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Monday, February 10, 2020

Soil science

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Soil_science 
 
A soil scientist examines horizons within the soil profile
 
Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.

Sometimes terms which refer to branches of soil science, such as pedology (formation, chemistry, morphology, and classification of soil) and edaphology (how soils interact with living things, especially plants), are used as if synonymous with soil science. The diversity of names associated with this discipline is related to the various associations concerned. Indeed, engineers, agronomists, chemists, geologists, physical geographers, ecologists, biologists, microbiologists, silviculturists, sanitarians, archaeologists, and specialists in regional planning, all contribute to further knowledge of soils and the advancement of the soil sciences. 

Soil scientists have raised concerns about how to preserve soil and arable land in a world with a growing population, possible future water crisis, increasing per capita food consumption, and land degradation.

Fields of study

Soil occupies the pedosphere, one of Earth's spheres that the geosciences use to organize the Earth conceptually. This is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Pedology is the study of soil in its natural setting. Edaphology is the study of soil in relation to soil-dependent uses. Both branches apply a combination of soil physics, soil chemistry, and soil biology. Due to the numerous interactions between the biosphere, atmosphere and hydrosphere that are hosted within the pedosphere, more integrated, less soil-centric concepts are also valuable. Many concepts essential to understanding soil come from individuals not identifiable strictly as soil scientists. This highlights the interdisciplinary nature of soil concepts. 

Research

Dependence on and curiosity about soil, exploring the diversity and dynamics of this resource continues to yield fresh discoveries and insights. New avenues of soil research are compelled by a need to understand soil in the context of climate change, greenhouse gases, and carbon sequestration. Interest in maintaining the planet's biodiversity and in exploring past cultures has also stimulated renewed interest in achieving a more refined understanding of soil.

Mapping

Most empirical knowledge of soil in nature comes from soil survey efforts. Soil survey, or soil mapping, is the process of determining the soil types or other properties of the soil cover over a landscape, and mapping them for others to understand and use. It relies heavily on distinguishing the individual influences of the five classic soil forming factors. This effort draws upon geomorphology, physical geography, and analysis of vegetation and land-use patterns. Primary data for the soil survey are acquired by field sampling and supported by remote sensing. 

Classification

Map of global soil regions from the USDA

As of 2006, the World Reference Base for Soil Resources, via its Land & Water Development division, is the pre-eminent soil classification system. It replaces the previous FAO soil classification.
The WRB borrows from modern soil classification concepts, including USDA soil taxonomy. The classification is based mainly on soil morphology as an expression pedogenesis. A major difference with USDA soil taxonomy is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics.

Many other classification schemes exist, including vernacular systems. The structure in vernacular systems are either nominal, giving unique names to soils or landscapes, or descriptive, naming soils by their characteristics such as red, hot, fat, or sandy. Soils are distinguished by obvious characteristics, such as physical appearance (e.g., color, texture, landscape position), performance (e.g., production capability, flooding), and accompanying vegetation. A vernacular distinction familiar to many is classifying texture as heavy or light. Light soil content and better structure, take less effort to turn and cultivate. Contrary to popular belief, light soils do not weigh less than heavy soils on an air dry basis nor do they have more porosity.

History

Contemporaries Friedrich Albert Fallou, the German founder of modern soil science, and Vasily Dokuchaev, the Russian founder of modern soil science, are both credited with being among the first to identify soil as a resource whose distinctness and complexity deserved to be separated conceptually from geology and crop production and treated as a whole. As a founding father of soil science Fallou has primacy in time. Fallou was working on the origins of soil before Dokuchaev was born, however Dokuchaev's work was more extensive and is considered to be the more significant to modern soil theory than Fallou's.
Previously, soil had been considered a product of chemical transformations of rocks, a dead substrate from which plants derive nutritious elements. Soil and bedrock were in fact equated. Dokuchaev considers the soil as a natural body having its own genesis and its own history of development, a body with complex and multiform processes taking place within it. The soil is considered as different from bedrock. The latter becomes soil under the influence of a series of soil-formation factors (climate, vegetation, country, relief and age). According to him, soil should be called the "daily" or outward horizons of rocks regardless of the type; they are changed naturally by the common effect of water, air and various kinds of living and dead organisms.
A 1914 encyclopedic definition: "the different forms of earth on the surface of the rocks, formed by the breaking down or weathering of rocks". serves to illustrate the historic view of soil which persisted from the 19th century. Dokuchaev's late 19th century soil concept developed in the 20th century to one of soil as earthy material that has been altered by living processes. A corollary concept is that soil without a living component is simply a part of earth's outer layer. 

