Mineral matter of the Soil
The completely weathered material like the alumino-silicates, free silica and various salts, and partly of unweathered particles of original rock and minerals like quartz, mica etc. The alumino-silicates together with free silica, oxides of iron, aluminium manganese titanium etc., and the insoluble salts like calcium and form the major bulk of the soil. The soluble salts of the alkali and alkaline earths constitute the soluble mineral matter and form only a small part of the soil.
The insoluble mineral matter may be divided into four main groups according to their particle size- gravel, sand, silt and clay.
Gravel
Gravel or stones consist of pieces of rock and mineral that have resisted weathering or by weathering agents. It has almost the same chemical composition as the original rock or mineral from which it is derived and as such does not contain any food nutrients that can be utilized by the plant. Both physically and chemically, it is an inert material. When it is present in small quantities it helps to keep the soil porous.
Sand
It is that portion of the mineral matter whose particles are smaller than 2 mm but bigger than 0.02 mm. It consists of fragments of rocks and minerals that have not yet weathered or have escaped weathering. Sand possesses a very poor water-absorbing capacity. Its water holding capacity is very poor. Due to lack of cohesion they cannot stick together and remain as more or less individual particles. They have no plasticity. Its power of absorbing and holding water and retaining manorial ingredients is very poor. Its heat-absorption capacity is relatively high.
Silt
Silt ranges in diameter from 0.02 to 0.002 mm. Almost the whole silt separate contains particles of primary minerals like feldspar, mica, quartz etc., that have escaped weathering. The finer particles of silt contain a few minerals of secondary origin such as glauconite. It is richer in plant food constituents than sand. It possesses a better water-holding capacity than sand. When these particles are suspended in water, they are readily flocculated on addition of salts. They also possess the power of slight cohesion and have very feeble cation exchange properties.
Clay
Clay particles that are smaller than 0.002mm in size. It consists of mainly of secondary minerals formed as a result of the decomposition of original minerals. The whole of the clay fraction is taken to represent particles of secondary origin that have undergone complete decomposition and the final product of weathering and soil formation. Sand and silt, which are inactive both chemically and physically, clay is the highly active portion of the soil. It has a very high water absorbing and water-holding capacity, much greater than that of sand or silt. Clay particles possess the poser of adhesion, cohesion and plasticity. Clay impede the movement of the water and air. On account of their great surface activity they hold and retain large quantities of water and plant food ingredients.
Gravel, sand and silt contain pieces and particles of original rock and minerals, while the last clay consists mainly of the weathered product of rocks.
- VIJAYKUMAR M. SARUR
M.Sc. Agro-chemical and Pest Management
Chief-Agronomist
Kothari Agritech Pvt. Ltd.
Mob. 9545552988