) is allowed to trip as high as 30 mA of current; this is known as an Equipment Protective Device (EPD). RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDs serve for equipment and fire protection instead of protection against the risks of electrical shocks.

In the United States the American Boat and Yacht Council requires both GFCIs for outlets and Equipment Leakage Circuit Interrupters (ELCI) for the entire boat. The difference is GFCIs trip on 5 mA of current whereas ELCIs trip on 30 mA after up to 100 ms. The greater values are intended to provide protection while minimizing nuisance trips.

Norway

In Norway, it has been required in all new homes since 2002, and on all new sockets since 2006. This applies to 32 A sockets and below. The RCD must trigger after a maximum 0.4 seconds for 230 V circuits, or 0.2 seconds for 400 V circuits.

South Africa

South Africa mandated the use of Earth Leakage Protection devices in residential environments (e.g. houses, flats, hotels, etc.) from October 1974, with regulations being refined in 1975 and 1976. Devices need to be installed in new premises and when repairs are carried out. Protection is required for power outlets and lighting, with the exception of emergency lighting that should not be interrupted. The standard device used in South Africa is indeed a hybrid of ELPD and RCCB.

Taiwan

Taiwan requires circuits of receptacles in washrooms, balconies, and receptacles in kitchen no more than 1.8 metres from the sink the use of earth leakage circuit breakers. This requirement also apply to circuit of water heater in washrooms and circuits that involves devices in water, lights on metal frames, public drinking fountains and so on. In principle, ELCBs should be installed on branch circuits, with trip current no more than 30 mA within 0.1 second according to Taiwanese law.

Turkey

Turkey requires the use of RCDs with no more than 30 mA and 300 mA in all new homes since 2004. This rule was introduced in RG-16/06/2004-25494.

United Kingdom

The current (18th) edition of the IEE Electrical Wiring Regulations requires that all socket outlets in most installations have RCD protection, though there are exemptions. Non armoured cables buried in walls must also be RCD protected (again with some specific exemptions). Provision of RCD protection for circuits present in bathrooms and shower rooms reduces the requirement for supplementary bonding in those locations. Two RCDs may be used to cover the installation, with upstairs and downstairs lighting and power circuits spread across both RCDs. When one RCD trips, power is maintained to at least one lighting and power circuit. Other arrangements, such as the use of RCBOs, may be employed to meet the regulations. The new requirements for RCDs do not affect most existing installations unless they are rewired, the distribution board is changed, a new circuit is installed, or alterations are made such as additional socket outlets or new cables buried in walls.

RCDs used for shock protection must be of the 'immediate' operation type (not time-delayed) and must have a residual current sensitivity of no greater than 30 mA.

If spurious tripping would cause a greater problem than the risk of the electrical accident the RCD is supposed to prevent (examples might be a supply to a critical factory process, or to life support equipment), RCDs may be omitted, providing affected circuits are clearly labelled and the balance of risks considered; this may include the provision of alternative safety measures.

The previous edition of the regulations required use of RCDs for socket outlets that were liable to be used by outdoor appliances. Normal practice in domestic installations was to use a single RCD to cover all the circuits requiring RCD protection (typically sockets and showers) but to have some circuits (typically lighting) not RCD protected. This was to avoid a potentially dangerous loss of lighting should the RCD trip. Protection arrangements for other circuits varied. To implement this arrangement it was common to install a consumer unit incorporating an RCD in what is known as a split load configuration, where one group of circuit breakers is supplied direct from the main switch (or time delay RCD in the case of a TT earth) and a second group of circuits is supplied via the RCD. This arrangement had the recognised problems that cumulative earth leakage currents from the normal operation of many items of equipment could cause spurious tripping of the RCD, and that tripping of the RCD would disconnect power from all the protected circuits.

Soil

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Soil

This is a diagram and related photograph of soil layers from bedrock to soil.

A, B, and C represent the soil profile, a notation firstly coined by Vasily Dokuchaev (1846–1903), the father of pedology. Here, A is the topsoil; B is a regolith; C is a saprolite (a less-weathered regolith); the bottom-most layer represents the bedrock.

