Soil – the importance and protection of a living soil
Soil – the importance and
protection of a living soil
- The functions of the soil
The soil performs many vital functions for the environment and society. A healthy soil is important
maintenance of the basic resources for food production: soil, clean water and stable climate
maintenance of terrestrial and aquatic biodiversity (soil life is the basis of much over-ground life;
healthy soil minimises agro-chemical pollution and nutrient leaching into watercourses)
regulating the flow of water on the planet, including reducing flooding
reducing water clean up costs (through reduction in pesticide and nutrient pollution)
reducing climate change (soil is a major carbon store and it reduces atmospheric methane;l
carbon dioxide and methane are major greenhouse gases)
reduction in the need for water for irrigation in agriculture
improvement in animal and human health through an increase in the nutrient content of food and
reduction in pesticide residues
- The key role of soil micro-organisms
Soil is often considered simply in physical or chemical terms. But, this overlooks the most important
component: the life in the soil. It is the soil biological life which delivers the soil’s main functions:
Soil micro-organisms create the soil’s structure: they convert organic matter into humus which
gives soil its physical properties of particle aggregation, protection against erosion, water
retention, good drainage, aeration, and compaction resistance
Biological activity is responsible for soil fertility: it mediates the organic nutrient cycle, releases
minerals from the sub-soil, fixes nutrients from the air, makes nutrients accessible and transports
nutrients directly into roots
A rich soil microbial life substantially contributes to the health and nutrient levels of crops
Soil microbes also add to the capacity of the soil to combat climate change by oxidising methane,
a more potent greenhouse gas than carbon dioxide
Soil microbial life is encouraged by the addition of organic matter, particularly composts, and
suppressed by the use of artificial fertilisers and pesticides.
To protect and improve the functions of the soil, it is necessary that the fundmental role of soil life is
widely recognised and that soil biological activity is maintained and developed. We welcome the
adoption of strategies for the protection of soil in the UK and EU and propose:
(i) the adoption of a strategic objective of increasing the biological activity of soils and the level
of organic matter, in particular humus.
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(ii) the development and adoption of indicators of soil micro-biological activity.
(iii) the use of new guiding principles for agricultural soil protectionto optimise soil biological
greater reliance for crop nutrition on the maintenance of inherent soil fertility (the organic
nutrient cycle) than the use of inorganic fertilisers
significant reduction in use of inorganic agro-chemicals
regular addition of organic matter to the soil, especially composts
non exploitation of soil nutrient content by the avoidance of intensive cropping or grazing of land
practices which reduce soil exposure to wind and water (e.g. hedgerows, less autumn sowing,
overwintering cover crops and green manures)
(iv) As organic farming is a system which uses all of the above, we recommend a greater
conversion to organic farming methods, through:
substantial investment in the wider adoption of organic farming
targeting of conversion to organic farming at vulnerable areas: where erosion, run-off, leaching
and flooding potential are high.
(v) Research and new projects which:
build on the work of the organic movement in understanding the role of soil life and the organic
nutrient cycle in plant nutrition and plant health
quantify the extent to which organic farming can contribute to a reduction in greenhouse gases by
building up soil carbon levels and improving soil methane oxidation rates.
Functions and management of biologically healthy soil
The Soil Association was founded in 1946. As indicated by our name, we believe that the soil is of
fundamental importance to man, and that its sound management is the basis for sustainable and
healthy food production. Organic farming is founded on good soil management, through treating and
nurturing the soil as a biologically active entity. This approach is based on research carried out by
the founders of the organic movement early last century. Unfortunately, although of great
significance to soil protection, agriculture and health, these findings have not yet been widely
recognised or fully investigated. Our aim is to address this. The Soil Association is the main
certifier and promoter of organic farming in the UK. Certified organic farming accounts for about
4% of UK agricultural land today.
