What is soil organic matter?
The term soil organic matter refers to all the organic matter in soil including living and non-living components.
The living components of soil organic matter includes active roots and living organisms, and non-living components include root exudates, decomposing plant and animal material, humus and charcoal. The amount of soil organic matter is determined by the net difference between new inputs and outputs of organic material. Outputs may include product removal through harvest or grazing, carbon dioxide (CO2) losses from decomposition, and any direct losses (erosion).
Soil organic matter is largely made of carbon, oxygen, and hydrogen, but also contains many nutrients essential to the growth of plants such as nitrogen, phosphorus, sulphur, and other nutrients. Plants cannot generally take up nutrients in organic forms. Soil organisms use the organic matter (including other organisms) as food, and in doing so release plant available nutrients and CO2. Depending on the soil and environmental conditions, 50 to 75% of the carbon in living organic material may be released as CO2.
Soil organisms such as earthworms, beetles and ants break large pieces of organic debris into smaller pieces and are capable of incorporating surface residues deeper into the soil. The greatest concentration of organic matter remains in the top ten centimetres of soil and is associated with surface residues and prolific root growth. The microbial population rapidly decomposes these organic residues and they themselves contribute to the organic matter. The organic matter loses mass and eventually, the more resistant components remain as humus (dark brown or black organic matter).

Soil organic matter: the essentials
An overview of why soil organic matter is so important to the function and fertility of agriculture and living systems. From Soil Quality: 2 Integrated Soil Management (Pluske et al. 2018). Video talent: Frances Hoyle, Murdoch University; video production: Lomax Media.
The carbon cycle
Soil organic matter cycles continuously between its living, actively decomposing, and more stable fractions. Soil represents a reservoir able to both store and release carbon within the global carbon cycle and as such is considered both a ‘sink’ and ‘source’ for atmospheric carbon. Soil contains carbon in both organic and inorganic forms, which with the exception of calcareous soil, is largely held as soil organic carbon. Soil organic carbon is in a constant state of flux, slowly responding to environmental or management changes and moving to reach a new equilibrium level after changes occur. At any one time, the amount of organic carbon in soil represents the balance between inputs and losses.
A significant amount of the organic carbon accumulated in soil has resulted from photosynthesis where plants convert atmospheric carbon dioxide (CO2) into above-ground shoot growth and below-ground root growth.
Carbon emissions from soil back to the atmosphere occur in the form of carbon dioxide, resulting primarily from the decomposition of organic matter largely as a result of agricultural practices driving changes in microbial processes. These emissions have, reflecting the historical declines that have been measured in soil organic matter for most agricultural soil, contributed to the measured increases in atmospheric carbon dioxide resulting from human activities.
Carbon cycling between the soil, plants and atmosphere involves the continuous transformation of organic and inorganic carbon compounds by plants and organisms, and has fast and slow components.
In the slow carbon cycle, it can take 100-200 million years for carbon to move between rocks, soil, ocean, and atmosphere. Carbon emitted as gases into the atmosphere results in warmer temperatures on Earth.
The fast carbon cycle results primarily from plants and phytoplankton photosynthesising and using solar energy to combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. Plants and animals including microorganisms break down these sugars for the energy they require to grow and function. When these organisms subsequently die and decay they get consumed by microorganisms or other soil fauna and so the cycle continues. In a similar chemical reaction, fire also consumes plant material to release water, carbon dioxide, and energy. In all cases, carbon dioxide usually ends up in the atmosphere.

Cycling of soil organic matter
Soil organic matter turnover is continuous, occurring at at annual rate of 2-4%. From Soil Quality: 3 Soil Organic Matter (Hoyle and Murphy 2018). Voice talent: Ed Barrett-Lennard, DPIRD; animation: Science with Style.
Factors influencing soil organic matter
Soil organic matter is always in a constant state of turnover, where it is decomposed and replaced by new organic material. The relative flux in soil organic matter content is determined by the balance between inputs and the rate of loss.
The primary method to increase soil organic matter is to increase the amount of organic residues entering the soil, and to limit losses due to erosion, cultivation or management. Plant production is limited by rainfall so even if all residues are returned to the soil, an upper limit remains determined by climate, soil type and management.