Surface-water-gley developed in glacial till in Northern Ireland

Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. (Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.) The term pedolith, used commonly to refer to the soil, translates to ground stone in the sense fundamental stone, from the ancient Greek word πέδον, meaning 'ground, earth'.

Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution). Accordingly, soil is a three-state system of solids, liquids, and gases. Soil is a product of several factors: the influence of climate, relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time. It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion. Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem.

Most soils have a dry bulk density (density of soil taking into account voids when dry) between 1.1 and 1.6 g/cm³, though the soil particle density is much higher, in the range of 2.6 to 2.7 g/cm³. Little of the soil of planet Earth is older than the Pleistocene and none is older than the Cenozoic, although fossilized soils are preserved from as far back as the Archean.

The pedosphere interfaces with the lithosphere, the hydrosphere, the atmosphere, and the biosphere. Collectively, Earth's body of soil, called the pedosphere, has four important functions:

as a medium for plant growth
as a means of water storage, supply and purification
as a modifier of Earth's atmosphere
as a habitat for organisms

All of these functions, in their turn, modify the soil and its properties.

Soil science has two basic branches of study: edaphology and pedology. Edaphology studies the influence of soils on living things. Pedology focuses on the formation, description (morphology), and classification of soils in their natural environment. In engineering terms, soil is included in the broader concept of regolith, which also includes other loose material that lies above the bedrock, as can be found on the Moon and other celestial objects.

Processes

Soil is a major component of the Earth's ecosystem. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, with effects ranging from ozone depletion and global warming to rainforest destruction and water pollution. With respect to Earth's carbon cycle, soil acts as an important carbon reservoir, and it is potentially one of the most reactive to human disturbance and climate change. As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback (amplification). This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover.

Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth, making it a critically important provider of ecosystem services. Since soil has a tremendous range of available niches and habitats, it contains a prominent part of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored. Soil has a mean prokaryotic density of roughly 10⁸ organisms per gram, whereas the ocean has no more than 10⁷ prokaryotic organisms per milliliter (gram) of seawater. Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration, leading to the depletion of soil organic matter. Since plant roots need oxygen, aeration is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores, which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.

Soils can effectively remove impurities, kill disease agents, and degrade contaminants, this latter property being called natural attenuation. Typically, soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide. Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins. Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.

Composition

Soil profile: Darkened topsoil and reddish subsoil layers are typical of humid subtropical climate regions.

Components of a silt loam soil by percent volume

Water (25%)

Gases (25%)

Sand (18%)

Silt (18%)

Clay (9%)

Organic matter (5%)

A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas. The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other. The pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.

Given sufficient time, an undifferentiated soil will evolve a soil profile which consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity. The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of parent material, the processes that modify those parent materials, and the soil-forming factors that influence those processes. The biological influences on soil properties are strongest near the surface, though the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon. It has been suggested that the pedon, a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon (the living part, where most soil organisms are dwelling, corresponding to the humus form), the copedon (in intermediary position, where most weathering of minerals takes place) and the lithopedon (in contact with the subsoil).

The soil texture is determined by the relative proportions of the individual particles of sand, silt, and clay that make up the soil. The interaction of the individual mineral particles with organic matter, water, gases via biotic and abiotic processes causes those particles to flocculate (stick together) to form aggregates or peds. Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction (acidity), etc.

Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed. The mixture of water and dissolved or suspended materials that occupy the soil pore space is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the dissolution, precipitation and leaching of minerals from the soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.

Soils supply plants with nutrients, most of which are held in place by particles of clay and organic matter (colloids) The nutrients may be adsorbed on clay mineral surfaces, bound within clay minerals (absorbed), or bound within organic compounds as part of the living organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.

Plant nutrient availability is affected by soil pH, which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acid) where weathering is more advanced.

Most plant nutrients, with the exception of nitrogen, originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia, but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to soil formation and soil fertility. Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by volatilisation (loss to the atmosphere as gases) or leaching.

Formation

Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time. These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars and analogous conditions in planet Earth deserts.

