Components of soil. Lecture 2

             Components of soil

Mineral soil consists of four major components i.e., inorganic or mineral materials, organic matter, water and air. In a representative loam surface soil, the solid mineral particles comprise about 45% of the soil volume and organic matter 5%. At optimum moisture for plant growth, the pore space is divided roughly in half, 25 %, of volume being water space and 25 % air. The proportions of air and water are subjected to rapid and great fluctuations. The four soil components occur in a thoroughly mixed condition in soil and this mixture encourages interactions within and between the groups and permits marked variations in the environment for the growth of plants. 

The proportion of different components in the diagram depicts the good soil condition for plant growth (Loam surface soil).The air and water are extremely variable and their proportions determine in large degree the soil’s suitability for the plant growth.

 Mineral Matter: -

                                  The Inorganic portion of soils is quite variable in size and composition. It is composed of small rock fragments and minerals of various kinds.                            The proportion of different sized particles (texture) determines the nutrient supplying power of the soil, considerably. Primary minerals (original) are prominent in sand and silt fractions; whereas the secondary minerals (formed) dominate in clay fraction and in some cases the silt fraction. The inorganic fraction of soil is the original source of most of the mineral elements that are essential for plant growth. 

Organic Matter :-

                                 It comprises an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by soil microbes as the decay occurs. It is a transitory soil constituent as it is continuously broken down by soil organisms and lasts from few hours to several hundred years. It requires maintenance by regular addition to the soil of plant and/ or animal residues .Organic matter content varies from 1.0 to 6.0 % by weight in top soil and very less in sub soil. In respect of soil productivity organic matter plays an indispensable role. 

Soil Water :-

                       Soil water is held in soil pores with varying degrees of tenacity depending on the amount of water present and size of the pores. Soil water with its soluble constituents (nutrients) makes up soil solution, which is the critical medium for supplying nutrients to growing plants. Soil water plays significant role in controlling energy balance of the soil and regulates the gaseousexchange in the upper layer of the soil. The presence of water in different amounts in soil governs its thermal, mechanical, physical, chemical and biological properties. 

Soil Air :-

 The content and composition of soil air are determined largely by the water content of the soil, since the air occupies those soil pores not filled with water. Soil air always differs from atmosphere air in composition because of moisture content, root and microbial activities.

 1. Relative humidity may approach 100% at optimum soil moisture

 2. CO2 content is often several hundred times higher than 0.033% 

3. Oxygen content usually <20%. In extreme cases only 5-10% 

In cases of low soil air, the diffusion rate of air in to and out of the soil would be slow and this leads to unsatisfactory conditions for optimum plant growth.