Climatic conditions (temperature and rainfall) control microbial activity and therefore soil organic matter decomposition. Moist, hot and well-aerated conditions favour microbial activity and the rapid decay of organic additions. An increase in the number of days after rainfall or irrigation when soil moisture is in the correct range to support biological activity (biologically active days) can result in faster decomposition of soil organic matter. Similarly, a decrease in the number of biologically active days (as might occur with prolonged waterlogging or dry conditions) would result in a decreased rate of breakdown.
If the rate of organic matter addition (plant residues, or application of organic inputs such as compost or manure) is greater than the rate of loss, soil organic matter will increase. Conversely, if the amount of organic matter retained is lower than the rate of loss, soil organic matter will decrease. The soil organic matter levels will always reflect the balance of inputs and losses over long periods of time, however the process to build soil organic matter is slower than the rate it can be to lose (or use) the soil organic matter.
The retention of soil organic matter is both soil and climate dependent. Because of the continued decomposition of soil organic matter, substantial amounts of additional organic material are required to have a measurable effect on soil organic matter over the long-term. The amount of extra plant material required can be calculated for different soil types. When considering the type of biomass grown to increase soil carbon, the amount of carbon input through plant root biomass (below ground biomass) is possibly of greater importance for increasing soil organic matter than above ground biomass, as it is largely protected from loss.
As a guide, an additional 10 t/ha of organic matter would be required each year for 10 years to increase soil organic matter by 1%, or more realistically in current farming systems – 2 t/ha each year for 20 years to achieve a 0.5% increase.
Clay platelets coat organic matter to form stable aggregates, physically protecting organic material from microbial decomposition. This means soil of higher clay content have a greater capacity to stabilise organic carbon. In comparison, rapid turnover of organic material occurs in soils with little or no clay content and explains why increasing organic carbon in coarse textured sandy soil is comparatively difficult. Organic matter breakdown is more rapid in soils with adequate pore structure, which allows the exchange of gases and infiltration of water.
Soil conditions that decrease microbial activity also decrease the mineralisation (breakdown) of organic matter.
For example, changes in soil pH can alter biological activity, function and survival of the microbial community – microorganisms prefer a relatively neutral soil pH (pHCaCl2 6.5-7.5). As soils become more acidic, microbial activity and organic matter decomposition slow down.
Benefits of soil organic matter
There are many benefits of soil organic matter on soil function. The relative benefit of soil organic matter can be influenced by soil type, season, and composition of soil organic matter.
Soil organisms break down organic matter and utilise carbon as an energy source and cellular building block, as well as nitrogen and other nutrients which they utilise for their growth and metabolism. In Australia, one of the main factors limiting microbial growth and activity (aside from soil moisture) is the absence of suitable (labile) carbon substrates. These organisms are essential for effective recycling of nutrients on and within the soil.
Plant and animal residues contain nutrients essential for plant growth, which are released as residues decompose. The quality of organic residues influences nitrogen supply, as well as phosphorus, sulphur and potassium supply to plants. The ratio between carbon and other nutrients in soil organic matter varies, but on average, for every tonne of carbon cycled, there is 80-100 kg of nitrogen, 15 kg of phosphorus and 15 kg of sulfur released as soil organic matter breaks down. Soil organic matter is cycled at 2-3% annually.
Crop stubbles are generally considered low quality (high C:N ratio) and as a consequence, immobilisation (microbial uptake) of soil nitrogen is often observed at the break of season resulting in lower plant availability. In contrast, high nitrogen content material will mineralise (release) nitrogen and increase plant-available nitrogen providing a reserve of nutrients available to plants. Although in some cases up to 80% of the crop nitrogen requirement can be supplied through biological cycling, nitrogen release from decomposition of soil organic matter and fresh residues can be slow and may not meet the peak demand of a growing crop (requiring the strategic application of fertilisers). Nutrients need to be added to ensure adequate levels of nitrogen for plant growth to overcome any deficiencies caused by microbial immobilisation (uptake) during decomposition of soil organic matter. Over the long-term, a gradual build-up of stable soil organic matter pools will contribute to a larger and more dynamic soil nitrogen pool, producing fewer instances of deficient soil nitrogen.
Read more: Organic matter and nutrient supply
Management of charcoal and humus provides some potential for altering cation exchange capacity (CEC), but almost all of the charges on organic colloids are pH dependent. This means that cation exchange capacity will increase as soil pH increases. Benefits from increasing cation exchange capacity are therefore limited on acid soils.
