Land is a major constituent of one of the life supporting system, the lithosphere. It is an important source of many materials essential to man and other organisms.
Area : Land forms about one fifth of the earth’s surface covering about 13,393 million hectares. About 36.6 percent of the land area is occupied by human dwellings, factories, roads, railways, deserts, mountains, rocks and polar ice marshes. About 30 percent of the total land mass is under forests. About 22 percent of land is occupied by meadows and pastures. Only 11 percent land is suitable for ploughing.
Soil : The fertile surface layer of earth capable, of supporting plant is called soil. Terrestrial plants obtain their water and mineral nutrients in the soil. Plants and animal materials decay and are released into the nutrient bank in the soil. Many micro-organisms and animals involved in detritus path way inhabit the soil. Soil covers about four fifth of the land area. The study of soil is called pedology (Gk pedion= ground, logos = discourse). It deals with the origin, formation and geographic distribution of soil.
Soil is formed by two processes (i) weathering : breaking down of rock into small particles and ii) pedogenesis : maturation of soil through development of humus. Weathering of rocks involves physical and chemical breakdown. Physical breakdown is caused by temperature variations, alternate ,drying and wetting, microbial activity, action of plant roots and burrowing animals. Chemical breakdown occurs by oxidation, reduction, hydration Ind other related reactions.
Composition of Soil : Soil is composed of five constituents: (i) mineral matter (ii) organic matter (iii) soil water (iv) soil air and (v) living organisms.
- Mineral matter : It is derived from the underlying parent rock by weathering and occurs in the soil as particles. The mineral particles are regular in outline and therefore enclose spaces called interstices for circulation of air and water.; Depending upon their size the mineral particles f soil are of following types
Gravel 2.00-5.00 mm
Coarse Sand 0.20-2.00 nun
Fine Sand 0.02-0.20 mm
Silt 0.002-0.02 mm
Clay Less than 0.002 mm
Sand and silt consist largely of quartz (Si02) and are chemically inert. Clay particles are chemically active. They enclose smaller but more numerous spaces, which can hold water but little air.
Soil Texture : The physical structure of a soil is called soil texture. It upon the percentage of its mineral particles. Soil texture determines the porosity and nutritional status of the soil. There are three important textural soil types — sandy, clayey and loamy.
- Sandy soils: They contain less than 10 percent each of clay and The remaining part is sand They are porous and well aerated. They, however, have little water holding capacity and are chemically inert. Sandy soils are generally called light soils because of the absence of moisture and crumbs formation. They are, therefore, unfit for plant growth.
(ii) Clayey Soils : They contain about 40 percent or more clay. They I are called heavy soils because the soils are compactly packed with little aeration. They have small sized pores, which retain water very firmly, little aeration. They have small sized pores, which retain Water very firmly. Clayey soils are rich in nutrients but do not support good plant growth due to poor aeration.
(iii) Loamy soils: They contain sand, silt and clay approximately in the ratio of 2 : 2 : l. They are ideally suited for plant growth because they possess good aeration, sufficient nutritive salts and good water retaining capacity.
Soil Crumbs : Soil crumbs are large, soft and spongy soil pieces, which have honey comb structure with high porosity. The functional structure of the soil is dependent upon the formation of soil crumbs. Soil crumbs are produced by the cementing of soil particles with the help of clay particles and gums present in the humus. Soil crumbs can hold both water and air besides containing nutrients.
2. Organic Matter : It is derived from plant refuse (leaves, twigs, roots), dead bodies of organisms and their excreta. The organic matter is broken down by microbes and is converted into dark amorphous substance called humus. Humus is very useful substance in the soil. It performs following functions:
(i) It acts as sponge and absorbs rain water and also increase water holding capacity.
(ii) It makes a soil porous and increases aeration of the soil.
(iii) It provides various minerals such as phosphorus, sulphur, calcium and potassium to the soil.
(iv) It contains several acids, which assist in weathering of rocks, cause solubilization of heavy mineral salts and functions as buffer against pH changes.
(v) Gums present in the humus act as binding agent.
3. Soil water: The soil particles are occupied by the water or air. The water in the soil is present as capillary water, hygroscopic water, combined water and water vapour. Small spaces serve as capillaries and allow the water to move against the pull of gravity. It is called capillary water. Some water form an extremely thin film around the soil particles and is called hydroscopic water. A small portion of soil water is chemically bound with soil materials called combined water. Some water vapour is also present in the pore spaces. Plants can draw only capillary water from the soil. It is called available water. If soil water is not replenished from time to time, a stage is reached, when the plants growing in it become permanently wilted. The amount of water in percentage left in the soil at the time of permanent wilting of the plants is called permanent wilting percentage (PWP) or permanent wilting coefficient (PWC). At this stage soil contains about 10 percent of water. This water is called non-available water and is held in the soil as hygroscopic water, combined water and water vapours.
Soil receives water either by rains or irrigation. Excess of water entering soil percolate down to permanent water table through larger pore spaces. It is called gravitation water. The maximum amount of water retained per unit dryweight of soil after the stoppage of gravitational flow is called water sliding capacity or field capacity of the soil. It is 25-35 percent in loam soil. Soil moisture beyond field capacity causes water logging. It is harmful to plants as it drives away soil air
4. Soil air: Soil contains air in the pour spaces. In a good soil such as about 20-25 percent of the total volume is soil air. The composition of is dependent upon ventilation or connection of the pore spaces with the surface. In a poorly ventilated soil, concentration of oxygen decreases while that of carbon dioxide increases because of the respiration of plant sand soil organisms. High concentration of carbon dioxide in the soil is to the soil organisms. Oxygen in soil is essential for humus formation, respiration of microorganisms and activities of roots (absorption and mineral salts). Lack of sufficient aeration reduce nitrates and s of the soil to form free nitrogen and toxic sulphides in the soil.
5. Soil organisms : A variety of living organisms occur in the soil include bacteria, actinomycetes, fungi, algae, protozoa, nematodes, worms and burrowing animals. The soil organisms perform following activities in the soil:
(i) Saprophytic soil microorganisms decompose the dead organic matter release the nutrients in the soil for their reuse by the plants.
(ii) A number of bacteria and cyanobacteria present in the soil, fix atmospheric nitrogen into the soil.
(iii) Burrowing animals increase aeration of the soil and also bring nutrient rich sub-soil to the surface.
(iv) Several soil organisms secrete mucus, which help in cementing soil particles to form soil aggregates.
The salt content, pH and organic and inorganic nutrients like nitrogen phosphorous and potassium determine its chemical properties. The topography, climate and biotic factors control the condition of the soil.
Depletion of Land Resources : A number of factors a:e posing danger to land resources and are causing depletion to the productive land. They include : (i) soil erosion due to wind or water (ii) salination due to improper irrigation and faulty drainage system (iii) acidification due to leaching of soluble bases (iv) deposition of salt due to floods and (v) developmental activities such as construction of dams, road, railway urban encroachment, industrialization and mining.