An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants) become established very quickly on basaltic lava, even though there is very little organic material. Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series. The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes, inselbergs, and glacial moraines.

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time. When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.

Physical properties

The physical properties of soils, in order of decreasing importance for ecosystem services such as crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity. Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: sand, silt, and clay. At the next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides, carbonates, clay, silica and humus, coat particles and cause them to adhere into larger, relatively stable secondary structures. Soil bulk density, when determined at standardized moisture conditions, is an estimate of soil compaction. Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil. These properties vary through the depth of a soil profile, i.e. through soil horizons. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.

Soil moisture

Soil water content can be measured as volume or weight. Soil moisture levels, in order of decreasing water content, are saturation, field capacity, wilting point, air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes the dry limit for growing plants.

Available water capacity is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of adherence and sorptivity to withdraw water. Irrigation scheduling avoids moisture stress by replenishing depleted water before stress is induced.

Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas. Subirrigation designs (e.g., wicking beds, sub-irrigated planters) rely on capillarity to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through salination.

Soil moisture measurement–measuring the water content of the soil, as can be expressed in terms of volume or weight–can be based on in situ probes (e.g., capacitance probes, neutron probes), or remote sensing methods.

Soil gas

The atmosphere of soil, or soil gas, is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO₂ concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration. Calcareous soils regulate CO₂ concentration by carbonate buffering, contrary to acid soils in which all CO₂ respired accumulates in the soil pore system. At extreme levels, CO₂ is toxic. This suggests a possible negative feedback control of soil CO₂ concentration through its inhibitory effects on root and microbial respiration (also called 'soil respiration'). In addition, the soil voids are saturated with water vapour, at least until the point of maximal hygroscopicity, beyond which a vapour-pressure deficit occurs in the soil pore space. Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction. Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including greenhouse gases) as well as water. Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space (porosity) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil. Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO₃ to the gases N₂, N₂O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called denitrification. Aerated soil is also a net sink of methane (CH₄) but a net producer of methane (a strong heat-absorbing greenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots, bacteria, fungi, animals. These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks playing a decisive role in the stability, dynamics and evolution of soil ecosystems. Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.

Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated, a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin.

Solid phase (soil matrix)

Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential, but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.

Chemistry

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties. A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling. Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Cation-exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil; cmol_c/kg). Similarly, positively charged sites on colloids can attract and release anions in the soil, giving the soil anion exchange capacity.

Cation and anion exchange

The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.

Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.
Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).
Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.

Cations held to the negatively charged colloids resist being washed downward by water and are out of reach of plant roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures.

There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another (ion exchange). If present in equal amounts in the soil water solution:

Al³⁺ replaces H⁺ replaces Ca²⁺ replaces Mg²⁺ replaces K⁺ same as NH⁺
₄ replaces Na⁺

If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers (potash, lime).

As the soil solution becomes more acidic (low pH, meaning an abundance of H⁺), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites (protonation). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH. Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile. Plants are able to excrete H⁺ into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.

Cation exchange capacity (CEC)

Cation exchange capacity is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. CEC is the amount of exchangeable hydrogen cation (H⁺) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40 ÷ 2) × 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g. The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (e.g. tropical rainforests), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils. Live plant roots also have some CEC, linked to their specific surface area.

Cation exchange capacity for soils; soil textures; soil colloids
Soil	State	CEC meq/100 g
Charlotte fine sand	Florida	1.0
Ruston fine sandy loam	Texas	1.9
Glouchester loam	New Jersey	11.9
Grundy silt loam	Illinois	26.3
Gleason clay loam	California	31.6
Susquehanna clay loam	Alabama	34.3
Davie mucky fine sand	Florida	100.8
Sands	—	1–5
Fine sandy loams	—	5–10
Loams and silt loams	—	5–15
Clay loams	—	15–30
Clays	—	over 30
Sesquioxides	—	0–3
Kaolinite	—	3–15
Illite	—	25–40
Montmorillonite	—	60–100
Vermiculite (similar to illite)	—	80–150
Humus	—	100–300

Anion exchange capacity (AEC)

Anion exchange capacity is the soil's ability to remove anions (e.g. nitrate, phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC, followed by the iron oxides. Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils. Phosphates tend to be held at anion exchange sites.

