PHYSICAL AND, CHEMICAL CHARACTERISTICS OF SOIL
Soils contain matter in three states.
(i) The solid portion is of two types. (a) The organic
part consists of living and decayed plant and animal materials like roots, leaves and worms. Humus is the endproduct of decay. It is black, amorphous organic matter, which is responsible for the fertility of soil. (b) The inorganic part (mineral part) of soil is made up of particles derived from the par~nt material, i.e., the rocks, which weather to form the soil.
(ii) The Liquid portion or soil water is the water temporarily held in the soil, derived from direct precipitation and from run-off, seepage and groundwater. Soil water is a dilute but complex chemical solution of substances like bicarbonates, sulphates, chlorides, nitrates, phosphates and silicates of calcium, potassium, magnesium, sodium and iron.
(Hi) The gaseous portion or soil air or soil atmosphere occupies the pore spaces of the soil when it is not saturated with water. Soil air has been analysed and found to contain an excess of carbon dioxide, but a deficiency of oxygen and nitrogen. Soil air and soil water are present in inverse proportion to each other.
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