The soil organic matter includes a variety of plant and animal debris in different stages of decomposition. The living plant roots and soil microorganisms are also often considered part of the soil organic matter. Soil organic matter is the smallest component of the soil. Although it is a small percentage of the overall soil, it is the heart of a healthy productive soil. Most of the soil organic matter is present near the soil surface (top six inches), rather than deeper in the soil. It is not possible to depict in detail the beneficial effects of organic matter in this short article. However, some prominent features have been highlighted.

Plant productivity is linked closely to organic matter. Consequently, landscapes with variable organic matter usually show variations in productivity. Plants growing in well-aerated soils are less stressed by drought or excess water. In soils with less compaction, plant roots can penetrate and flourish more readily. High organic matter increases productivity and, in turn, high productivity increases organic matter.

The chemical and nutritional benefits of organic matter are related to the cycling of plant nutrients and the ability of the soil to supply nutrients for plant growth. Organic matter retains plant nutrients and prevents them leaching to deeper soil layers. Microorganisms are responsible for the mineralization and immobilization of N, P and S through the decomposition of organic matter. Thus, they contribute to the gradual and continuous liberation of plant nutrients. Available nutrients that are not taken up by the plants are retained by soil organisms. In organic-matter depleted soils, these nutrients would be lost from the system through leaching and runoff.

Crop yields in systems with high soil organic matter content are less variable than those in soil that are low in organic matter. This is because of the stabilizing effects of favorable conditions of soil properties and microclimate. Improvements in crop growth and vigor stem from direct and indirect effects. Direct effects stem from improvements in nutrient and water content. Indirect effects stem from a favorable rooting environment and possible weed suppression and a reduction in pests and diseases.

Immobilization of N may occur in systems with crop residue management, especially where C: N ratios of the residues are high (tough, woody materials). This can cause a decrease in maize yield. The preservation of wheat residues/other organic materials on the soil surface led to immobilization of N, which was overcome through the application of N fertilizer. Based on an experimental data, maize with oats, lupine and vetch as a winter cover crop (without fertilization) can produce a yield that is comparable with or higher than those obtained with conventional tillage and a fertilizer treatment of 90 kg/ha. In this case, the yield increase was highly correlated with the P content of the leaves and the P availability in the soil. This occurred because of the higher moisture content in the soil under the mulch layer, which led to a higher P uptake by plant roots. Grain crops can also have residual effects on each other through the decomposition of chemical compounds in the residues.

Conventional agriculture tends to reduce aboveground and belowground diversity. Thus, it brings about significant changes in the vegetation structure, cover and landscape. The change in vegetal cover during the conversion of forest and pastures to cropping affects plants, animals and micro-organisms. Through increasing specialization of certain plant species (food and fibre crops, pasture and fodder crops, and tree crops) and livestock species, some functions may be affected severely, e.g. nutrient cycling and biological control. Some non-harvested or associated species profit from the change and become pests. However, many organisms either disappear completely or their numbers are reduced drastically, e.g. pollinators and beneficial predators, unless efforts are made to retain a suitable habitat.

Associated species can be managed to a certain extent. Through appropriate crop rotations, crop-livestock interactions and the conservation of soil cover, a habitat can be created for a number of species that feed on pests. This will in turn attract more insects, birds and other animals. Thus, rotations and associations of crops and cover crops as well as hedgerows and field borders promote biodiversity and ecological functions.

In most agricultural soils, organic matter is increased by leaving residue on the soil surface, rotating crops with pasture or perennials, incorporating cover crops into the cropping rotation, or by adding organic residues such as animal manure, litter, or sewage sludge.It appears that increasing organic matter in soil has a host of benefits from both an agricultural and environmental prospective. Thus, we will be well served to enhance those factors that result in soil organic matter accumulation in the soil and, as much as possible, to moderate those factors that result in losses of soil organic matter.

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