- The findings of the organic movement
Early last century, the founders of the organic movement made and brought together several
important discoveries about the role of soil biological life in plant nutrition:
global soil erosion and the steady desertification of agricultural soils was noticeable already in
the 1940s. The rate of change was due to man’s activities
the key to a healthy soil, both its structure and fertility is humus. Humus is created as a result of
the activities of soil biological life on organic matter
nutrients are not simply held in solution in the soil in an inorganic form, but are bound up in
organic complexes in the various stages of breakdown of organic matter and in the soil organisms
plants do not simply absorb nutrients into their roots by diffusion but naturally rely heavily on a
close association with soil micro-organisms for the breakdown and absorption of nutrients
soil life is encouraged by the addition of composted organic matter
there is a direct relationship between the vigour and health of plants and animals which feed on
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them, and the management of the soil for its biological activity
These findings led to the development of principles and practices for the maintenance of a
biologically rich soil and the harnessing of the organic nutrient cycle for crop nutrition, ie. organic
farming. Further research and development has taken place since then, but this remains the basis of
organic farming today.
- Soil biological life
Soil is not simply the physical material on the earth’s surface: probably the most important
component is the living organisms within it. Healthy soil contains extremely large numbers: typically
600 million bacteria/gram (when no agro-chemicals are applied). A typical arable soil may
contains100 million bacteria/g while a desert soil, with little structure and fertility, has ‘only’ c.1
million bacteria/g. The rhizosphere, the thin layer immediately next to a plant root, typically has 1
million, million bacteria/g. Diversity is also important: in 1g of healthy soil there can be 15-20,000
different species of bacteria, with perhaps 10,000 in a typical arable soil and 5-8000 in a desert soil.
Fungi are also very important, especially mycorrhiza which form close associations with plant roots.
Fungi greatly exceed the volume of bacteria in soil, with 1km of fungal hyphae have been detected in
1g of soil..
Whilst much remains to be understood of the soil’s biological life, several key aspects are known,
showing that the soil life plays a central role in the soil’s many functions.
- Soil biological life and soil structure
Good soil structure is essential. If soil has a healthy structure, the likelihood of erosion, compaction,
run-off, flooding and leaching are low; its drainage, aeration and water retention are good; and its
fertility is enhanced. Soil organisms and humus are the key components that need to be recognised,
not just organic matter, as it is the humification of organic matter that creates good soil structure.
A government survey suggested that 44% of arable land is prone to erosion and that total annual
losses may be up to 2.3 million tonnes of soil every year, an average of 1t/ha/year. Probably the
largest problem is water erosion on sloping land during high intensity rainfall. Soil loss is therefore
rightly identified as an important concern. However, it would be misleading to assume that this
erosion occurs naturally. While the processes involved are clearly natural, the rate and scale of soil
loss in the UK is not natural. Soil would not have built up in the first place if this was the case.
Soil erosion and soil quality are not two separate subjects. Erosion is simply the final stage of the
degradation of soil quality. To reduce the potential for erosion, the particles need to be aggregated
together. This is achieved by the presence of humus and soil organisms. Humus is made up of
organic complexes that result from the breakdown of solid organic matter (eg. manure, crop remains)
by micro-organisms. It is humus together with polysaccharide gums that are produced by the soil’s
micro-organisms that glue the soil particles together, thus avoiding erosion and forming the soil’s
crumb structure. Practices that reduce water and wind movement at the soil surface are good
additional practices. But unless the basic health of the soil is addressed, soil will remain prone to
Other structural functions
Good crumb structure delivers several other important functions. It means the soil is able to resist
compaction, it improves the ability of roots to penetrate the soil and it means the soil is well aerated,
which is important for soil fertility and plant health. It also enables the soil to drain properly which
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Other important physical properties are also related to the level of humus, and thus dependent on
micro-organisms. The ideal soil contains roughly 45% minerals (particles), 25% water, 25% air, and
5% organic matter. However, it is the humus in the organic component which gives the soil its
capacity to retain this large percentage of water. Good drainage and water retention properties
reduce the risk of flooding. The flooding incidents of Autumn-Winter 2000 showed that poor soil
health carries significant economic consequences for certain areas, both short-term and long-term.
With flooding expected to become an increasingly regular phenomenon through climate change, the
need for biologically healthy soils in all areas prone to run-off and flooding should be recognised as a
high priority. Increased water retention also contributes to soil fertility and reduces the potential for
All the structural functions required of the soil depend on the level and activity of its biological life,
particularly through their role in decomposing organic matter to humus.