Humus (decomposed organic matter) is very effective at holding nutrients due to a large negatively charged surface area. Like clay particles, humus attracts positively charged ions (cations) and contributes to the cation exchange capacity of a soil. For example in a soil with 25% clay and 2.5% organic matter, approximately 1/3 of the cation exchange capacity (of topsoil) can be attributable to the soil organic matter. This compares to a sandy soil, with little or no clay content in which soil organic matter accounts for close to 100% of the cation exchange capacity of topsoil. This ability to hold and release nutrients (that would otherwise leach deeper into the profile) in the upper soil layers where they are available for plant uptake is a key benefit of soil organic matter.
When considering the cation exchange capacity of a whole soil profile accessible by the plant roots, however, it is often the mineral fraction that provides the majority of the cation exchange capacity of a soil. An exception to this would perhaps be the pale deep sands in which clay content does not increase at depth.
The presence of higher amounts of soil organic matter can help water infiltrate the soil profile because of its more open structure. Surface plant residues, provided they are in sufficient quantities, can reduce evaporation.
Humus holds several times its own weight in water and thus if it accumulates in sufficient quantities can increase the capacity of soils low in clay content to hold water for gradual release as plants take it up. However, this only applies to the surface 10 cm of soil where organic matter accumulates and is unlikely to have a large impact on a soil profile that may be a metre deep.

Water-holding capacity of soil organic matter
Organic matter is increasingly important as an influence on water holding capacity as clay content declines. From Soil Quality: 3 Soil Organic Matter (Hoyle and Murphy 2018). Video: Frances Hoyle, Murdoch University; video and editing: Lomax Media.
Soil aggregation is promoted by binding agents and by-products of biological origin including fungal hyphae, bacteria, living roots and root exudates, all part of soil organic matter. In forming stable aggregates, organic matter not only contributes to holding mineral soil particles together but also improves soil porosity for the exchange of gas and water.
Organic residues which form mulch on the soil surface can protect the soil from raindrop impact, minimise the risk of both wind and water erosion, reduce evaporation and buffer the soil from extremes of temperature. Mulches also promote root growth in the topsoil where nutrients tend to be concentrated and protect seedlings from wind-blast damage.
Increased soil organic matter levels reflect greater storage (sequestration) of carbon. This generally reflects a greater level of organic inputs but does not necessarily reflect a decrease in total greenhouse gas emissions. Accumulation of soil organic matter (particularly as humus) reduces the amount of carbon dioxide gas evolved to the atmosphere. This makes a small but important contribution to a slowing of global warming associated with greenhouse gas emissions.
Increasing ground cover and intact roots (part of the soil organic matter) will contribute to stabilising soil and can reduce erosion significantly (each increase of 10% in ground cover up to 50%, will reduce the average rate of erosion significantly). Decreasing erosion will help retain non-anchored organic material and soil organic matter that would otherwise be lost.
Acidity is temporarily created in the soil when plants take up more cations than anions, and this must be neutralised by the decomposition of the plant residues when returned to the soil. If the residues are removed, then the original acidity is not neutralised and the soil becomes more acid. This results in a classic, yet uncommonly recognised problem in agriculture that adding organic matter from another site can raise the soil pH, but the site from which the organic matter was removed becomes more acid.
In very acidic soils, soil organic carbon can build up. This is thought to be associated with an interaction between low soil pH and increasing water repellence.
Low soil pH may cause a physiological stress on the soil microbes which results in a small microbial population that requires less soil organic carbon to function.
Water repellence increases as soil pH decreases. At low soil pH, severe water repellence can restrict microbes from being able to colonise and decompose soil organic matter causing it to build up. In this case the hydrophobic (water repellent) nature of the soil organic matter physically protects it from microbial decomposition.
The net effects of building up soil organic matter are generally positive, but increasing surface residues and soil organic matter may also result in some of the following outcomes, which require management.
- Result in an uneven seed bed for seeding
- Harbour plant disease and insect pests
- Immobilise (tie-up) nitrogen making it less available to plants in the short term
- Adversely affect crop growth by producing phytotoxins during early decomposition
- Induce water repellence in sandy soils (organic coatings on sand grains produced by microorganisms during the decomposition of organic matter repel water)
- Affect the efficacy of herbicides and pesticides, such as reduced soil contact
Assessing soil organic matter
Soil organic matter (SOM) is commonly, but incorrectly used interchangeably with soil organic carbon (SOC). Soil organic matter differs in that it includes other elements such as hydrogen, oxygen, phosphorous, sulphur and nitrogen whereas soil organic carbon is a measure of the organic carbon content of soils.
