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Current Category » Principles of Agronomy

Soil- Components Of Soil Or Phases Of Soil

Minerals soils consist of 4 major components: Mineral materials, OM, water and air in various proportions. Approximately 50% of the total volume of the surface horizon of many soils is made up of inorganic Materials (mineral matter) and OM (5%) and the remaining volume is per space between the soil particles. Water and air occupy these pore spaces in various proportions. The proportion of air and water varies from one season to another. At optimum moisture for plant growth, the 50% of pore space possessed is divided roughly in half 25% of water space and 25% or air.
The soil may be described as the three phase system: Soil solid, Liquid and gaseous phase.

1. Solid phase: Soil material less than 2 mm size constitutes the soil sample. It is broadly composed of inorganic and organic constitutes. Soils having more than 20% of org. constitutes are arbitrarily designated organic soils. Where inorganic constituents dominate, they are called mineral soils. The majority of the soils of India are mineral soils. It accounts for nearly 50% of the total volume and 95% without of the solid phase is made up of inorganic or mineral matter. The remaining 5% weight comprises of OM which is mainly derived from dead parts of the vegetation an animals.In inorganic constituents consist of silicates, certain preparation of carbonates, soluble salts, an free oxides of iron, aluminium and silicon. The humus and humus like fractions of the solid phase constitute the soil organic matter. Soil is the habitat for enormous number of living organisms like roots of higher plants (Soil Macro flora), bacteria, fungi, actinomycetes and algae (Soil Micro flora). A gram of fertile soil contains billions of these micro-organisms. The live weight of the micro-organisms may be about 4000 kg/ha may constitute about 0.01 to 0.4% of the total soil mass. Soil also consists of protozoa and nematodes (Soil Micro Fauna).

2. Liquid phase: About 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. A considerable part of the rain which falls on soil is absorbed by the soil and stored in it to be returned to the atmosphere by direct evaporation or by transpiration through plants. The soil acts as the reservoir for supplying water to plants for their growth. The soil water keeps salts in solution which act as plant nutrients. Thus, the liquid phase is an aqueous solution of salts, when water drains from soil pores are filled with air.

3. Gaseous phase: The air filled pores constitutes the gaseous phase of soil system and dependent on that of the liquid phase. The N and O2 contents of soil air are almost the atmospheric air but the concentration of CO2 is much higher (8 – 10 times more) which may be toxic to plant roots. This phase supplies O2and thereby prevents CO2 toxicity.
The 3 phases of the soil system have definite roles to play. The solid phase provides mechanical support for and nutrients to the plants. The liquid phase supplies water and along with it dissolved nutrients to plant roots. The gaseous phase satisfies the acration (O2) need of plants.

Current Category » Principles of Agronomy