- Soil biological life and fertility
Fertility is a description of the soil’s nutrient content and the level of nutrient supply to plants .
Fertile soils are the basis of sustainable and healthy food production. Plant nutrition is
conventionally considered in chemical terms alone, ie. simply the level of each nutrient in the soil,
but actually plants naturally rely heavily on soil biological processes for their nutrition. As with soil
structure, fertility depends on the soil organisms both directly and indirectly through the formation of
humus. While nitrate, phosphate and potassium are considered to be the major nutrients, no less
important are the trace or micro-nutrients which are used for example to create plant protein and
In the soil, nutrients are held in both the organic and mineral fractions. A large amount of nutrients
are bound up in the various stages of the organic cycle. This is the process of decomposition of
organic matter from large complex molecules to smaller, simpler products, carried out by the soil
organisms. Soil micro-organisms themselves provide an essential reservoir of nutrients. Bacteria and
fungi are the most concentrated form of nutrients of any life form: for example, they have one
nitrogen for every 20 carbons (humans have only one nitrogen for every 30-40 carbons). They also
contain phosphorus, sulphur, magnesium, calcium and iron. The organisms break down organic
matter to humus which both contains nutrients and increases the nutrient holding capacity of the soil
through its water retention property. In addition, bacteria produce acids and enzymes which release
minerals from the sub-soil and they fix nitrogen and other substances (carbon and sulphur) from the
air, thus adding to the nutrients in the system received from organic matter.
Even more important is the availability of nutrients to plants. Without the activity of
micro-organisms, the nutrients in the sub-soil, the mineral fractions of the top-soil and in organic
matter would never become available to plants. Plants do not have digestive systems; they do not
produce their own enzymes to breakdown substances. Instead they rely on the diverse range of
biological life in the soil for this function.
Larger organisms like worms, snails and small arthropods break up organic matter into small pieces
with a high surface area and take them below the soil surface. The various species of bacteria and
fungi then secrete very specific enzymes which ‘chop up’ long chain molecules at specific locations
to form simpler molecules. The specificity of the activity of these enzymes means that a vast
diversity of micro-organisms are required to completely break down organic matter. In this process,
excess nutrients become available for the plant. Furthermore, the predatory activity of protozoa on
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bacteria and fungi releases large quantities of nutrients from the bacteria and fungi themselves. In
particular, protozoa have a low requirement for nitrogen, so this digestion process releases large
excesses of nitrogen: it has been estimated that the interaction between protozoa and bacteria in a
healthy soil can release 80% of the nitrogen required by the crop.
Through these processes, plant-available nutrients collect in the humus and colloids on the surfaces
of fine clay particles, where they are held by chemical attraction. The nutrients are then gradually
released for absorption by two processes: the action of exudates from the plant roots (organic acids)
and the activity of micro-organisms.
Overall, the provision of nutrients by microbes is so important to plants that they actively nurture the
development of rich microbial communities around their roots. The energy rich organic acids
released by the root feed the microbial populations, and plants spend from 10-90% of their energy
supporting microbes in this way.
Nutrient and water absorption
Not only do plants have no digestive system, they are also inefficient at nutrient absorption and,
again, naturally rely heavily on soil organisms for this. Particularly important is the intimate
symbiotic relationship between a type of soil fungi called mycorrhiza and plant root cells.
Mycorrhizae are composed of long thin threads called hyphae which both substantially increase the
area for absorption and also transfer nutrients and water directly from the soil into the plant roots .
Mycorrhizae are associated with almost all plant species. They live in the root cells and send out
hyphae up to 4cm into the soil. These act as a living bridge for transporting water and nutrients into
the plant. The total mass of hyphae connected with a root can increase the surface area for
absorption up to ten times. The end result is that plants are assured of a much higher level of nutrient
supply. This has been confirmed by analyses, for example of the mineral content of winter bean
shoots: those with mycorrhizae had mineral levels 36-118% higher than plants without mycorrhizae,
depending on the mineral (Neil Fuller, 1997).
