Components
Soil consists of a mixture of: (a) particles of sand or clay, (b) humus, (c) water, (d) air, (e) dissolved salts, (f) bacteria.
(a)Inorganic particles are formed from rocks which have been weathered and broken down. Particles from 2 to 0-02 mm diameter constitute sand, 0-02 to 0-002 mm form silt and less than 0-002 mm clay. Chemically, sand is silicon Oxide while clay may be various complexes of aluminium and silicon oxides. Iron oxide may give a red or brown coating to the particles.
Aggregates of these inorganic particles together with humus produce crumbs up to about 3 mm in diameter, which form the Skeleton of the soil. The crumb structure of a soil depends on the proportion of clay, sand and humus and the activities of plant roots; a good crumb structure is one of the most important attributes of a soil.
(b) Humus is the finely divided organic matter incorporated into the crumbs; it originates mainly from decaying plant remains. The presence of humus in the crumbs affects the colour and physical properties of the soil. Humus is black, structure less, often forms a coating round sand particles and may be important in glueing" particles together to form soil crumbs.
A sandy soil deficient in humus tends to have a poor crumb structure and is easily blown away if exposed by ploughing. The exclusive use of chemical fertilizers on certain soils in dry climates may lead to the formation of dust bowls or the advance of desert margins. The bacterial decay of humus and the organic matter from which it originates produces the nitrates and other mineral salts needed for plant growth.
(C) Water is spread over the sand particles or clay aggregates as a thin film which adheres by capillary attraction. It may also penetrate the aggregates and be held to the clay particles by chemical forces. When a soil contains as much water as it can hold by capillary and chemical attraction (1.e. any more would drain away by the force of gravity), it is said to be at field capacity. Capillary attraction will tend to distribute water from regions above field capacity to drier regions. The forces holding water in the soil also set up considerable opposition to the suction of plant roots when the soil begins to dry out.
(d) Air occurs in the spaces between the aggregates or sand particles unless the soil is water -logged, in which case the air spaces are blocked up. A supply of oxygen is essential for the respiration of roots and some soil organisms, e.g. bacteria.
(e) Mineral salts. Salts in the soil water are dissolved out either from the surrounding rock or from the humus in the soil. They make a very dilute solution with the soil water but are vital for plant growth as explained.
f)Bacteria. Many microscopic plants, fungi and animals live in the soil, but among the most important to plant life are the bacteria which break down the organic matter and humus to form soluble salts which can be taken up in solution by roots.
Other bacteria convert atmospheric nitrogen to organic compounds of nitrogen. For further details, Heavy and light soils
Heavy And Light Soils
Heavy soils A soil in which clay particles predominate and which has a poor crumb structure will be sticky and difficult to dig or plough. This results partly from chemical and capillary forces acting on the very large surface area of the minute clay particles, making them difficult to separate. When dry, the soil forms hard clods which do not break up readily during cultivation.
The small distances between particles tend to produce poor aeration and drainage, but the large surface presented by the particles retains a high proportion of water in conditions of drought. There is also less tendency for soluble minerals to be washed away as they are held chemically to the clay particles.
A heavy soil can be made lighter, more workable and permeable to water and air by adding organic matter or lime. The lime makes the particles clump together or flocculation, the clumps of particles behaving as the larger particles of a light soil. The crumb structure of a clay soil can be improved by growing grass on it for a year or two.
Light soils. The large inorganic particles of a light soil give it its sandy texture. The wider separation of the particles leads to better aeration and drainage but there is a smaller surface for the water film. Such a reduced surface lessens the Surface forces of the water and makes it easier to separate the particles in ploughing and digging, and the clumps break up easily when dry.
The mineral salts are more liable to be washed out from a lighter soil and it loses water rapidly in dry conditions. Its water-holding properties and nitrogen content can be improved by adding farmyard manure or compost.
Loam. A soil with a balanced mixture of particle sizes, a good humus content and stable crumb structure is called a loam are the most productive soils in agriculture.
Laterite soils. A laterite soil is formed in tropical conditions, where high temperatures cause very rapid decomposition of organic matter and the heavy rain washes out many of the minerals including much of the silica. The soil consists mainly of alumina particles coloured red with iron oxide, and is deficient in humus and mineral nutrients. When it dries, the alumina and iron oxide particles stick together forming a hard layer which is poorly aerated and difficult to plough or dig.
Soil pH (acidity). The pH of soils varies. A soil on limestone or chalk may be alkaline, up to pH8. Some clay soils and soils containing much organie matter may be acid, Acid conditions in the soil often lead to a deficiency of minerals by making them more soluble and easily washed out by rain. Alkaline conditions, on the other hand, may make some minerals so insoluble that they cannot be taken up in solution by the plant.
A pH of about 6:5 is considered favourable for most crops and cultivation methods can be used to adjust the pH. For example, application of lime (calcium hydroxide) will raise the pH of an acid soil, while addition of ammonium sulphate will lower the pH of an alkaline soil.