Further refinement of the soil concept is occurring in view of an appreciation of energy transport and transformation within soil. The term is popularly applied to the material on the surface of the Earth's moon and Mars, a usage acceptable within a portion of the scientific community. Accurate to this modern understanding of soil is Nikiforoff's 1959 definition of soil as the "excited skin of the sub aerial part of the earth's crust".

Areas of practice

Academically, soil scientists tend to be drawn to one of five areas of specialization: microbiology, pedology, edaphology, physics, or chemistry. Yet the work specifics are very much dictated by the challenges facing our civilization's desire to sustain the land that supports it, and the distinctions between the sub-disciplines of soil science often blur in the process. Soil science professionals commonly stay current in soil chemistry, soil physics, soil microbiology, pedology, and applied soil science in related disciplines.

One interesting effort drawing in soil scientists in the USA as of 2004 is the Soil Quality Initiative. Central to the Soil Quality Initiative is developing indices of soil health and then monitoring them in a way that gives us long term (decade-to-decade) feedback on our performance as stewards of the planet. The effort includes understanding the functions of soil microbiotic crusts and exploring the potential to sequester atmospheric carbon in soil organic matter. The concept of soil quality, however, has not been without its share of controversy and criticism, including critiques by Nobel Laureate Norman Borlaug and World Food Prize Winner Pedro Sanchez.

A more traditional role for soil scientists has been to map soils. Most every area in the United States now has a published soil survey, which includes interpretive tables as to how soil properties support or limit activities and uses. An internationally accepted soil taxonomy allows uniform communication of soil characteristics and soil functions. National and international soil survey efforts have given the profession unique insights into landscape scale functions. The landscape functions that soil scientists are called upon to address in the field seem to fall roughly into six areas:
There are also practical applications of soil science that might not be apparent from looking at a published soil survey.

Fields of application in soil science

Related disciplines


Depression storage capacity

Depression storage capacity, in soil science, is the ability of a particular area of land to retain water in its pits and depressions, thus preventing it from flowing. Depression storage capacity, along with infiltration capacity, is one of the main factors involved in Horton overland flow, whereby water volume surpasses both infiltration and depression storage capacity and begins to flow horizontally across land, possibly leading to flooding and soil erosion. The study of land's depression storage capacity is important in the fields of geology, ecology, and especially hydrology.
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Humus

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Humus
 
Humus has a characteristic black or dark brown color and is an accumulation of organic carbon. Besides the three major soil horizons of (A) surface/topsoil, (B) subsoil, and (C) substratum, some soils have an organic horizon (O) on the very surface. Hard bedrock (R) is not in a strict sense soil.
 
In soil science, humus (derived in 1790–1800 from the Latin humus for earth, ground) denominates the fraction of soil organic matter that is amorphous and without the "cellular cake structure characteristic of plants, micro-organisms or animals". Humus significantly affects the bulk density of soil and contributes to its retention of moisture and nutrients.

In agriculture, "humus" sometimes also is used to describe mature or natural compost extracted from a woodland or other spontaneous source for use as a soil conditioner. It is also used to describe a topsoil horizon that contains organic matter (humus type, humus form, humus profile).

Humus is the dark organic matter that forms in soil when dead plant and animal matter decays. Humus has many nutrients that improve the health of soil, nitrogen being the most important. The ratio of carbon to nitrogen (C:N) of humus is 10:1. 

Description

It is difficult to define humus precisely because it is a very complex substance which is not fully understood. Humus is different from decomposing soil organic matter. The latter looks rough and has visible remains of the original plant or animal matter. Fully humified humus, on the contrary, has a uniformly dark, spongy, and jelly-like appearance, and is amorphous; it may gradually decompose over several years or persist for millennia. It has no determinate shape, structure, or quality. However, when examined under a microscope, humus may reveal tiny plant, animal, or microbial remains that have been mechanically, but not chemically, degraded. This suggests an ambiguous boundary between humus and soil organic matter. While distinct, humus is an integral part of soil organic matter.