Among other factors, leaching is greatly affected by whether the soil’s nutrients are mainly held in an
organic or inorganic form. The most leachable form of nitrogen (N) is inorganic nitrate, i.e. the form
of N fertiliser used in non-organic farming. The least leachable form is bacteria and fungi, i.e. the
soil micro-biological life. In this form, the nutrients are only gradually released during the life cycles
of the organisms, rather than being all the time in solution in the soil water and thus prone to
leaching. This explains why in non-organic farming systems typically about 20% of nitrogen applied
The soil is not simply a physical “medium” for crop growth but the means by which plant nutrition
naturally occurs through various stages and processes. These all depend on the soil’s biological
activity: soil nutrient content; soil nutrient availability and efficient plant nutrient absorption.
- Soil biological life and plant and human health
Soil biological life is considered to promote plant health in three ways: (i) through the control of soil
pathogens, (ii) through enhanced nutrient supply, and (iii) indirectly through the creation of a good
crumb structure. These in turn influence animal and human health through the food chain.
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Plant pathogen control
Soil biological life plays a major role in the control of soil-borne pathogens. The dense and diverse
community of micro-organisms around the root surface acts as a barrier to pathogens, both physically
and also by out-competing opportunistic pathogens for resources. The microbial community also
sensitises the immune system of the plant, in a process similar to vaccination, so that plants are
prepared for any pathogenic attacks. Finally, the micro-organisms are producing a vast array of
chemicals between them and it is possible that some have an anti-pathogenic effect, though little is
known about this as yet.
Good oxygen diffusion through the soil is ensured by a good crumb structure. This is important
because anaerobic conditions support pathogenic organisms and decomposition in anaerobic
conditions results in the formation of plant toxins such as alcohol.
Nutrition and disease resistance
As the micro-organisms enhance the supply of nutrients to plants from the soil in so many ways, it is
not surprising that nutrient levels in plants can be related to the level of biological activity in the soil .
This is shown by the different results of organic and non-organic farming. Organic farming harnesses
the activities of soil biological life. In non-organic farming, soil biological activity is suppressed by
agro-chemicals and plant nutrition relies mainly on artificial, inorganic fertilisers, mainly N,P,K. For
other trace minerals, aerial deposition and the inherent soil fertility are relied on for the main supply,
even though the biological activity that maintains and supplies these nutrients is not nurtured.
Supplements are only added depending on the crop or if deficiencies are identified. However, Plant
nutrition in non-organic farming is thus very different from nature and organic farming, relying
largely on the simple absorption of minerals in solution via the roots.
Clearly, in Western Europe, non-organic systems have surpassed organic systems in total yield terms.
However, the suppression of biological activity appears to have resulted in important reductions in
the levels of nutrients in crops, for both main nutrients and trace minerals. Since the adoption of non
organic practices, there has been a major reduction in the level of minerals in fruit and vegetables: a
UK study found that six minerals have reduced by between 15 and 76% between 1940 and
1991(“The composition of foods”, 1991, MAFF and the Royal Society of Chemistry). In contrast, a
reviewin 2001 of all the available comparative studies on crops produced with organic mattrer and
inorganic fertilisers for the first time quantified the higher nutrient levels of organically produced
food. The results were statistically significant for magnesium (29% more), iron (21%), phosphorus
(14%) and Vitamin C (27%) (Worthington, 2001).
At some stage this reduction in minerals must affect plant health, their vigour and ability to resist
disease. The current dependence on a high use of pesticides suggests that this may be occurring
already. It is certainly a concern for the longer term if levels continue to fall. Even if these trends are
not yet affecting plant health or this can be addressed in other ways, this decline carries major
implications for human health because of our reliance on crops for our nutrition.
The soil life has a major role in the control of soil plant pathogens and in the maintenance of nutrient
levels in crops and thus the human diet. Nutrient levels in food appear to have significantly fallen in
recent decades suggesting that the current levels of soil biological activity are already dangerously
low. This needs to be recognised and addressed as a high priority.
- Soil as a bank for greenhouse gases
Biologically healthy soils have an important role to play in the efforts to combat climate change.