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH⁻) for other anions. The order reflecting the strength of anion adhesion is as follows:

H
₂PO⁻
₄ replaces SO²⁻
₄ replaces NO⁻
₃ replaces Cl⁻

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil. As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).

Reactivity (pH)

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.

At 25 °C an aqueous solution that has a pH of 3.5 has 10^−3.5 moles H₃O⁺ (hydronium ions) per litre of solution (and also 10^−10.5 moles per litre OH⁻). A pH of 7, defined as neutral, has 10⁻⁷ moles of hydronium ions per litre of solution and also 10⁻⁷ moles of OH⁻ per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10^−9.5 moles hydronium ions per litre of solution (and also 10^−2.5 moles per litre OH⁻). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 (9.5 − 3.5 = 6 or 10⁶) and is more acidic.

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese. As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH, although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5. Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms, it has been suggested that plants, animals and microbes commonly living in acid soils are pre-adapted to every kind of pollution, whether of natural or human origin.

In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual rain acidity against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests. Once the colloids are saturated with H₃O⁺, the addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity. In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil. In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10. Beyond a pH of 9, plant growth is reduced. High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit. Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.

Base saturation percentage

There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called base saturation. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids (20 − 5 = 15 meq) are assumed occupied by base-forming cations, so that the base saturation is 15 ÷ 20 × 100% = 75% (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH). It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity). The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.

Buffering

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high buffering capacity. Buffering occurs by cation exchange and neutralisation. However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too. More generally, according to pH levels, several buffer systems take precedence over each other, from calcium carbonate buffer range to iron buffer range.

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydronium ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.

The addition of a small amount of lime, Ca(OH)₂, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO₂ and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.

Redox

Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. Reduction potential is measured in volts or millivolts. Soil microbial communities develop along electron transport chains, forming electrically conductive biofilms, and developing networks of bacterial nanowires.

Redox factors in soil development, where formation of redoximorphic color features provides critical information for soil interpretation. Understanding the redox gradient is important to managing carbon sequestration, bioremediation, wetland delineation, and soil-based microbial fuel cells.

Nutrients

Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake
Element	Symbol	Ion or molecule
Carbon	C	CO₂ (mostly through leaves)
Hydrogen	H	H⁺, H₂O (water)
Oxygen	O	O²⁻, OH⁻, CO²⁻ ₃, SO²⁻ ₄, CO₂
Phosphorus	P	H ₂PO⁻ ₄, HPO²⁻ ₄ (phosphates)
Potassium	K	K⁺
Nitrogen	N	NH⁺ ₄, NO⁻ ₃ (ammonium, nitrate)
Sulfur	S	SO²⁻ ₄
Calcium	Ca	Ca²⁺
Iron	Fe	Fe²⁺, Fe³⁺ (ferrous, ferric)
Magnesium	Mg	Mg²⁺
Boron	B	H₃BO₃, H ₂BO⁻ ₃, B(OH)⁻ ₄
Manganese	Mn	Mn²⁺
Copper	Cu	Cu²⁺
Zinc	Zn	Zn²⁺
Molybdenum	Mo	MoO²⁻ ₄ (molybdate)
Chlorine	Cl	Cl⁻ (chloride)

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl). Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation, the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant. A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.

Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.

The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.

Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter. However, despite the great capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability. All in all, small amounts of humus may remarkably increase the soil's capacity to promote plant growth.

Soil organic matter

The organic material in soil is made up of organic compounds and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 lb of earthworms, 2400 lb of fungi, 1500 lb of bacteria, 133 lb of protozoa and 890 lb of arthropods and algae.

A few percent of the soil organic matter, with small residence time, consists of the microbial biomass and metabolites of bacteria, molds, and actinomycetes that work to break down the dead organic matter. Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute to carbon sequestration in the topsoil through the formation of stable humus. In the aim to sequester more carbon in the soil for alleviating the greenhouse effect it would be more efficient in the long-term to stimulate humification than to decrease litter decomposition.