Soil organic matter is approximately 58% carbon. Laboratories typically measure soil organic carbon and use a conversion factor to estimate soil organic matter.
Soil organic carbon (%) x 1.72 = Soil organic matter (%)
What is a good level of soil organic matter?
Soil organic matter levels in agricultural systems vary considerably depending on climate, vegetation, soil type and land use. Land converted from a natural system to a cropping or grazing system experiences changes in the amount of soil organic matter (typically, agricultural soils experience organic matter declines of between 25 and 75% with continued cultivation).
The optimal level of soil organic matter for any given soil is one which supports the functional capacity of the soil to hold and supply plant available water, store plant nutrients, provide energy for soil fauna, improve crop/biomass yields, and moderate net greenhouse gas emissions. Irrespective of soil type, studies suggest that if the soil organic matter is below 1.7% (equivalent to 1% soil organic carbon), water-limited potential yield may not be achieved.
The impact of improving soil organic matter will be greatest on soils with poor soil structure and those in which biological activity has been constrained by the lack of a suitable food source. Additional gains will occur where yields are improved from a better nitrogen supply (assuming no constraint to uptake). The economic basis for improving soil organic matter may be strongest where practices providing multiple production benefits, such as stubble retention or growing a vigorous, deep-rooted crop/pasture species in a rotation are employed.
Aside from soil testing, indicators of soil organic matter include:
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Soil colour – for a particular soil type, an increase in soil organic matter can be related to a darkening of soil colour. This is particularly evident on the soil surface and in the ‘A’ horizon
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Soil smell – earthy (particularly strong after fresh rain)
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Softness of soil – soils high in organic matter are often more ‘spongy’
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A high proportion of ground cover (e.g. stubble, residues, and living plants)
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Presence of roots, earthworms, fungi, and other below-ground organisms

Monitoring soil organic matter
Soil organic matter changes in broadacre agricultural systems occur very slowly, usually over decades. Most changes in soil organic carbon can be measured in the soil surface. This is because low disturbance systems such as minimum tillage concentrate plant and animal residues on the surface and roots are more concentrated in the surface soil layer. Spatial and temporal variability can also mean carbon content measured across a paddock, or between paddocks can vary widely.
Soil organic carbon is a standard measurement in most soil tests. This can infer soil organic matter associated nutrient release and buffering of that soil layer. The level of soil organic carbon always reflects the history of that site. For example, if there has been deep soil mixing, it would be expected to see a lower organic carbon percentage in the top 10 cm with the organic matter redistributed through the profile due to the tillage. Long-term pasture systems tend to have higher organic matter at depth than cropping paddocks due to the overall balance of organic matter inputs and outputs.
Managing soil organic matter
To increase soil organic matter, the rate at which plant and animal residues are added to the soil must exceed the rate at which these are lost through microbial decomposition, erosion or removal. Cropping and pasture management practices that generate adequate amounts of high-quality residues are critical to rebuilding and sustaining soil organic matter, with an underpinning requirement for farming systems to remain profitable. Extreme climatic conditions such as drought constrain the ability of land managers to improve soil organic matter status largely through its impact on availability and management of organic inputs.
Converting agricultural soils into carbon ‘sinks’ requires the continued addition and maintenance of organic inputs at a rate faster than removal. The challenge is to ensure that once stored this carbon remains in the soil for decades to millennia rather than just cycling through the soil and returning to the atmosphere as carbon dioxide.
A commonly asked question in regard to increasing soil organic matter is if there is more soil organic matter in my soil does it have a benefit and what is that benefit? The type of organic matter, the soil conditions, and management objectives all influence the outcome from increasing soil organic matter content. For example, adding charcoal to soil in sufficient quantities would make both the carbon content greater (if added at a high enough rate) and the colour darker, but it would not directly increase the soil’s fertility. However, if applied at sufficiently high rates in a low cation exchange capacity (CEC) soil, charcoal could increase the ability of the soil to hold more nutrients in the upper layers of soil. Alternatively, fresh inputs of crop residues are relatively labile and can enhance soil fertility in the short term, but are unlikely to have a significant role in sequestering carbon over longer times. Therefore, in managing soils for soil organic matter it is important to determine your objectives.