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The soil is a major store of carbon. The soils in England are reported to contain twice as much
carbon as the atmosphere. This presumably means that a decrease in soil carbon content has the
potential to result in a two-fold increase in the level of atmospheric carbon (and vice versa). Though
agriculture has been slowly releasing soil carbon for a long time, most of the emissions have occurred
over the last 100 years and remain in the atmosphere. The losses are generally mainly attributed to
ploughing. However, there could be other or more important reasons as to why soil emissions
suddenly increased so much.
Between 1945 and 1986, the amount of carbon being released from the land tripled, with the most
dramatic increase being between the mid ‘70s and mid ‘80s (Houghton et al ). This coincides with the
period of intensification of crop production. We suggest that the transfer of reliance for crop
nutrition away from soil organic matter and biodiversity and onto the use of inorganic fertilisers
during this period may be a major culprit, as maintenance of the soil’s organic matter levels were no
longer important to agriculture, which they had been throughout history before.
This proposition is supported by the proposal thatorganic farming techniques have the potential to
recapture the carbon already released from the soil (Rattan Lal, Ohio State University). After all,
organic farming includes most of the traditional practices for the maintenance of soil fertility. We
note that the potential for carbon sequestration is considered to be highest in humid-temperate areas.
Methane has a warming effect 63 times stronger than carbon dioxide and its atmospheric
concentration has more than doubled over the past 100 years. It is produced by anaerobic
decomposition and ruminants, but is also destroyed by oxidation. Whilst most oxidation takes place
in the atmosphere (85%), research by the IACR-Rothamsted has discovered that a significant amount
is carried out by soil microbes, which use it as an energy source (Willlison et al , 1995) . While the
application of cattle manure apparently has little effect, the researchers found that the use of
ammonium based Nitrogen fertilisers results in a major reduction in soil oxidation rates.
Properly managed soil has the potential to have a significant effect on the level of greenhouse gases
in the atmosphere. The ability of the soil to reduce the level of these gases depends on building up
soil organic matter levels and maintaining the activity of methane oxidising bacteria. This could be
done through changing reliance away from artificial fertilisers and onto the maintenance of soil
fertility through organic matter applications.
- Maintaining/developing healthy, biologically active soils
We can recommend specific practices for the maintenance and development of biologically active
soils. However, it is also helpful if the overall approach of the agriculture industry to the soil is
Approaches to agricultural soil management
Organic and non-organic farming are two very different approaches with important implications for
soil protection. Modern farming has lost touch with the concept of a healthy soil: the soil has been
increasingly seen simply as a substrate for the receipt of synthetic chemicals, and the structure and
inherent fertility of the soil has been neglected. Agro-chemicals suppress soil life and thus directly
suppress all the functions of the soil. Ironically, the resulting physical and biological deterioration is
usually addressed by the addition of more fertilisers and pesticides to make up for the failure of the
soil in these aspects, only exacerbating the problems further.
Organic farming is based on good soil management with its practices founded on the fact that soils
© Soil AssociationPolicy Document Document: Soil – the importance and protection of a living soil 8
are a biological system. Organic farming uses both traditional techniques and new practices derived
from the discoveries about soil life to nurture the soil for long term productivity and stability.
During the compilation of a 1980 report on organic farming, the United States Department of
Agriculture (USDA) found little evidence of soil erosion on organic farms and noted that many of the
practices were those highly recommended by the USDA for soil productivity. The risk of nutrient
leaching has been shown to be less: in all published calculations in Europe, the N,P,K surpluses of
organic farms are significantly lower than conventional farms (Stolze et al , 1999). In the UK, ADAS
research has shown that N leaching rates of typical mixed organic farms are equivalent to
conventional farms which are adhering to the rules of Nitrate Vulnerable Zones (DEFRA, 1997). A
21-year Swiss trial (by the Research Institute of Organic Agriculture) comparing organic, integrated
management and conventional systems found that soil microbial biomass was far higher under the
organic system: up to 85% higher than the conventional fields and 40% higher than the integrated
management fields .
The following are some of the main practices used in organic farming for the maintenance of a
healthy, biologically active soil, and which we recommend for use in all UK farming.
(i) Avoidance of artificial fertilisers and pesticides
This is of great importance. Nematicides are among the most toxic to soil organisms, but
fungicides, insecticides and inorganic fertilisers all have negative effects on microbial
populations, including the development of mycorrhiza.
