The upper-most layer of the ground, soil is a complex mixture of minerals and micro-organisms, which has been described as both a micro-ecosystem, and the most complex of any ecosystem. Soil is essential for most plant growth, providing both nutrients and a substrate to anchor the roots. Depleted soils are those in which the microbiological and/or nutritional components have been seriously reduced. Depleted soils can best be restored to good condition with organic manures. Soil conservation
Practices which are designed to prevent or reduce soil erosion. The principle methods are minimum tillage, contour ploughing, terracing, and mulching. The planting of trees to make windbreaks is also helpful. Soil conservation is one of the more important aspects of sustainable agriculture. Soil erosion
The loss of soil to either wind or water. During the 1930s drought in the American mid-west, the wind-erosion was so severe that this area became known as the 'dust bowl'.
Similarly, the water-erosion in the Tigris-Euphrates Valley during some five millennia has extended the mouth of the combined rivers about one hundred miles into the Arabian Gulf. Soil erosion can be prevented by suitable soil conservation practices and this is one of the more important concerns of sustainable agriculture. It is notable that rice paddies conserve soil very well, and that ancient rice-based cultures still thrive while ancient wheat-based cultures are often extinct, as in the Tigris-Euphrates Valley.. Soil inoculation
Soil can be inoculated with either beneficial or parasitic microorganisms. The most commonly used beneficial inoculants involve Rhizobium to encourage nitrogen fixation by leguminous plants. Inoculation with soil-borne parasites is undertaken for the purpose of plant breeding and screening a heterogeneous plant population for horizontal resistance. The most effective method of soil inoculation is to transplant inoculated seedlings. Soil nutrients
Except for iron, plants absorb all their nutrients as inorganic chemicals, and most of them are extracted from the soil. Carbon dioxide, oxygen, and some water, are extracted from the atmosphere, but all others come from the soil. The three major nutrients are nitrogen, phosphates, and potash (NPK). Minor nutrients are calcium, sulphur, iron, boron, and magnesium. Trace elements include zinc, copper, manganese, etc. See also: Deficiency diseases; Mobile and immobile nutrients. Soil pasteurisation
Soil pasteurisation means that the soil has been heated to only 80°C. This kills most pests and pathogens without a complete sterilisation. Pasteurised soil can be used as soon as it is cool, whereas sterilised soil must be kept for some three weeks for its microbiological activity to be restored. Soil science
See: Pedology. Soil sieves
Soil sieves are designed for the mechanical analysis of soils, separating the soil particles on the basis of their size. The sieves come in sets which fit securely into each other, with the coarsest at the top, and the finest at the bottom. A set of soil sieves can be very useful for separating small seeds, which may be either wet or dry, from the debris of their extraction.
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