Loss of soil organic matter
Organic matter is the most important component of soils. It is formed predominately by a mixture of the decomposition of plant material that falls to the surface of the soil, the decaying parts of the rooting systems of plants within the soil, and the remains of any of the huge population of organisms in the soil once they die. It is a vital 'control box' of nature's nutrient cycle in which: (i) plant remains that fall to the soil surface are broken down by soil animals and microorganisms to form amorphous organic matter; (ii) the nutrients within the organic matter are transformed to an available form by the many soil organisms and chemical processes in the soil; and (iii) the nutrients are taken up by the roots of plants and passed upwards to the above-ground growing vegetation to enable it to continue its growth. This key to soil fertility is one of the amazing and most valuable cycles in nature.
In addition to having the key role in the nutrient cycle, soil organic matter is also used as a source of energy and sustenance for most of the huge number of organisms that occur in the soil. It is a carbon supply for many of the microbes in the soil, enabling them to fulfil other useful functions and it buffers against any rapid change in acidity which would affect the soil biodiversity. It is a critical factor in maintaining this hugely diverse population of creatures. Organic matter also helps to improve several important soil properties. For example, it encourages the development of well structured soils and thus movement of air through the soil. Favourable forms of structure such as crumb, granular and subangular blocky are all associated with soils that have reasonable levels of good quality organic matter. Organic matter also allows the soil to hold more water at field capacity and increases the available waterholding capacity of soils. This is particularly important in sandy soils which tend to have difficulty in holding water. Organic matter also contributes to good aeration of the soil and allows the soil to carry out healthy gas exchange with the atmosphere.
The type of organic matter varies depending on the use to which the land is put. For example, under a coniferous forest where the soils are acid there will be a reasonably well defined series of layers with the least decomposed, most identifiable plant remains (e.g., pine needles) at or near the surface. Immediately below this layer are layers in which the organic remains are more poorly preserved and eventually become unrecognisable as to origin as they are converted into black, amorphous humus. Under grassland, it is the blades of grass and the many grass roots that contribute to the organic matter and the organic rich layer is usually somewhat thicker and better integrated with the mineral soil. Under arable farming, the crop residues provide most of the organic matter, supplemented at times by farmyard manure, but here, due to ploughing and cultivation, the organic matter is spread more or less evenly throughout the top 20 cm or so. Different land uses such as those above give rise to different types of organic matter which are a function of the relationship between the prevailing vegetation and the soil. Soils vary in their content of organic matter, from less than 1 per cent in most desert soils to over 50 percent in peat soils. Most agricultural soils contain about 1-3 percent. Although a small fraction in most soils, organic matter enables the soil to perform its many functions.