Healthy soils have a greater capacity to grow more organic material and are able to retain their healthy status (positive feedback) – these soils are more resilient in the face of environmental extremes. The more typical situation in continuous cropping systems is that soils and soil organic matter are in decline and have negative feedback – with more erosion, poorer structure, more runoff and less biomass produced.
Consider a combination of the following factors when developing soil management plans that aim to improve soil organic matter.
Growing more above- and below-ground biomass increases the amount of photosynthetic carbon gained over time by plants. Optimal agronomic and soil management practices that increase water use efficiency and grain yield should result in higher plant biomass (as long as harvest index is maintained).
While a significant amount of carbon dioxide is fixed during plant photosynthesis, the vast majority of plant carbon is converted back to carbon dioxide and respired to the atmosphere as a result of microbial decomposition. This means that changes in soil organic matter are generally decadal, since only a small amount of fixed carbon remains in the soil and accumulates in more stable soil organic matter fractions.
The law of limiting constraints (Liebig’s law) states that plant growth in agricultural systems is controlled, not by the total amount of resources available, but by the most limiting factor.
Crop and pasture management
- Ameliorating production constraints such as acidic, sodic or compacted soils and nutrient toxicities and deficiencies should result in increased plant biomass production and support a more profitable farming base.
- Removing constraints to plant growth allows development of more expansive and deeper root systems that assist in stabilising soil and contribute to the below-ground contribution of soil organic matter.
- Slow incremental increase in measured carbon is expected (e.g. less than 0.2 tonnes per hectare for each tonne of extra yield). Often, small production increases result in little or no change.
- Increasing rotational diversity can result in a more diverse and/or functional microbial community associated with a range of food substrates.
Retirement of non-productive areas
Returning marginal agricultural land to native vegetation and excluding areas from grazing can have significant benefits to soil organic matter because nearly all plant production is returned to the soil.
Fallow
- The longer a soil is left devoid of plants as bare fallow, the less soil organic matter it will contain. Characterised by declining soil organic carbon, sites where tillage occurs will experience an increased rate of loss.
- Bare fallow increases erosion risk associated with low plant cover.
Irrigation
- Though unavailable to most broadacre grain production areas, irrigating crops or pasture removes water as a primary constraint limiting biomass production in dryland systems. Irrigated systems can increase plant productivity where there are no underlying soil constraints and are capable of reaching their attainable soil carbon potential as determined by soil type.
- Irrigation may also allow for a cover crop or second crop/pasture to be sown over the normally dry summer period, adding root and above ground biomass to the system and capturing nutrients resulting from the breakdown of organic matter. Counter balancing this, higher decomposition rates during warmer periods where moisture is not a limiting factor to biological processes may result in faster losses, such as may occur in irrigated pastures under intensive agriculture.
Improved pasture systems where residues are retained can positively influence the quantity and quality of soil organic matter in Western Australian agricultural systems. Changes in carbon are incremental and often take a number of years to be able to measure as each tonne of extra pasture biomass grown is likely to add less than 0.3 tonnes of soil organic carbon per hectare per year. Mixed (grass and legume) pasture systems provide the best quality of organic inputs.
Maintaining a minimum 40% pasture cover under grazing, introducing legumes into grass pastures and applying nutrients to maintain pasture production will increase soil organic matter in pasture systems. Managing the duration and intensity of grazing is also required to avoid overgrazing and to minimise erosion, which can dramatically decrease soil organic carbon levels by removing topsoil.
Perennial pasture systems
- Well established, diverse pastures provide the greatest potential for increasing soil organic matter – particularly in high rainfall environments.
- Pastures generally have more roots compared to grain crops for the same shoot biomass, with underground biomass less susceptible to loss.
- Perennial pasture species are able to make greater use of ‘out of season’ rainfall.
- Pasture systems are often subjected to less soil disturbance than cropping and are less susceptible to erosion.
- An unintended consequence of long-term pasture can be the development of water repellence.
- Unimproved, poorly established or native pastures systems often limit plant growth leading to lower biomass production and lower inputs, restricting contributions to the soil organic matter pool.
Annual pasture systems
- Annual pasture systems contribute organic matter to soil when managed for optimum biomass production and where not removed from the paddock.
- Establishing ground cover is a good strategy to protect surface soil from erosion events and direct loss of organic matter.
- Grown in rotation with crops, annual pastures can be slow to establish and early growth can be slow. Under shorter, more limited growing season conditions this may result in limited opportunities to increase soil organic matter.