Humification

Microorganisms decompose a large portion of the soil organic matter into inorganic minerals that the roots of plants can absorb as nutrients. This process is termed "mineralization". In this process, nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) in the decomposed organic matter are recycled. Depending on the conditions in which the decomposition occurs, a fraction of the organic matter does not mineralize, and instead is transformed by a process called "humification" into concatenations of organic polymers. Because these organic polymers are resistant to the action of microorganisms, they are stable, and constitute humus. This stability implies that humus integrates into the permanent structure of the soil, thereby improving it.

Humification can occur naturally in soil or artificially in the production of compost. Organic matter is humified by a combination of saprotrophic fungi, bacteria, microbes and animals such as earthworms, nematodes, protozoa, and arthropods. Plant remains, including those that animals digested and excreted, contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins, and organic acids. Decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which decompose easily as detritivores initially invade the dead plant organs, while the remaining cellulose and lignin decompose more slowly. Simple proteins, organic acids, starches, and sugars decompose rapidly, while crude proteins, fats, waxes, and resins remain relatively unchanged for longer periods of time. Lignin, which is quickly transformed by white-rot fungi, is one of the primary precursors of humus, together with by-products of microbial and animal activity. The humus produced by humification is thus a mixture of compounds and complex biological chemicals of plant, animal, or microbial origin that has many functions and benefits in soil. Some judge earthworm humus (vermicompost) to be the optimal organic manure.

Stability

Much of the humus in most soils has persisted for more than 100 years, rather than having been decomposed into CO2, and can be regarded as stable; this organic matter has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles, or tightly sorbed or complexed to clays. Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or more labile. Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure. A very stable form of humus is formed from the slow oxidation of soil carbon after the incorporation of finely powdered charcoal into the topsoil. This process is speculated to have been important in the formation of the very fertile Amazonian terra preta do Indio.

Horizons

Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon. Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons: the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. The master horizon (E) is used for subsurface horizons that have significantly lost minerals (eluviation). Bedrock, which is not soil, uses the letter R.

Benefits of soil organic matter and humus

The importance of chemically stable humus is thought by some to be the fertility it provides to soils in both a physical and chemical sense, though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease. It helps the soil retain moisture by increasing microporosity, and encourages the formation of good soil structure. The incorporation of oxygen into large organic molecular assemblages generates many active, negatively charged sites that bind to positively charged ions (cations) of plant nutrients, making them more available to the plant by way of ion exchange. Humus allows soil organisms to feed and reproduce, and is often described as the "life-force" of the soil.

  • The process that converts soil organic matter into humus feeds the population of microorganisms and other creatures in the soil, and thus maintains high and healthy levels of soil life.
  • The rate at which soil organic matter is converted into humus promotes (when fast) or limits (when slow) the coexistence of plants, animals, and microorganisms in the soil.
  • Effective humus and stable humus are additional sources of nutrients for microbes: the former provides a readily available supply and the latter acts as a longeval storage reservoir.
  • Decomposition of dead plant material causes complex organic compounds to be slowly oxidized (lignin-like humus) or to decompose into simpler forms (sugars and amino sugars, and aliphatic and phenolic organic acids), which are further transformed into microbial biomass (microbial humus) or reorganized, and further oxidized, into humic assemblages (fulvic acids and humic acids), which bind to clay minerals and metal hydroxides. The ability of plants to absorb humic substances with their roots and metabolize them has been long debated. There is now a consensus that humus functions hormonally rather than simply nutritionally in plant physiology.
  • Humus is a colloidal substance and increases the cation exchange capacity of soil, hence its ability to store nutrients by chelation. While these nutrient cations are available to plants, they are held in the soil and prevented from being leached by rain or irrigation.
  • Humus can hold the equivalent of 80–90% of its weight in moisture, and therefore increases the soil's capacity to withstand drought.
  • The biochemical structure of humus enables it to moderate, i.e. buffer, excessive acidic or alkaline soil conditions.
  • During humification, microbes secrete sticky, gum-like mucilages; these contribute to the crumby structure (tilth) of the soil by adhering particles together and allowing greater aeration of the soil. Toxic substances such as heavy metals and excess nutrients can be chelated, i. e., bound to the organic molecules of humus, and so prevented from leaching away.
  • The dark, usually brown or black, color of humus helps to warm cold soils in Spring.
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Climate-friendly gardening

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Climate-friendly_gardening

Climate-friendly gardening is gardening in ways which reduce emissions of greenhouse gases from gardens and encourage the absorption of carbon dioxide by soils and plants in order to aid the reduction of global warming. To be a climate-friendly gardener means considering both what happens in a garden and the materials brought into it and the impact they have on land use and climate. It can also include garden features or activities in the garden that help to reduce greenhouse gas emissions elsewhere.

