Fundamentals of Soil Science (1+1)

Land and Soil

Land and Soil:

• Land and soil are often confused by students as synonymous. Land is broadly defined as total natural environment of the areas of the earth not covered by water. In addition to soil, its attributes include all the living organisms, the air and water bodies with in or on it and rocks below.
• Soil forms a part of an ecosystem which is the base functional unit of ecology including both biotic and abiotic environment mutually influencing each other to maintain dynamics of life on earth.
• The biotic component comprises all the living organisms whereas the abiotic component consists of the solid mineral matter on earth, the water in the oceans, lakes, river etc., the gaseous mixture in the air and radiant solar energy.
• Soil as natural body: The scientist considers the soil to be a natural body having both depth and surface (L X B) area. In fact, the soil is product of nature resulting from both destructive and synthetic forces. The weathering of rocks and minerals and decomposition of the organic matter are the examples of destructive processes. Whereas, formation of various minerals, clays and development of different horizons are synthetic processes. Thus, a scientist considers the soil as a habitat for the plants. He recognizes the contribution of plants in the development of soil and also its importance in crop production. The term soil is derived from Latin word “Solum” which means the floor.

Major component of soil:

• Soil is composed of partly weathered, unweathred, transformed products of rocks, rock minerals and organic matter.
• The mineral soil consists of four major components/phases: mineral material and organic matter (solid), water (liquid) and air (gases). In an ideal surface soils these components are observed in amounts (by volume,Fig.2) as follows,

Soild Phase (Mineral matter): The solid phase is broadly composed of inorganic and organic constituents. The inorganic constituents which forms bulk of solid phase of soil includes silicates, carbonates, soluble salts and free oxides of Fe, Al and Si in addition to some amorphous silicates. Only a small fraction of the solid phase is of organic origin. The sources of organic constitutes are plant and animals. Of total volume, about half is solid space, 45 per cent mineral matter and 5 % organic matter.

Liquid phase (Soil water): Forty to fifty per cent of the bulk volume of the soil body is occupied by soil pores, which may be completely and partially filled with water. The soil acts as a reservoir for supplying water to plants for their growth. The soil water keeps salts in solution which act as plant nutrients. Thus, liquid phase is an aqueous solution of salts.

Gaseous Phase (Soil air): The air filled pores constitutes the gaseous phase of the soil system. The volume of the gaseous phase is thus dependent on that of liquid phase. The nitrogen and oxygen contents of soil air are almost same as that of atmospheric air but concentration of carbon dioxide is much higher.

The four major components of a typical soil exist mainly in an intimately mixed condition. The proportion of these components may vary from time to time and from place to place. The volume composition of sub- soil is different from the surface soil. Compared to top soils they are lower in organic matter content, lower in total pore space and contain a higher percentage of small pores. This means they have a higher percentage of mineral and water and considerable lower content of organic matter and air.