- Where inputs are not sustained long-term or under high grazing pressure that leads to loss of ground cover and erosion, any gains in organic matter content may be lost.
- Prior to returning to a cropping phase, pastures can be used as a green or brown manure, contributing organic residues to soil and in addition providing effective weed control when done in spring.
Other pasture-based systems
- Pasture cropping or summer active systems provide an opportunity, where conditions allow, for an extended period of plant growth and more biologically active days, increasing the capability of soil to accumulate soil organic matter.
- The influence of summer active species on plant available water, where soil water stores are used prior to a cropping phase, needs to be considered; as does the potential for build-up of pathogens associated with a ‘green bridge’ and reduced opportunity for weed control.
Green and brown manure phases are sometimes used strategically to address soil fertility constraints, but more often are associated with managing high weed burdens as a component of an integrated weed management approach, used during the last phase of a pasture rotation prior to cropping, or in a failed crop. A green manure is a crop or pasture grown and returned to the soil in situ either by incorporation, desiccation, slashing or rolling—with the term ‘green manure’ reflecting the stage of plant growth at which the crop is manured (flowering).
Where suited to production, the use of cover crops in rotation can potentially increase soil organic matter and, depending on the plant species, deliver other beneficial functions to farming systems. However the winter dominant growing climate of the south-western agricultural region, suggests growing a cover crop is largely opportunistic in regards to whether soil temperatures are sufficiently warm and sufficient rainfall is received over the spring and summer months to establish a cover crop. For the south-western agricultural region, drying of the soil profile by a cover crop (or pasture) and low or infrequent rainfall prior to sowing the following cash crop is likely to result in lower grain yields.
Where used persistently, these systems can contribute slowly to increased soil organic matter, with a range of options for implementation. Combined with low soil disturbance systems, increasing the diversity of residues and quantity of soil organic matter using legume and cereal cover crops or green manure phases has the potential to increase the population and diversity of soil biota.
Green manures
- Implemented only occasionally (for example, once every five years) it is unlikely that green manures will result in measurable changes to stable soil organic matter.
- While growing green manures act as a ‘catch crop’— absorbing and holding nutrients such as nitrogen that might otherwise leach away, the nutrients are later released as residues decompose.
- Green manuring in the last year of a pasture phase before returning to cropping can minimise costs and maximise potential benefits. As costs increase, the requirement for a benefit to be realised in subsequent years increases. This presents a risk because the magnitude of the eventual benefits will depend on seasonal conditions.
- Green manuring should coincide with flowering and completed prior to weed seed set. Spraying with a non-selective herbicide before manuring will decrease the risk of regrowth.
Common techniques of green manuring:
- Discs, plough or other mechanical means are used to incorporate plant residues into soil.
- Plant residues are desiccated (brown manuring) with a non-selective herbicide and left to decompose naturally over time. Under high residue loads, seeding operations can be compromised with pinning of stubble around tines and variable influences on nutrient supply.
- Residues are slashed, chopped or rolled, and left in a layer on the soil surface as a mulch.
A green manure calculator available on soilquality.org.au provides a comparison of relative benefits between a green or brown manure phase and a grain crop within a defined rotational sequence.
Cover crops
- Crops and pastures grown during what is normally a ‘non-growth’ period over the summer months will help anchor soil and minimise wind and water erosion.
- Plants grown out of season can take up excess nutrients in the soil and prevent leaching, or even fix nitrogen in the soil for future use (e.g. leguminous species).
- Increasing the total net primary productivity of a paddock by increasing the number of crops or pastures grown in a year, provides additional organic matter to soils providing a habitat for beneficial insects and other organisms.
- Risks associated with cover crops and pastures exist in relation to climate, with establishment often dependent on warming soils and sufficient rainfall, and a risk of dry conditions limiting yield both in the cover phase, but also in the subsequent crop.
Retaining crop and pasture residues on a paddock contributes to the long-term maintenance of soil organic matter levels over a number of decades, but with few exceptions is unlikely to add or ‘build’ soil organic carbon levels in dryland systems. This is because a well-functioning soil turns over organic matter as it recycles carbon and nutrients essential for biological function.
- In soils that have experienced losses in organic matter due to historical farming practices, there may be some capacity to restore these levels by retaining residues (with no soil disturbance), resulting in small incremental gains.