Orchard garden showing orchard trees, herbaceous perennials and ground-cover plants, at Hergest Croft Gardens, Herefordshire, Britain.

Land use and greenhouse gases

Most of the excess greenhouse gases causing climate change have come from burning fossil fuel. But a special report from the Intergovernmental Panel on Climate Change (IPCC) estimated that in the last 150 years fossil fuels and cement production were responsible for only about two-thirds of climate change: the other third has been caused by human land use.

The three main greenhouse gases produced by unsustainable land use are carbon dioxide, methane, and nitrous oxide. Black carbon or soot can also be caused by unsustainable land use, and, although not a gas, can behave like greenhouse gases and contribute to climate change.

Carbon dioxide

Carbon dioxide, CO
2, is a natural part of the carbon cycle, but human land uses often add more, especially from habitat destruction and the cultivation of soil. When woodlands, wetlands, and other natural habitats are turned into pasture, arable fields, buildings and roads, the carbon held in the soil and vegetation becomes extra carbon dioxide and methane to trap more heat in the atmosphere.

Gardeners may cause extra carbon dioxide to be added to the atmosphere in several ways:
Gardeners will also be responsible for extra carbon dioxide when they buy garden products which have been transported by vehicles powered by fossil fuel.

Methane

Methane, CH4, is a natural part of the carbon cycle, but human land uses often add more, especially from anaerobic soil, artificial wetlands such as rice fields, and from the guts of farm animals, especially ruminants such as cattle and sheep.

Gardeners may cause extra methane to be added to the atmosphere in several ways:
  • Compacting soil so that it becomes anaerobic, for example by treading on soil when it is wet;
  • Allowing compost heaps to become compacted and anaerobic;
  • Creating homemade liquid feed by putting the leaves of plants such as comfrey under water, with the unintended consequence that the plants may release methane as they decay;
  • Killing pernicious weeds by covering them with water, with the unintended consequence that the plants may release methane as they decay;
  • Allowing ponds to become anaerobic, for example by adding unsuitable fish species which stir up sediment that then blocks light from and kills submerged oxygenating plants.

Nitrous oxide

Nitrous oxide, N2O, is a natural part of the nitrogen cycle, but human land uses often add more.

Gardeners may cause extra nitrous oxide to be added to the atmosphere by:
  • Using synthetic nitrogen fertilizer, for example "weed and feed" on lawns, especially if it is applied when plants are not actively growing, the soil is compacted, or when other factors are limiting so that the plants cannot make use of the nitrogen;
  • Compacting the soil (for example by working in the garden when the soil is wet) which will increase the conversion of nitrates to nitrous oxide by soil bacteria;
  • Burning garden waste on bonfires.

Black carbon

Black carbon is not a gas, but it acts like a greenhouse gas because it can be suspended in the atmosphere and absorb heat.

Gardeners may cause extra black carbon to be added to the atmosphere by burning garden prunings and weeds on bonfires, especially if the waste is wet and becomes black carbon in the form of soot. Gardeners will also be responsible for extra black carbon produced when they buy garden products which have been transported by vehicles powered by fossil fuel especially the diesel used in most lorries. 

Gardening to reduce greenhouse gas emissions and absorb carbon dioxide

There are many ways in which climate-friendly gardeners may reduce their contribution to climate change and help their gardens absorb carbon dioxide from the atmosphere.

Climate-friendly gardeners can find good ideas in many other sustainable approaches:

Protecting and enhancing carbon stores


Protecting carbon stores in land beyond gardens

Woodland and wetland in the New Forest, Hampshire
 
Woodland and trees in Herefordshire
 
Kitchen garden at Charles Darwin's home, Down House, Kent, showing greenhouse, waterbutt, box hedging and vegetable beds.
 
Alliums, lavender, box and other water-thrifty plants in the dry garden at Cambridge Botanic Garden

Climate-friendly gardening includes actions which protect carbon stores beyond gardens. The biggest carbon stores in land are in soil; the two habitat types with the biggest carbon stores per hectare are woods and wetlands; and woods absorb more carbon dioxide per hectare per year than most other habitats. Climate-friendly gardeners therefore aim to ensure that nothing they do will harm these habitats.