Organic farming avoids pesticide use as far as possible, replacing all possible uses with alternative
approaches (particularly prevention through promoting good plant health via a healthy soil and
ecological pest control). Integrated farming methods (which only reduce use by a limited amount) do
not minimise pesticide use.
(ii) Feeding soil biological life
Populations of soil organisms depend on adequate levels of organic matter. The use of grass leys,
green and farmyard manures all add organic matter to the system in organic farming.
Research has found that the development of mycorrhiza is especially encouraged by the addition
of composted organic matter. Composting kills soil pathogens and creates a stable microbial
community and composition.
(iii) Maintaining nutrient levels
The application of composted manures and other organic agricultural wastes in organic farming helps
the recycling of nutrients, a key principle in organic farming. Other nutrients are added in an organic
form: Legumes in the crop rotation, as part of a grass/clover ley, provide the main supply of nitrogen to
the system. Care needs to be taken in the management of the leys to minimise leaching.
Manures and slurry are mainly used for the supply of phosphorus and potassium, but provide
Green manures are used
In these ways, organic farming optimises the supply of nutrients that is available via the organic
nutrient cycle. Only if there are deficiencies, for example if the sub-soil is particularly low in certain
minerals, are inorganic minerals allowed.
(iv) Not exploiting fertility
Great efforts are made to maintain, even improve, fertility and not exploit it, ie. not to ‘cash in’ on the
soil’s reserves simply for short term gain. The aim is to keep nutrient input and nutrient utilisation in
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Mono-cropping is prohibited (grassland and glasshouses excepted). C are is taken in the design of
the rotation and all organic farmers have to submit their crop rotations to the organic certifiers.
Grazing levels are lower than in non-organic systems; stocking densities are limited by the
Nutrient budgeting is often used to verify the farm’s nutrient balance.
(v) Reducing wind and water exposure
In addition to the creation of a healthy soil, erosion is further avoided by practices which reduce
exposure to the wind or the flow of water at the soil surface:
Hedges and trees at the field margins are used by organic farmers as habitats for the natural
predators that control crop pests, with smaller field sizes to ensure predators can access the whole
crop. This also provides good wind breaks.
High percentage of grassland. Grassland avoids the ploughing and exposure to the wind that
occurs in cropping. Organic farming rotations include about 50-60% of grassland on the farm at
any one time, including higher levels of permanent grassland.
Reductions in autumn sowing. The use of crop rotations and the greater diversity of crops grown
results in less autumn sowing.
Overwintering cover crops and the use of green manures also help.
All of the soil’s main functions can be related to the level of biological activity in the soil. Organic
matter levels are important but without soil organisms to break down the organic matter to humus,
there is little point increasing organic matter levels alone. It is therefore essential to consider the role
of soil biological life when analysing the causes of the loss of key soil properties and in making
policy recommendations. This requires the introduction of measurements of micro-biological activity
and also some radical decisions, such as recognition of the need to significantly reduce the use of
pesticides and inorganic fertilisers in agriculture and the wider adoption of organic farming
techniques. However, the economic and social benefits of such decisions are clearly enormous:
sustainable and nutritious food production, reduction in flooding and a major contribution to the
halting of climate change and to the reversal of the decline in farmland biodiversity.
Dr Elaine Ingham, Oregon State University, presentation at Soil Association conference, 2001
Rothamsted IACR, Annual Report, 2001-02
MAFF and the Royal Society of Chemistry, 1991,“The composition of foods”
Worthington V, 2001, “Nutritional Quality of Organic Versus Conventional Fruits, Vegetables, and
Grains”, The Journal of Alternative and Complimentary Medicine, Vol. 7, No. 2, 2001, 161-173.
Houghton et al , “Carbon flux to the atmosphere from land-use changes: 1850 to 1990”
Willlison et al , 1995,“Farming, Fertilizers and the Greenhouse Effect”
Stolze et al , 1999, “Environmental and resources use impacts of organic farming in Europe”
DEFRA, 1997, “Assessment of relative nitrate losses from organic and conventional farming systems
based on recent measurements”
Soil Association Campaigning for organic food and farming and sustainable forestry
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