Soil organic matter
Date: 16 Aug 2004 Author: Rebecca Lines-Kelly
Organic matter is the lifeblood of fertile, productive soil. Without it, agricultural production is not sustainable.
Organic matter is any living or dead animal and plant material. It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions.
On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures. Earthworms and microorganisms decompose these materials. The process of decomposition releases nutrients which can be taken up by plant roots. The end product of decomposition is humus, a black crumbly material resistant to further decomposition. A complex chemical substance, humus stores plant nutrients, holds moisture and improves soil structure.
The rate of decomposition of organic matter depends on the soil's temperature, moisture, aeration, pH and nutrient levels.
The warmer and wetter the climate, the faster the rate of organic matter breakdown. Cooler areas have higher levels of soil organic matter because it does not break down as quickly in low temperatures.
Waterlogged organic matter breaks down very slowly because microorganisms necessary for decomposition cannot exist where there is no oxygen. Soils formed from waterlogged organic matter are known as peats, and contain a high percentage of organic matter.
Acid soils with low pH usually contain greater quantities of organic matter because microorganisms become less active as soil acidity increases.
Benefits of organic matter
- Improve soil structure
As organic matter decays to humus, the humus molecules 'cement' particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and fungal strands in the decomposing organic matter, makes the soil aggregates stable in water.
- Improves drainage
These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily.
- Holds moisture
The aggregates are also very effective in holding moisture for use by plants. Humus molecules can absorb and hold large quantities of water for use by plant roots.
- Provides nutrients
Organic matter is an important source of nitrogen, phosphorus and sulfur. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients. If crops are continually removed from the soil, there is no organic matter for microbes to feed on and break down into nutrients, so fewer nutrients are available to plants.
- Improves cation exchange capacity
Humus molecules are colloids, which are negatively charged structures with an enormous surface area. This means they can attract and hold huge quantities of positively charged nutrients such as calcium, magnesium and potassium until the plant needs them. Clays also have this capacity, but humus colloids have a much greater CEC than clays.
(For more explanation, see Cation exchange capacity.)
How to increase soil organic matter levels
- Grow perennial pasture
A period under perennial, grass-dominant pasture is an effective way of increasing organic matter in farm soils. Short-lived annual grasses are a source of dead roots; perennial grasses are a source of leaf matter. Even short periods (1–2 years) under pasture can improve soil structure, even though the actual increase in organic matter may be small.
- Grow cereal crops
Cereal crops leave significant amounts of organic matter in their dead roots and stubbles after harvest.
- Grow green manure crops
Green manure crops provide protective cover until they are ploughed into the soil. Initially they provide a large increase in organic matter levels, but they break down rapidly to give only a small increase in long-term organic matter levels; also, the ploughing operation can do more harm than the good done by the organic matter.
- Spread manure
Bulky organic manures will increase organic matter, but frequent and heavy applications are needed to produce significant changes.
- Use organic fertilisers
Organic fertilisers applied in large amounts can boost organic matter levels but are generally less cost-effective as supplies of nutrients than inorganic fertilisers. Applied in small quantities, they are unlikely to have a significant effect on organic matter levels.
- Keep cultivation to a minimum
Cultivation breaks down the stable aggregates, exposing humus in the aggregates to air and faster decomposition. Direct drill techniques allow you to sow seed while leaving stubble residues on top of the soil, and leaving aggregates intact.
- Concentrate organic matter
An alternative to increasing inputs is to make more effective use of what is already there. Retain all organic additions, whether roots, stubble or manure, close to the surface. The stability of soil structure is related to the concentration of organic matter at the surface, not the total quantity present in the soil.
Problems with incorporation
Incorporation of organic matter can present some problems.
- It is difficult to incorporate large quantities by cultivation.
- Green manure crops break down quickly and provide only a small increase in soil organic matter levels. Ploughing hastens the breakdown of humus and may counteract the small benefit from the crop itself.
- If organic matter is incorporated when the soil is wet, the soil may compact so that there is not enough oxygen available for microroganisms to decompose the organic matter. This may affect crop growth and nitrogen supply.
- Chemicals released from organic matter may reduce the rate of plant growth for a short time or have a toxic effect on young seedlings.
- Incorporating straw can also lead to a temporary shortage of available nitrogen for the planted crop, as the microorganisms will draw on the limited nitrogen in the decomposing straw.