According to Morison and Morecroft (eds.)'s Plant Growth and Climate Change, the net primary productivity (the net amount of carbon absorbed each year) of various habitats is:
The Intergovernmental Panel on Climate Change's Special Report Land use, land-use change, and forestry  lists the carbon contained in different global habitats as:
  • Wetlands: 643 tonnes carbon per hectare in soil + 43 tonnes carbon per hectare in vegetation = total 686 tonnes carbon per hectare;
  • Tropical forests: 123 tonnes carbon per hectare in soil + 120 tonnes carbon per hectare in vegetation = total 243 tonnes carbon per hectare;
  • Temperate forests: 96 tonnes carbon per hectare in soil + 57 tonnes carbon per hectare in vegetation = total 153 tonnes carbon per hectare;
  • Temperate grasslands: 164 tonnes carbon per hectare in soil + 7 tonnes carbon per hectare in vegetation = total 171 tonnes carbon per hectare;
  • Croplands: 80 tonnes carbon per hectare in soil + 2 tonnes carbon per hectare in vegetation = total 82 tonnes carbon per hectare.
The figures quoted above are global averages. More recent research in 2009 has found that the habitat with the world's highest known total carbon density - 1,867 tonnes of carbon per hectare - is temperate moist forest of Eucalyptus regnans in the Central Highlands of south-east Australia; and, in general, that temperate forests contain more carbon than either boreal forests or tropical forests.

Carbon stores in Britain

According to Milne and Brown's 1997 paper "Carbon in the vegetation and soils of Great Britain",[30] Britain's vegetation and soil are estimated to contain 9952 million tonnes of carbon, of which almost all is in the soil, and most in Scottish peatland soil:
  • Soils in Scotland: 6948 million tonnes carbon;
  • Soils in England and Wales: 2890 million tonnes carbon;
  • Vegetation in British woods and plantations (which cover only 11% of Britain's land area): 91 million tonnes carbon;
  • Other vegetation: 23 million tonnes carbon.
A 2005 report suggested that British woodland soil may contain as much as 250 tonnes of carbon per hectare. 

Many studies of soil carbon only study the carbon in the top 30 centimetres, but soil is often much deeper than that, especially below woodland. One 2009 study of the United Kingdom's carbon stores by Keith Dyson and others gives figures for soil carbon down to 100 cm below the habitats, including "Forestland", "Cropland" and "Grassland", covered by the Kyoto Protocol reporting requirements.
  • Forestland soils: average figures in tonnes carbon per hectare are 160 (England), 428 (Scotland), 203 (Wales), and 366 (Northern Ireland).
  • Grassland soils: average figures in tonnes carbon per hectare are 148 (England), 386 (Scotland), 171 (Wales), and 304 (Northern Ireland).
  • Cropland soils: average figures in tonnes carbon per hectare are 110 (England), 159 (Scotland), 108 (Wales), and 222 (Northern Ireland).

Protecting carbon stores in wetland

Permeable paving of wood chip with birch-log edging at the Royal Horticultural Society garden at Wisley
 
A ground-cover and rain-garden plant - Symphytum grandiflorum, creeping comfrey (with Cotinus coggygria)
 
Climate-friendly gardeners choose peat-free composts because some of the planet's biggest carbon stores are in soil, and especially in the peatland soil of wetlands. 

The Intergovernmental Panel on Climate Change's Special Report Land Use, Land-Use Change and Forestry gives a figure of 2011 gigatonnes of carbon for global carbon stocks in the top 1 metre of soils, much more than the carbon stores in the vegetation or the atmosphere.

Climate-friendly gardeners also avoid using tapwater not only because of the greenhouse gases emitted when fossil fuels are burnt to treat and pump water, but because if water is taken from wetlands then carbon stores are more likely to be oxidised to carbon dioxide.

A climate-friendly garden therefore does not contain large irrigated lawns, but instead includes water-butts to collect rainwater; water-thrifty plants which survive on rainwater and do not need watering after they are established; trees, shrubs and hedges to shelter gardens from the drying effects of sun and wind; and groundcover plants and organic mulch to protect the soil and keep it moist.

