Soil is one of the most important national resources of any country. The soil not only grows a variety of food and fodder crops required for men and animals but also produces raw materials for various agro-industries viz., sugar and starch factories, textile mills, canning and food processing units. Soil is a habitat for plant growth bears certain physical, chemical and biological properties, which determined degree of workability, suitability to the specific crop varieties, physical and chemical capacities as well as productivity. The physical capacities of a soil are influence by the size, proportion, and arrangement on mineral composition of the soil particles. The physical and biological properties of soil need careful studies because soil is a natural medium for the plant growth and gives mechanical support to plant.
Physical properties of soil
Some of the important physical properties of soil connected with growing of crops,are as under:
Soil texture refers to the relative amounts of inorganic particles viz. Sand, Silt and Clay. Sand grains are large and coarse, clay particles are vary fine and smooth, and silt particle intermediate.
The way in which soil particles are grouped or bound together to form lumps or aggregates is known as soil structure. There are two main types of soil structure viz. single grained and compound structure. Soil structure can be changed or modified and improved or damage by adopting various soil management practices like tillage, manuring, liming, rotation of crops, irrigation, drainage etc.
The density of soil that is weight per unit volume can be expressed in two ways viz; the density of solid (particle density), particles of the soil and the density of the whole (Bulk density) soil that is inclusive of pore space. Generally soils with low bulk density have better physical condition than those with higher bulk densities. Texture and structure of a soil, its total pore space and organic matter content are all related to bulk densities.
In between the particles there are empty spaces which are occupied by air and water and are termed as pore spaces. Pore spaces between the aggregates of soil particles are macro-pores and those between the individual particles of the aggregates are micro-pores. The percentage of macro pores is more in sandy soils these soils never remain water logged and allow water to percolate down so rapidly that their moisture content is very low,crops suffer due to shortage of soil moisture. The percentage of micro pore is more in clayey or fine texture soils. In this soil water logging condition causes adverse effect on respiration of roots and bacterial activities. A proper balance between the macro and micro pores can be maintain by timely cultivation and addition of organic matter so as to that crop neither suffers from shortage of excess of water.
It is a combination of properties that determine the resistance of the dry soil to crushing or pulverising action by implements and when wet its ability to be moulded or changed in shape. All soils have cohesive and adhesive properties. Sandy soils have these properties to a much lesser degree than fine texture clay soils. A good tilth soils has both the macro and micro pores in more or less equal proportion, ensures adequate retention of water and also free drainage of excess water. The effect of such physical condition of soil on germination of seed and growth of plant is very beneficial.
Soils have various shades of black, yellow, red and grey colours useful in soil classification. Parent material e.g. red sandstone, organic mater, presence of certain minerals e.g.titanium compounds imparts darker colour, hematite give red colour, limonite-yellow colour, predominace of silica or lime. Soil colour is indirectly helpful in indicating many other properties of soils e.g. a dark brown or black coloured soil indicates its high organic matter content and fertility. A red or yellowish soil shows good aeration and proper drainage. A white or black colour due to accumulation of certain salts of alkali indicates deterioration of soil fertility and its unsuitability for normal growth of many crops
Soil micro-organisms show maximum growth and activity at optimum soil temperature range. All crops practically slow down their growth below the temperature of about 90C and above the temperature of about 500 C. The biological processes for nutrient transformations and nutrient availability are controlled by soil temperature and soil moisture. Soil temperature has a profound influence on seed germination, root and shoot growth, and nutrient uptake and crop growth. Seeds do not germinate below or above a certain range of temperature but Micro-organisms functioning in the soil are very active while a certain range of temperature, which is about 270 to 320C. It is necessary to know whether the soil temperature is helpful to the activities of plants and micro-organisms and the temperature could be suitably controlled and modified. The various factors that control the soil temperature are soil moisture, soil colour, slope of the land, vegetative cover and general tilth of the soil. Soil temperature can be controlled by regulating soil moisture, proper soil management practices viz. good drainage, proper mulch, good crumby structure, addition of sufficient organic matter help in keeping the soil sufficiently warm and help in the chemical and biological activities in the soil.
