Definition of Soil

Soil

composition of soil

Definition of Soil

Definition of soil in a dynamic natural body developed as a result of the pedogenic process during and after weathering of rocks, consisting of the mineral and organic constituents, processing definite chemical, physical and biological properties, having variable depth over the surface on the earth and providing a medium for the the growth of land plants.

Components of Soil

Volumetric components of representative silt loam surface soil shows that it contains about 50 % pore space (air and water ) and 50 % of organic and inorganic matters. It is solid phase. The phase made up to 45 % of inorganic matter and 5 % of organic matter . At optimum moisture for plant growth, the 50 % pore space possessed by this representative soil is roughly divided in half 25 % water space and 25 % air space. The proportion of air and water is subject to great fluctuation under natural condition depending on weather and other factors.

Soil is described as the three phases of the system solid, liquid and gases phases.

1. Solid phase

The solid phase is broadly composed of inorganic constituents. The soil having more than above 20 % of organic constituents are arbitrarily designated is called organic soil, where inorganic constituents dominate in soil, they are called mineral soil or inorganic soil.

The inorganic constituents from the bulk of the solid phase of the soil and consist of minerals of various kinds. Quartz, mica and feldspar have persisted with little change in composition since they were deposited in molten lava and are known as primary minerals. Other minerals such as silicate clay and iron oxides have been formed by weathering of less resistant minerals and are known as secondary minerals.

In general, the primary minerals dominate the coarser fractions of soil whereas secondary minerals are most dominant in the fine materials especially in the clays. The soil components a small fraction of the solid phase is of organic origin.

The main sources of organic matter are plants and animals. billions of macro and micro organisms thrive on the debris of animals and plants add themselves to the soil when they die. The organic matters is often is firmly combined with mineral particles forming aggregates.

2. Gaseous phase

The air filled pores constitute the gaseous phase of the soil system. The volume of the gaseous phase is that dependent on that of liquid phase. At any stage, the sum of the volume of liquid and gaseous phase remains constant for a particular soil.

The nitrogen and oxygen contents of the soil air are almost the same as that of the atmospheric air but the concentration of carbon dioxide is much higher (0.1%) . gaseous diffusion tends to reduce the differences in composition.

The three phases of the soil system have a definite role to play. The soil phase provides physical, mechanical, chemical support and nutrients to the plants .

The liquid phase supplies the water and also supply dissolved nutrients to plant roots. The gaseous phase of soil important needs of plants the aeration satisfies. The three phases share the soils responsibility to sustain the plant growth.

3. Liquid phase

The liquid phase about 40 to 50 % of the bulk volume of the soil body is generally occupied by voids or soil pores which may be completely or partially filled with water. The soil acts as a water holding or reservoir for supplying water to plants for theirs growth. The soil water keeps salts in solution. Thus, the liquid phase is an soil colloids (aqueous ) solution of salts.

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