Climate-friendly gardeners will ensure that any paved surfaces in their gardens (which are kept to a minimum to increase carbon stores) are permeable, and may also make rain gardens, sunken areas into which rainwater from buildings and paving is directed, so that the rain can then be fed back into groundwater rather than going into storm drains. The plants in rain gardens must be able to grow in both dry and wet soils.

Protecting carbon stores in woodland

Wetlands may store the most carbon in their soils, but woods store more carbon in their living biomass than any other type of vegetation, and their soils store the most carbon after wetlands. Climate-friendly gardeners therefore ensure that any wooden products they buy, such as garden furniture, have been made of wood from sustainably managed woodland. 

Protecting and increasing carbon stores in gardens

Juglans elaeopyren, an American walnut, at Cambridge Botanic Garden
 
After rocks containing carbonate compounds, soil is the biggest store of carbon on land. Carbon is found in soil organic matter, including living organisms (plant roots, fungi, animals, protists, bacteria), dead organisms, and humus. One study of the environmental benefits of gardens estimates that 86% of carbon stores in gardens is in the soil.

Wild strawberries in flower below a British hedge.

The first priorities for climate-friendly gardeners are, therefore, to:
  • Protect the soil's existing carbon stores;
  • Increase the soil's carbon stores.
To protect the soil, climate-friendly gardens:
Mulch of woodchips protecting soil at the Royal Horticultural Society garden at Wisley in Surrey.
 
Climate-friendly gardeners avoid things which may harm soil. They do not tread on the soil when it is wet, because it is then most vulnerable to compaction. They dig as little is possible, and only when the soil is moist rather than wet, because cultivation increases the oxidation of soil organic matter and produces carbon dioxide.

To increase soil carbon stores, climate-friendly gardeners ensure that their gardens create optimal conditions for vigorous healthy growth of plants, and other garden organisms above and below ground, and reduce the impact of any limiting factors. 

In general, the more biomass that the plants can create each year, the more carbon will be added to the soil. However, only some biomass each year becomes long-term soil carbon or humus. In Soil Carbon and Organic Farming, a 2009 report for the Soil Association, Gundula Azeez discusses several factors which increase how much biomass is turned into humus. These include good soil structure, soil organisms such as fine root hairs, microorganisms, mycorrhizas and earthworms which increase soil aggregation, residues from plants (such as trees and shrubs) which have a high content of resistant chemicals such as lignin, and plant residues with a carbon to nitrogen ratio lower than about 32:1.

Nitrogen-fixing nodules on Wisteria roots (hazelnut for scale)
 
Climate-friendly gardens therefore include:
  • Hedges for shelter from wind;
  • A light canopy of late-leafing deciduous trees to let in enough sunlight for growth but not so much that the garden becomes too hot and dry (this is one of the principles behind many agroforestry systems, such as Paulownia's use in China partly because it is late-leafing and its canopy is sparse so that crops below it get shelter but also enough light);
  • Groundcover plants and organic mulches (such as woodchips over compost made from kitchen and garden "waste") to keep soil moist and at relatively stable temperatures;
  • Nitrogen-fixing plants, because soil nitrogen may be a limiting factor (but climate-friendly gardeners avoid synthetic nitrogen fertilizers, because these may cause mycorrhizal associations to break down);
  • Many layers of plants, including woody plants such as trees and shrubs, other perennials, groundcover plants, deep-rooted plants, all chosen according to 'right plant, right place', so that they are suited to their growing conditions and will grow well;
  • A wide diversity of disease-resistant, vigorous plants for resilience and to make the most of all available ecological niches;
  • Plants to feed and shelter wildlife, to increase total biomass, and to ensure biological control of pests and diseases.
  • Compost made from garden and kitchen "waste".
Lawns, like other grasslands, can build up good levels of soil carbon, but they will grow more vigorously and store more carbon if besides grasses they also contain nitrogen-fixing plants such as clover, and if they are cut using a mulching mower which returns finely-chopped mowings to the lawn. More carbon, however, may be stored by other perennial plants such as trees and shrubs. They also do not need to be maintained using power tools. 

Climate-friendly gardeners will also aim to increase biodiversity not only for the sake of the wildlife itself, but so that the garden ecosystem is resilient and more likely to store as much carbon as possible as long as possible. They will therefore avoid pesticides, and increase the diversity of the habitats within their gardens. 