The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or humic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete. Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.

Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition which rate is dependent on temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by protozoa, which in turn are fed upon by nematodes, annelids and arthropods, themselves able to consume and transform raw or humified organic matter. This has been called the soil food web, through which all organic matter is processed as in a digestive system. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile.^[153] In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.

Humus

Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to soil health and plant growth. Humus also feeds arthropods, termites and earthworms which further improve the soil. The end product, humus, is suspended in colloidal form in the soil solution and forms a weak acid that can attack silicate minerals by chelating their iron and aluminum atoms. Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus. As the residues break down, only molecules made of aliphatic and aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus. Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure. Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.

Lignin is resistant to breakdown and accumulates within the soil. It also reacts with proteins, which further increases its resistance to decomposition, including enzymatic decomposition by microbes. Fats and waxes from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers. Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay. Proteins normally decompose readily, to the exception of scleroproteins, but when bound to clay particles they become more resistant to decomposition. As for other proteins clay particles absorb the enzymes exuded by microbes, decreasing enzyme activity while protecting extracellular enzymes from degradation. The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years. A study showed increased soil fertility following the addition of mature compost to a clay soil. High soil tannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.

Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present. Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility. Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity. Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia. Charcoal is a source of highly stable humus, called black carbon, which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis of Amazonian dark earths, has been renewed and became popular under the name of biochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.

Climatological influence

The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when a thawing event occurs, the flux of soil gases with atmospheric gases is significantly influenced. Temperature, soil moisture and topography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where decomposer activity is impeded by low temperature or excess moisture which results in anaerobic conditions. Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities. Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.

Plant residue

Typical types and percentages of plant residue components

Cellulose (45%)

Lignin (20%)

Hemicellulose (18%)

Protein (8%)

Sugars and starches (5%)

Fats and waxes (2%)

Cellulose and hemicellulose undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate. Brown rot fungi can decompose the cellulose and hemicellulose, leaving the lignin and phenolic compounds behind. Starch, which is an energy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of polymers composed of 500 to 600 units with a highly branched, amorphous structure, linked to cellulose, hemicellulose and pectin in plant cell walls. Lignin undergoes very slow decomposition, mainly by white rot fungi and actinomycetes; its half-life under temperate conditions is about six months.

Horizons

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions. No soil profile has all the major horizons. Some, called entisols, may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed mining waste deposits, moraines, volcanic cones, sand dunes or alluvial terraces. Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage. The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died. By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in sediment layers. Sampling pollen, testate amoebae and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, land use change) which occurred in the course of soil formation. Soil horizons can be dated by several methods such as radiocarbon, using pieces of charcoal provided they are of enough size to escape pedoturbation by earthworm activity and other mechanical disturbances. Fossil soil horizons from paleosols can be found within sedimentary rock sequences, allowing the study of past environments.

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils. The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts (leaf litter) or are directly produced belowground for subterranean plant organs (root litter), and then release dissolved organic matter. The remaining surficial organic layer, called the O horizon, produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live. After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.

Classification

One of the first soil classification systems was developed by Russian scientist Vasily Dokuchaev around 1880. It was modified a number of times by American and European researchers and was developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on soil morphology instead of parental materials and soil-forming factors. Since then, it has undergone further modifications. The World Reference Base for Soil Resources aims to establish an international reference base for soil classification.

Uses

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. Agricultural soil science was the primeval domain of soil knowledge, long time before the advent of pedology in the 19th century. However, as demonstrated by aeroponics, aquaponics and hydroponics, soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.

Soil material is also a critical component in mining, construction and landscape development industries. Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in surface mining, road building and dam construction. Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. Many building materials are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of subsistence agriculture.

Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans. Soil provides minerals and water to plants according to several processes involved in plant nutrition. Soil absorbs rainwater and releases it later, thus preventing floods and drought, flood regulation being one of the major ecosystem services provided by soil. Soil cleans water as it percolates through it. Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil, in the form of earthworms, woodlice, millipedes, centipedes, snails, slugs, mites, springtails, enchytraeids, nematodes, protists), bacteria, archaea, fungi and algae; and most organisms living above ground have part of them (plants) or spend part of their life cycle (insects) below-ground. Above-ground and below-ground biodiversities are tightly interconnected, making soil protection of paramount importance for any restoration or conservation plan.

The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even in deserts, cyanobacteria, lichens and mosses form biological soil crusts which capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in greenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.

Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Land application of waste water relies on soil biology to aerobically treat BOD. Alternatively, Landfills use soil for daily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells. Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.

Organic soils, especially peat, serve as a significant fuel and horticultural resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production. However, wide areas of peat production, such as rain-fed sphagnum bogs, also called blanket bogs or raised bogs, are now protected because of their patrimonial interest. As an example, Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now candidate for being included in the World Heritage List. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature, a contention which is still under debate when replaced at field scale and including stimulated plant growth.

Geophagy is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes. It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits), in order to alleviate tannin toxicity.

Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests (viruses) and pollutants, such as persistent organic pollutants (chlorinated pesticides, polychlorinated biphenyls), oils (hydrocarbons), heavy metals (lead, zinc, cadmium), and excess nutrients (nitrates, sulfates, phosphates) are filtered out by the soil. Soil organisms metabolise them or immobilise them in their biomass and necromass, thereby incorporating them into stable humus. The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.

Degradation

Land degradation is a human-induced or natural process which impairs the capacity of land to function. Soil degradation involves acidification, contamination, desertification, erosion or salination.

Acidification

Soil acidification is beneficial in the case of alkaline soils, but it degrades land when it lowers crop productivity, soil biological activity and increases soil vulnerability to contamination and erosion. Soils are initially acid and remain such when their parent materials are low in basic cations (calcium, magnesium, potassium and sodium). On parent materials richer in weatherable minerals acidification occurs when basic cations are leached from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming nitrogenous fertilizers and by the effects of acid precipitation. Deforestation is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence of tree canopies.

Contamination

Soil contamination at low levels is often within a soil's capacity to treat and assimilate waste material. Soil biota can treat waste by transforming it, mainly through microbial enzymatic activity. Soil organic matter and soil minerals can adsorb the waste material and decrease its toxicity, although when in colloidal form they may transport the adsorbed contaminants to subsurface environments. Many waste treatment processes rely on this natural bioremediation capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. Remediation of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore soil functions and values. Techniques include leaching, air sparging, soil conditioners, phytoremediation, bioremediation and Monitored Natural Attenuation. An example of diffuse pollution with contaminants is copper accumulation in vineyards and orchards to which fungicides are repeatedly applied, even in organic farming.

Microfibres from synthetic textiles are another type of plastic soil contamination, 100% of agricultural soil samples from southwestern China contained plastic particles, 92% of which were microfibres. Sources of microfibres likely included string or twine, as well as irrigation water in which clothes had been washed.

The application of biosolids from sewage sludge and compost can introduce microplastics to soils. This adds to the burden of microplastics from other sources (e.g. the atmosphere). Approximately half the sewage sludge in Europe and North America is applied to agricultural land. In Europe it has been estimated that for every million inhabitants 113 to 770 tonnes of microplastics are added to agricultural soils each year.

Desertification

Desertification, an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such as overgrazing or excess harvesting of firewood. It is a common misconception that drought causes desertification. Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover. These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification. It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.

Erosion

Erosion control

Erosion of soil is caused by water, wind, ice, and movement in response to gravity. More than one kind of erosion can occur simultaneously. Erosion is distinguished from weathering, since erosion also transports eroded soil away from its place of origin (soil in transit may be described as sediment). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitable land use practices. These include agricultural activities which leave the soil bare during times of heavy rain or strong winds, overgrazing, deforestation, and improper construction activity. Improved management can limit erosion. Soil conservation techniques which are employed include changes of land use (such as replacing erosion-prone crops with grass or other soil-binding plants), changes to the timing or type of agricultural operations, terrace building, use of erosion-suppressing cover materials (including cover crops and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes. Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-called dust bowl) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching.