Biological properties of soil
A variety of organisms inhabit the soil. They decompose organic matter, fix atmospheric nitrogen, cause denitrification and plant disease. Cultivated soils harbour bacterial, actinomycetes, fungi, algae, protozoa, nematodes, worms, insects and rodents. Specific groups of organisms are responsible for specific activities in the soil. Such activities may be beneficial or harmful to the crop or its yield potential.
They decompose the organic matter and release plant food nutrients like nitrogen, phosphorus etc. nitrogen fixing bacteria like rhizobium and azotobacter, and phosporus-solublising bacteria. Nitrosomonas and nitrobacter bacteria, which oxidize ammonia to nitrate and nitrate to nitrite compounds respectively, the process is called nitrification. These are generally confined to the surface 20 to 30 cm.layer and work best when there is good aeration, a neutral reaction, soil moisture content at about half of the water holding capacity and temperature between 250 c and 380 c. Phosphate fertilization of legumes also helps to increase the yield of green matter and more nitrogen is fixed to the soil.
These organisms produce microscopic threads called mycelia and ae found in the organic matter of plant roots. Fungi help in breaking down the somewhat resistant parts of the organic matter like cellulose, lignin, gums etc. A large part of slowly decomposing soil humus is made up of the dead remains of fungi.
They can grow in deeper layers even under dry conditions of soil, and require less nitrogen. Their main function lies in decomposing the resistant parts of organic matter like cellulose.
They are microscopic or very minute sized plants having chlorophyll and are usually found on the surface of wet soils of paddy fields. They help in adding organic matter to soil, improving the soil aeration and fixing atmospheric nitrogen e.g. blue-green algae.
Texture and other soil properties and plant growth
Many of the important soil properties are related to texture. Clayey soils show high water holding capacity, high plasticity, and stickiness and swelling whereas sandy soils are conspicuous by the absence of these properties. The most important way in which soil texture affects plant growth is water and with it the nutrient supply. The available water holding capacity of soil is related to soil texture.
Soil structure and plant growth
Soil structure influences plant growth rather indirectly. The pores are the controlling factors governing water, air and temperature in soil, which in turn, govern plant growth. One of the best e.g. of the effect of soil structure on plant growth is the emergence of seedlings in the seedbed. The seedlings are very sensitive to soil physical condition so that there should not be any hindrance to the emergence of tender seedlings and there should be optimum soil water and soil aeration. The soil in the seedbed should have a crumb structure so that the peds are soft and porous and roots of the seedling can penetrate it easily. The hard compact layer impedes root growth.
Water is essential for plant growth. Soil is capable of being a storehouse of water and becoming the main source of water for land plants. Soil water plays a significant role in several natural processes- evaporation, infiltration and drainage of water, diffusion of gases, conduction of heat, and movement of salts and nutrients are all dependent upon the amount of water present in soil.
Plants meet their water requirement from water stored in soil.
Effect of crops and cropping practices on soil structure
Crops affect soil structure through their vegetative canopy above the ground and their roots below the ground. Grasses are conducive to well structured soil. Organic residues left by the grassroots, root pressure, pores due to decayed roots and microbial activity in the rhizosphere produce ideal crumb structure. The vegetative canopy protects the soil from the beating action of rain drops and destruction of the structure of the surface soil and prevents crusting. The role of legumes in building up soil fertility is well known. As legumes have place in sound crop rotation practices, the beneficial effect is usually attributed to nitrogen added to the soil by legumes.
Soil Aeration and plant growth
Oxygen is required by microbe and plants for respiration. Oxygen taken up and carbon dioxide evolved are stoichiometric. Under anaerobic conditions, gaseous carbon compounds other than carbon dioxide are evolved. Root elongation is particularly sensitive to aeration. Oxygen deficiency disturbs metabolic processes in plants, resulting in the accumulation of toxic substances in plants and low uptake of nutrients. Certain plants such as rice are adapted to grow under submerged condition. These have large internal air spaces, which facilitate oxygen transport to the roots.
Soil compaction is the process of increasing dry bulk density of soil, reducing the pore space by expulsion of air through applied pressure on a soil body. Soil compaction creating problems for seed germination, water transmission and aeration. Crusting of soil is a form of soil compaction. The crusts present a serious barrier for seedling emergence. Lowering the exchangeable sodium percentage and incorporation of organic matter prevent crust formation.