Reducing greenhouse gas emissions

Climate-friendly gardeners can directly reduce the greenhouse gas emissions from their own gardens, but can also use their gardens to indirectly reduce greenhouse gas emissions elsewhere. 

Using gardens to reduce greenhouse gas emissions

Climate-friendly gardeners can use their gardens in ways which reduce greenhouse gases elsewhere, for example by using the sun and wind to dry washing on washing lines in the garden instead of using electricity generated by fossil fuel to dry washing in tumble dryers.

From farmland

Walnut, Juglans regia, with ripening walnuts

Food is a major contributor to climate change. In the United Kingdom, according to Tara Garnett of the Food Climate Research Network, food contributes 19% of the country's greenhouse gas emissions.

Soil is the biggest store of carbon on land. It is therefore important to protect the soil organic matter in farmland. Farm animals, however, especially free-range pigs, may cause erosion, and cultivation of the soil increases the oxidation of soil organic matter into carbon dioxide. Other sources of greenhouse gases from farmland include: compaction caused by farm machinery or overgrazing by farm animals can make soil anaerobic and produce methane; farm animals produce methane; and nitrogen fertilizers can be converted to nitrous oxide. 

Most farmland consists of fields growing annual arable crops which are eaten directly by people or fed to farm animals, and grassland used as pasture, hay or silage to feed farm animals. Some perennial food plants are also grown, such as fruits and nuts in orchards, and watercress grown in water.
Although all cultivation of the soil in arable fields produces carbon dioxide, some arable crops cause more damage to soil than others. Root crops such as potatoes and sugar-beet, and crops which are harvested not just once a year but over a long period such as green vegetables and salads, are considered "high risk" in catchment-sensitive farming.

Climate-friendly gardeners therefore grow at least some of their food, and may choose food crops which therefore help to keep carbon in farmland soils if they grow such high-risk crops in small vegetable plots in their gardens, where it is easier to protect the soil than in large fields under commercial pressures. Climate-friendly gardeners may grow and eat plants such as sweet cicely which sweeten food, and so reduce the land area needed for sugar-beet. They may also choose to grow perennial food plants to not only reduce their indirect greenhouse gas emissions from farmland, but also to increase carbon stores in their own gardens.

Grassland contains more carbon per hectare than arable fields, but farm animals, especially ruminants such as cattle or sheep, produce large amounts of methane, directly and from manure heaps and slurry. Slurry and manure may also produce nitrous oxide. Gardeners who want to reduce their greenhouse gas emissions can help themselves to eat less meat and dairy produce by growing nut trees which are a good source of tasty, protein-rich food, including walnuts which are an excellent source of the omega-3 fatty acid alpha-linolenic acid.

Researchers and farmers are investigating and improving ways of farming which are more sustainable, such as agroforestry, forest farming, wildlife-friendly farming, soil management, catchment-sensitive farming (or water-friendly farming). For example, the organisation Farming Futures assists farmers in the United Kingdom to reduce their farms' greenhouse gas emissions.

Farmers are aware that consumers are increasingly asking for "green credentials". Gardeners who understand climate-friendly practices can advocate their use by farmers.

From industry

Nitrogen-fixing and edible - Elaeagnus umbellatus at the Agroforestry Research Trust forest garden in Devon
 
Climate-friendly gardeners aim to reduce their consumption in general. In particular, they try to avoid or reduce their consumption of tapwater because of the greenhouse gases emitted when fossil fuels are burnt to supply the energy needed to treat and pump it to them. Instead, gardeners can garden using only rainwater.

Greenhouse gases are produced in the manufacture of many materials and products used by gardeners. For example, it takes a lot of energy to produce synthetic fertilizers, especially nitrogen fertilizers. Ammonium nitrate, for example, has an embodied energy of 67000 kilojoules/kilogramme, so climate-friendly gardeners will choose alternative ways of ensuring the soil in their gardens has optimal levels of nitrogen by alternative means such as nitrogen-fixing plants. 

Climate-friendly gardeners will also aim to follow "cradle-to-cradle design" and "circular economy" principles: when they choose to buy or make something, it should be possible to take it apart again and recycle or compost every part, so that there is no waste, only raw materials to be made into something else. This will reduce the greenhouse gases otherwise produced when extracting raw materials. 

