Soil organic matter

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Soil Organic Matter

The beneficial effects of soil organic matter have been mentioned under various sections, relating to nutrient supply, soil texture and water management. The importance of maintaining soil organic matter cannot be overstressed, particularly in relation to low-input, rainfed cropping systems.

Soil fertility decline under intensive cropping is often associated with decline of soil organic matter. Mechanised agriculture tends to ignore this resource, substituting its services with more fertilisers and irrigation. However, changing the farming system to promote soil organic matter can make it more efficient and more sustainable.

Increasing the organic matter content of the soil has a number of beneficial effects. The gradual decomposition of this material provides a steady supply of plant-available nutrients. The organic particles may also provide a suitable substrate on which soil nutrients can be held in an available form. Organic matter increases the soil’s ability to resist acidification. It also increases water retention so that the soil takes longer to dry out, and gives the soil an open texture so that more air can get to the roots.

Organic matter is increased by leaving crop or fallow residue on the field, without burning, or by bringing plant material from another site. If the need for field sanitation prevents leaving the crop residue, then options may be to compost it and return it later, or to use it to mulch another crop which is not at risk from the pest. Often sweet potato vines are removed and used to feed stock. Returning the animal manure to the field replaces some of the nutrients, but it will not maintain the organic matter content of the soil unless supplemented by plant material from fallow or green manure crops.

Improved fallows

The long fallows of traditional swidden farming systems allow organic matter to accumulate in the soil, and this forms the basis for restoring soil fertility for cropping. However, in tropical conditions, organic matter is quickly degraded during cropping, and this source of fertility is lost within two or three years. In many areas where swidden farming was practiced, land pressure has meant that fallows have become shorter, and often the tree-dominant fallows have been replaced by grasses or woody weeds, which are less effective in restoring fertility (although grass fallows can be quite effective in restoring soil texture).

In these situations, consideration needs to be given to improving the quality of fallow vegetation, and reducing the losses of organic matter. Burning of fallows is one of the biggest sources of loss, which can be avoided. Communities which have traditionally burnt fallows will find it difficult to move away from this practice. To overcome these prejudices, non-burning should be introduced with planted fallows, selecting fallow species which are easy to clear, are non-weedy and suppress weeds during the fallow. The concept that gardens should be “clean”, with no debris lying on the ground, should be challenged, and the benefits of surface mulches demonstrated.

Improved fallow species are most commonly legumes, which have the ability to replenish soil nitrogen by “fixing” nitrogen from the atmosphere. Legumes may be either trees or herbaceous species, and a large body of literature is available on the merits of various species in particular situations. However, non-legume species may also be included in an improved fallow, particularly species which provide some other utility, such as production of firewood or pole timber, fruits, fibres or medicinal substances, or which suppress pests or diseases.

Typically an improved fallow may consist of an herbaceous legume “cover crop”, which will establish quickly and suppress weeds, together with one or more tree species, which may later take over from the cover crop. Trees which can be propagated from pole cuttings are easy to establish among grasses and weeds, but may not root as deeply as trees grown from seed, as they do not produce a true tap root. Deep rooting may be an advantage in accessing nutrient reserves and water from the subsoil, but this depends on local soil conditions. Tree seedlings may be established during the cropping cycle, to give them a head-start in the fallow. However, this may be difficult where mechanical tillage is used.

Very short fallows (one year or less) may be restricted to herbaceous species only. This becomes in effect a continuous cropping system, where one crop in the rotation is dedicated to soil improvement.

It should be understood that the nitrogen fixed by a leguminous plant is contained in the plant tissues. It does not replenish soil nitrogen until the plant is decomposed in the soil (or recycled through the droppings of animals or insects eating the plant). Growing a leguminous crop, such as peanut, does not appreciably increase soil nitrogen as the nitrogen is removed with the harvested crop.

Contributed by: Jane O'Sullivan 

Further topics on Soil Management:

Soil management

Soil structure

Plant nutrients


Causes of nutritional disorders

Diagnosing nutritional disorders

Correcting nutritional disorders

Other topics on Crop Management:

Land preparation

Production of planting materials


Soil management

Vine lifting

Integrated pest management


Postharvest practices