- Stubble retention promotes water entry and conservation, and decreases erosion risk. Over time decomposition of residues (and associated carbon) occurs.
- Removing or burning crop residues causes a more rapid loss of carbon and decreases the amount of fresh organic matter entering soil, with remaining carbon after burning (char) less decomposable and relatively inert. Nutrient loss from burning crop residues is also high, with up to 80%, of nitrogen, 25% of phosphorus, 50% of sulfur and 20%, of potassium lost.
- Differences in organic matter can sometimes be measured associated with windrow burning compared to whole paddock burning since organic matter is concentrated in a smaller area.
Changes in the allocation of carbon to different soil organic matter fractions is often reflected in a decrease to biological activity, microbial biomass and the diversity of soil organisms.
In Western Australia it is often difficult to detect measurable differences in soil organic carbon between retained and burnt stubble systems with the same history. This is explained because while there is an immediate loss of carbon when burning stubble, decomposition in the soil by microorganisms will also generate a similar loss — just over a longer time frame.
Most organic matter and soil biota is concentrated towards the soil surface. In Western Australia approximately 60%, of the total organic matter within the top 30 centimetres of soil is in the surface 10 centimetres and, if lost, can take decades to replace. Organic residues on the soil surface protect the soil from raindrop impact, minimise the risk of wind and water erosion, reduce evaporation, buffer the soil from extreme temperatures, promote root growth in the topsoil and protect seedlings from wind damage.
- Soil forms at a very slow rate, typically about one millimetre every 100 years.
- Loss of topsoil during an erosion event will result in a direct loss of soil organic matter.
- Although uncommon, wind and water erosion associated with high-intensity storms can erode up to 300 tonnes of soil per hectare in a single event. In a dry summer large amounts of topsoil (up to 80 tonnes per hectare) can also be lost in a single year, dramatically decreasing soil organic matter in the soil surface.
- Annual soil erosion losses under established crops can be up to 8 tonnes per hectare. At 1.5%, carbon content, this loss is equivalent to 0.12 tonnes of carbon per hectare.
Cultivation of agricultural soils causes an immediate and rapid loss of soil organic matter, followed by a slower rate of loss lasting several decades. Losses are often greater on sandy coarse textured soils, where there is little or no physical protection of organic matter due to poor soil aggregation.
Minimum tillage is often promoted as a way to build soil organic matter. Small, if any, benefits to total soil organic matter levels have been measured associated with minimum tillage systems in Western Australia in the medium term. Long-term adoption can promote soil aggregation and slow decomposition rates of soil organic matter, possibly leading to greater accumulation. Its placement on the surface makes any newly acquired carbon vulnerable to losses and limits its contribution to stable soil organic matter.
- Direct drilling decreases the risk of erosion and maintains soil structure, slowing soil organic matter decomposition.
- Greater stratification of organic matter is associated with low disturbance systems with organic residues remaining primarily on the soil surface. These residues tend to decompose with only minor contributions to stable soil organic carbon fractions.
- Cultivation exposes ‘new’ organic matter that was previously protected within aggregates to decomposition. The effect on structured or aggregated soils is greater than on coarse textured sand.
- Cultivation can be associated with an immediate and rapid loss of up to a third of soil organic matter within a 12-month period, followed by a slower rate of loss lasting several decades. Longer term, soil organic matter levels can be depleted by as much as 85% of the original stock regardless of climate, soil type or vegetation.
- Decomposition rates of organic matter incorporated with increasing levels of disturbance, increases as methods become more disruptive.
While in higher rainfall areas some farmers are able to generate enough plant residue biomass on the farm to increase soil organic matter, for many production systems only additional external sources of organic matter will lift soil organic matter levels — and only with regular inputs. At sufficiently high rates and when applied regularly, soil properties such as nutrient supply and soil organic matter can increase but consideration of relative cost and practicality needs to be considered.
Many organic soil amendments, with some reporting agronomic benefits, are now available to Australian farmers. Until recently, organic amendments have largely been used in intensive horticultural industries and organic farming systems. Increasingly, however, these products are being targeted at broadacre farming systems to supply plant nutrients, control pests and diseases and improve soil health. However such products often have little independent or local evidence of their benefits, and where prior constraints such as soil acidity exist their benefit is likely to be limited.
Despite manufacturer claims of benefits, high application rates (and cost), practicality of application, and for some the associated transport costs, required to produce soil or productivity benefits mean organic amendments can be uneconomic and result in little or no benefit.