A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the Loess Plateau region of northwest China.

Soil piping is a particular form of soil erosion that occurs below the soil surface. It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient. The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.

Salination

Soil salination is the accumulation of free salts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic. All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. Soil salinity control involves watertable control and flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.

Reclamation

Soils which contain high levels of particular clays with high swelling properties, such as smectites, are often very fertile. For example, the smectite-rich paddy soils of Thailand's Central Plains are among the most productive in the world. However, the overuse of mineral nitrogen fertilizers and pesticides in irrigated intensive rice production has endangered these soils, forcing farmers to implement integrated practices based on Cost Reduction Operating Principles.

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of shifting cultivation for a more permanent land use. Farmers initially responded by adding organic matter and clay from termite mound material, but this was unsustainable in the long-term because of rarefaction of termite mounds. Scientists experimented with adding bentonite, one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the International Water Management Institute in cooperation with Khon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of 200 kg bentonite per rai (6.26 rai = 1 hectare) resulted in an average yield increase of 73%. Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.

If the soil is too high in clay or salts (e.g. saline sodic soil), adding gypsum, washed river sand and organic matter (e.g.municipal solid waste) will balance the composition.

Adding organic matter, like ramial chipped wood or compost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.

Special mention must be made of the use of charcoal, and more generally biochar to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenic pre-Columbian Amazonian Dark Earths, also called Terra Preta de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus. However, the uncontrolled application of charred waste products of all kinds may endanger soil life and human health.

History of studies and research

The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.

Studies of soil fertility

The Greek historian Xenophon (450–355 BCE) is credited with being the first to expound upon the merits of green-manuring crops: 'But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung.'

Columella's Of husbandry, circa 60 CE, advocated the use of lime and that clover and alfalfa (green manure) should be turned under, and was used by 15 generations (450 years) under the Roman Empire until its collapse. From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European Middle Ages, Yahya Ibn al-'Awwam's handbook, with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence. Olivier de Serres, considered the father of French agronomy, was the first to suggest the abandonment of fallowing and its replacement by hay meadows within crop rotations. He also highlighted the importance of soil (the French terroir) in the management of vineyards. His famous book Le Théâtre d'Agriculture et mesnage des champs contributed to the rise of modern, sustainable agriculture and to the collapse of old agricultural practices such as soil amendment for crops by the lifting of forest litter and assarting, which ruined the soils of western Europe during the Middle Ages and even later on according to regions.

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century. In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight. John Woodward (d. 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.

As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about 1778 that plants and animals must combust oxygen internally to live. He was able to deduce that most of the 165-pound (75 kg) weight of van Helmont's willow tree derived from air. It was the French agriculturalist Jean-Baptiste Boussingault who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil. Justus von Liebig in his book Organic chemistry in its applications to agriculture and physiology (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced. Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.

The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station, founded by the former, and (re)discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the superphosphate, consisting in the acid treatment of phosphate rock. This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of coke was recovered and used as fertiliser. Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms was still not understood.

In 1856, J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates, and twenty years later Robert Warington proved that this transformation was done by living organisms. In 1890 Sergei Winogradsky announced he had found the bacteria responsible for this transformation.

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.

Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.

Studies of soil formation

The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate. However, soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic processes. After studies of the improvement of the soil commenced, other researchers began to study soil genesis and as a result also soil types and classifications.

In 1860, while in Mississippi, Eugene W. Hilgard (1833–1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic, and considered the classification of soil types. Unfortunately, his work was not continued. At about the same time, Friedrich Albert Fallou was describing soil profiles and relating soil characteristics to their formation as part of his professional work evaluating forest and farm land for the principality of Saxony. His 1857 book, Anfangsgründe der Bodenkunde (First principles of soil science) established modern soil science. Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to western Europe until 1914 through a publication in German by Konstantin Glinka, a member of the Russian team.

Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka's publication into English, and, as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system.

Search This Blog