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What is soil organic matter?

Soil organic matter (OM) is at the very core of successful organic farming and is vital for a healthy, living, productive soil that will sustain plant, crops and animal production. In general terms OM provides food and energy for microorganisms, which in turn release nutrients from the OM which can then be used by growing plants.

Note that soil is not essential for plant growth - plants will grow hydroponically in nutrient solutions, its importance is all about making a healthy productive soil. Decomposing OM may either increase or decrease the amount of nutrients available to plants, depending on the approach used to manage the soil.

What does OM do in the soil?

OM is highly complex and its components have only been partly identified. It has important effects on the following:

  • Increases soil stability.
  • Increases soil water reserves.
  • Darkens soil colour - absorbs more heat from the sun.
  • It increases Cation Exchange Capacity (CEC).
  • Increases nutrient supply, especially N, P, S.
  • It prevents leaching of nutrients.
Where does organic matter come from?

It comes either directly from plant and crop residues (stems, leaves, shoots), or indirectly from plants that have been through animals (manure, sewage, slurry).

The “living” part of OM comes from living organisms such as plant roots, bacteria, fungi, and insects. The “non-living” part of the OM comes from recently dead plant and animal matter and excreta from soil organisms and grazing stock.

The age of OM in soil varies widely from very recent additions a few hours old, to humus that could more than 1000 years old. The amount present at any one time depends on the balance of these two. The very old material my not contribute much to stable soil structure.

What are soil microorganisms?

Elaine Ingham of Oregon State University suggested that in a spoonful of healthy agricultural soil you will find up to 60 million bacteria, 150-500 feet of fungal hyphae, 10,000 protozoa, 20-30 beneficial nematodes (not root feeders) and several thousand mites, springtails and other micro arthropods. She points out that in many heavily cropped soils many of these life forms have disappeared.

The table below shows the variety of life in the soil - often called the “biomass”. It shows the number of organisms and mass in a typical 15cm of surface soil.

Organism No /g of dry soil mass kg/ha
Bacteria 100 million 1600
Actinomycetes  2 million  1600
Fungi  0.2 million  2000
Algae  25,000  320
Protozoa   30,000 380
Nematodes  1.5 120
Earthworms  1/kg  800

In a typical soil, most of the microorganisms stay in the same position and only move when disturbed by root growth, water movement, cultivations, earthworms and beetles. Their number can drop sharply in adverse conditions and may change to resistant forms and remain dormant for long periods.

When conditions are right, numbers can increase rapidly. An example of this is when they live next to plant roots where there is a continual supply of leaked carbohydrate, to feed on. Worm action and cultivation stimulate more microbial activity and OM decomposition.

The main product from OM decomposition is Carbon Dioxide, but it’s the residue left behind - humus, that is so important to the chemical and physical properties of the soil.

What is humus?

Humus is a very stable form of OM that remains when most of the animal residues in the soil have been decomposed by microorganisms.

What does humus do in the soil?
  • It is relatively resistant to further attack by microorganisms.
  • It can increase the ability of the soil to store water.
  • Soils rich in humus are darker in colour.
  • It can absorb and deactivate a number of chemicals such as herbicides added to the soil.
What affects the activity of soil microorganisms?

Decomposition of OM added to the soil depends on these things:

  • Temperature (ideal is 25-30C, poor is below 10C)
  • A pH below 5 decreases OM
  • Soil texture. OM increases as the clay content in mineral soils increases. It combines with clay to form more stable structures protected from more microorganism attack.
What else is released from OM breakdown?
  • Carbon dioxide is the main component.
  • Minerals used for plant growth.
  • The elements N, P, S.
How does OM break down?

As the microorganisms in the soil decompose OM feed themselves with energy and minerals and as a result some of these are released into the soil solution. This process is called “mineralisation”. It converts nutrients from an organic form available to plants, into a soluble inorganic form that plants can use.

Where do soluble soil nutrients go?

There is great competition for these soil nutrients from:

  • Absorbed by plant root hairs
  • React with other soil constituents.
  • Be leached out of the root zone.
  • Be reabsorbed by soil organisms.
Is there a downside to organic residues?

Yes. In the early stages of decomposition, acids may be produced that can be toxic and hinder germination. For example ploughed-in straw may cause a temporary nitrogen shortage to young seedlings as the microorganisms are using up the N.

How does OM affect soil structure?

Plant residues that have not decomposed benefit the soil as they protect the surface from rain batter. They also can help surface drainage by forming gaps in the soil.

Some carbohydrates (called polysaccharides) are formed when plant material decays, and act as “organic gums” that bond soil particles together to form crumbs or aggregates. Fungi that grow in long filaments also enmesh soil particles to help form aggregates.

How can you increase soil OM?

The amount of OM present in the soil depends on additions and decomposition. In arable cropping, after 1-2 years in grass the OM will improve, but it may take up to 10 years to make a significant difference. Increasing OM will largely depend on management techniques.

In organic farming, OM can be increased by adding compost, worm casts, and farm manures. Note that effluent varies greatly in nutrient content and may contain some heavy metals. Also remember that adding small amounts of “organic fertilisers” may not cause rapid rises in OM levels.

Endeavor to keep any applied OM in the top layers of the soil. There’s no point in putting on OM, and then cultivate or plough it in deep. On large farms where OM application is not practical, use management practices that protect the OM already present, and then enhance it, by for example using fertilisers that are not antagonistic to the soil biomass.

How can you tell if soil OM is deficient?

Check the soil surface for “capping”. Here fine soil grains have separated out forming slurry during rain. When the surface dries out, if forms a crust or cap. Then check the soil surface for erosion. Here the sand particles that have separated out, blow away.

But there is always the wider issue, that if a soil is not working well because of low OM, then the products from the soil (the animal production) will reflect this in poor performance. The challenge is that we tend to look only as far as the animal and not to the soil.

Information provided by:
Mr Denis Cadwallader, Organic Farming Specialist. 22 May Avenue, Napier, New Zealand
Phone (06) 834-3405, Fax (06) 834-3406, Mobile 025-481-782, Email This email address is being protected from spambots. You need JavaScript enabled to view it.

Mr Cadwallader is guest tutor in Organic Farming at the Waikato Polytechnic.
Phone (07) 834-8806 for further information on courses.

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