Comparing amendments
Careful consideration of the costs, risks and application rates required should be undertaken before investing in organic amendments. On-farm trials are a good idea to improve knowledge of potential responses in your local situation before considering a large investment in organic amendments. Strip trials incorporating both treated and untreated areas and measured responses in both soil and plant variables are recommended (see Understanding trial results and on-farm experimentation).
A comparison of organic amendments that may increase soil organic matter
Resource stream | What is it? | Potential benefits | Potential risks |
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Animal manures and green wastes | Municipal, industrial and agronomic waste products (e.g. olive and paper mill waste, green waste, treated sewage sludge, bio-solids and un-composted animal manures). |
Wastes and animal manures can improve soil condition depending on degradability of material and rate of breakdown but will need to be continuously replaced.
Source of nutrients Diversity of inputs can contribute to a more diverse microbial community by providing a transient source of carbon and nutrients. |
Rate required and transport costs for broadacre agriculture may not be economic.
May contain unwanted material or contaminants (e.g. manure derived from feedlots can contain high levels of sodium which may promote subsoil constraints). |
Compost | Commercially available compost is composted or partially decomposed organic matter, usually produced from crop residues, municipal waste materials and manures from intensive animal production, such as beef and chicken industries. |
In contrast to fresh plant residues or animal manure, composted organic matter decomposes slowly when added to soil because it has already undergone a significant amount of decomposition and stabilisation during the composting process.
Diversity of inputs can contribute to a more diverse microbial community by providing a transient source of carbon and nutrients. |
Contamination by weed seeds, heavy metals, salts and pathogens is possible, depending on quality control of producer.
Few independent scientific evaluations have shown productivity benefits for broadacre agriculture. Rates required and transport costs for broadacre agriculture may not be economic. |
Biochar | Solid, fine, granular, black charcoal produced by pyrolysis of organic biomass. |
Potential to improve cation exchange capacity and nutrient acquisition.
Benefits vary based on feedstock and pyrolysis temperature. Ties up some mobile pesticides and nutrients that are a risk to the environment. Decreases greenhouse gas emissions. |
Rates required and transport costs for broadacre agriculture may not be economic.
Can intercept pesticides and decrease efficacy in an agricultural context. May contain toxins and heavy metals dependent on feedstock. |
Organic extracts and animal by-products | Liquid extract produced via extraction or steeping methods. Blood and bone by-products. |
Source of plant nutrients.
Vector for beneficial microorganisms. |
Variable quality due to unregulated industry and methods.
Difficult to determine potential impact. |
From Soil Quality: 3 Soil Organic Matter (Hoyle and Murphy 2018).
Dempster DN, Gleeson DB, Solaiman ZM, Jones DL and Murphy DV (2012). Decreased soil microbial biomass and nitrogen mineralisation with Eucalyptus biochar addition to a coarse textured soil. Plant and Soil 354: 311–324. https://doi.org/10.1007/s11104-011-1067-5
Edmeades DC (2002). The effects of liquid fertilisers derived from natural products on crop, pasture and animal production: A review. Australian Journal of Experimental Agriculture 53: 965–976. https://doi.org/10.1071/AR01176
Jones DL, Edwards-Jones G and Murphy DV (2011). Biochar mediated alterations in herbicide breakdown and leaching in soil. Soil Biology & Biochemistry 43: 804–813. https://doi.org/10.1016/j.soilbio.2010.12.015
Krull E, Daniel VM, Macdonald LM, Farrell M, Solaiman Z, Kookana R, Dempster D, Gleeson D, Maccarone L (2012). A fundamental understanding of biochar – implications and opportunities for the grains industry. Final GRDC Report (CSO 00041 and UWA00130). CSIRO Australia and University of Western Australia.
Quilty J and Cattle S (2011). The use and understanding of organic amendments in Australian agriculture – a review. Soil Research 49: 1–26. https://doi.org/10.1071/SR10059
Page references and acknowledgements
Material on this page adapted from:
- Hoyle FC (2007). Soil Health Knowledge Bank.
- Pluske W, Boggs G and Leopold M (2018). Soil Quality: 2 Integrated Soil Management. SoilsWest, Perth, Western Australia. [Access]
- Hoyle FC and Murphy D (2018). Soil Quality: 3 Soil Organic Matter. SoilsWest, Perth, Western Australia. [Access]
Last updated July 2024.