From transport

Gardeners can reduce not only their food miles by growing some of their own food, but also their "gardening miles" by reducing the amount of plants and other materials they import, obtaining them as locally as possible and with as little packaging as possible. This might include ordering plants by mail order from a specialist nursery if the plants are sent out bare-root, reducing transport demand and the use of peat-based composts; or growing plants from seed, which will also increase genetic diversity and therefore resilience; or growing plants vegetatively from cuttings or offsets from other local gardeners; or buying reclaimed materials from salvage firms.

From houses

Climbers as insulation - Boston ivy, Parthenocissus tricuspidata, Boston ivy, in autumn

Climate-friendly gardeners can use their gardens in ways which reduce greenhouse gas emissions from homes by:
  • Using sunlight and wind to dry washing on washing lines instead of fossil fuel-generated electricity to run tumble dryers;
  • Planting deciduous climbers on houses and planting deciduous trees at suitable distances from the house to provide shade during the summer, reducing the consumption of electricity for air conditioning, but also such that at cooler times of year, sunlight can reach and warm a house, reducing heating costs and consumption;
  • Planting hedges, trees, shrubs and climbers to shelter houses from wind, reducing heating costs and consumption during the winter (as long as any planting does not create a wind-tunnel effect).
Climate-friendly gardeners may also choose to reduce their own personal greenhouse gas emissions by growing and eating carminative plants such as fennel and garlic which reduce intestinal gases such as methane.

Reducing greenhouse gas emissions from gardens

Slow-growing yew, Taxus baccata, as hedge at Charles Darwin's home, Down House, Kent
 
Nitrogen-fixing red and white clover (Trifolium) as lawn plants
 
Leaf cage, compost heap and wormery at the Royal Horticultural Society garden at Wisley
 
There are some patent sources of greenhouse gas emissions in gardens and some more latent.

Power tools which are powered by diesel or petrol, or electricity generated by burning other fossil fuels, emit carbon dioxide. Climate-friendly gardeners may therefore choose to use hand tools rather than power tools, or power tools powered by renewable electricity, or design their gardens to reduce or remove a need to use power tools. For example, they may choose dense, slow-growing species for hedges so that the hedges only need to be cut once a year.

Lawns need[?] to be cut by lawn mowers and, in drier parts of the world, are often irrigated by tapwater. Climate-friendly gardeners will therefore do what they can to reduce this consumption by:
  • Replacing part of or all lawns with other perennial planting such as trees and shrubs with less ecologically demanding maintenance requirements;
  • Cut some or all lawns only once or twice a year, i.e. convert them into meadows;
  • Make lawn shapes simple so that they may be cut quickly;
  • Increase the cutting height of mower blades;
  • Use a mulching mower to return organic matter to the soil;
  • Sow clover to increase vigour (without the need for synthetic fertilisers) and resilience in dry periods;
  • Cut lawns with electric mowers using electricity from renewable energy;
  • Cut lawns with hand tools such as push mowers or scythes.
Greenhouses can be used to grow crops which might otherwise be imported from warmer climates, but if they are heated by fossil fuel then they may cause more greenhouse gas emissions than they save. Climate-friendly gardeners will therefore use their greenhouses carefully by:
  • Choosing only annual plants which will only be in the greenhouse during warmer months, or perennial plants which do not need any extra heat during winter;
  • Using water tanks as heat stores and compost heaps as heat sources inside greenhouses so that they stay frost-free in winter.
Climate-friendly gardeners will not put woody prunings on bonfires, which will emit carbon dioxide and black carbon, but instead burn them indoors in a wood-burning stove and therefore cut emissions from fossil fuel, or cut them up to use as mulch and increase soil carbon stores, or add the smaller prunings to compost heaps to keep them aerated, reducing methane emissions. To reduce the risk of fire, they will also choose fire-resistant plants from habitats which are not prone to wildfires and which do not catch fire easily, rather than fire-adapted plants from fire-prone habitats which are flammable and adapted to encourage fires and then gain a competitive advantage over less resistant species.

Climate-friendly gardeners may use deep-rooted plants such as comfrey to bring nutrients closer to the surface topsoil, but will do so without making the leaves into a liquid feed, because the rotting leaves in the anaerobic conditions under water may emit methane.

Nitrogen fertilizers may be oxidised to nitrous oxide, especially if fertilizer is applied in excess, or when plants are not actively growing. Climate-friendly gardeners may choose instead to use nitrogen-fixing plants which will add nitrogen to the soil without increasing nitrous oxide emissions.
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