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Soil Profile Soil profile is the term used for the vertical section of earth crust generally up to the

depth of 1.83 meter or up to the parent material to show different layers or horizons of soil for the study of soil in its undisturbed state. It is made up of a succession of horizontal layers or horizons, each of which varies in thickness, colour, texture, structure, consistency, porosity, acidity and com-position. A pedon is thesmallest threedimensional volume of soil needed to give full representation of horizontal variabilityof soil. In general, soils have following four horizons an organic or Ohorizon and three mineral(A,B,C) horizons. A and B-horizons form the true soil or solum. Each horizon of soil profile is further subdivided. Horizon subdivisions are indicated by arabic numbers, e.g, O1, O2, A1, A2, etc. Different horizons of soil-profile have following characteristics : 1. O-horizon. The O-horizon, once designated as Ao and Aoo, is the surface layer forming above the mineral layers and composed of fresh or partially decomposed organic material, as found in temperate forest soils. It is usually absent in cultivated soils and grasslands. O-horizon contains both kinds of humus (e.g., mull and mor) and is subjected to the greatest changes in soil temperatures and moisture conditions and contains most organic carbon. O-horizon and upper part of A horizon is the region where life is most abundant. The O horizon is divided into following two sub-layers: (i) O1 (Aoo) region. It is the uppermost layer which consists of freshly-fallen dead leaves, branches, flowers and fruits, dead remains of animals, etc. All these do not show evident breakdown. (ii) O2 (Ao) region. Below the O1 region is the O2 layer of partly decomposed organic matter. The process of decomposition of the litter is started in O2 region. Thus, organic matter is found under different stages of decomposition and microorganisms such as bacteria, fungi, actinomycetes are frequently found in it. Upper layers contain detritus in initial stage of decomposition, in which material can be faintly recognized, whereas the lower layers contain fairly decomposed matter, the duff. 2. A-horizon. It is characterized by major organic matter accumulation, by the loss of clay, iron and aluminium and by the development of organo-mineral complexes, granular crumbs or platy structures. The A-horizon is divided into following two sublayers : (i) A1 region. This region is dark and rich in organic matter and is called humic or melanized region. The amorphous, finely-divided organic matter here becomes mixed with the mineral matter,which is now known as humus and is dark brown or black-coloured. (ii) A2 region. It is light-coloured region where the mineral particles of large size as sand (silica) are more with little amount of organic matter. In areas of heavy rainfall, the

mineral elements and organic chemicals are rapidly lost downwards in this region, making it light-coloured. A2 region is, thus, also called podsolic or eluvial zone or zone of leaching. 3. B-horizon. It lies below A-horizon and also called sub-soil or illuviation or illuvial zone, since, the nutrients received from A-horizon due to leaching are accumulated in this region. B-horizon is dark-coloured and coarse textured due to the presence of silica rich clay, organic compounds, hydrated oxides of aluminium, iron, etc. It is poorly developed in dry areas. B-horizon can be divided into B1 (A3), B2 and B3 regions, depending upon the stages of soil development in the area. 4. C-horizon. Below B-horizon and above the surface of weathered parent rock, is the zone of regolith or C-horizon. It is a light coloured horizon containing weathered parent material. 5. R-horizon. Below all these horizons may lie the R-horizon, which is the parent, unweathered bedrock. The percolated soil water tends to collect at the surface of the bedrock Texture of soil. The texture of a soil is determined by the production of different sized soil particles. Mineral fractions or particles of soil are called “soil fractions” or “soil separates”. The soil particles have been classified into gravel, sand, silt and clay on the basis of their size differences. Gravel consists of coarse particles larger than 2.0 mm. Sand ranges from 0.02 to 2.0 mm in diameter and is easily seen and feels gritty. Silt consists of particles from 0.002 to 0.02 mm in diameter, which scarcely can be seen by the naked eye, feels and looks like flour. Clay particles range below 0.002 mm in diameter, are too fine to be seen even under the ordinary microscope, and are colloidal in nature. Clay controls the most important properties of soils, including plasticity and exchange of ions between soil particles and soil solution. Physical Properties of Soil Soil possesses many characteristic physical properties such as density, porosity, permeability, temperature, water and atmosphere, each of which can be studied under following separate headings: 1. Soil density. Average density of soil is 2.65 gms. per ml. Density of soil varies greatly depending upon the degree of weathering. 2. Porosity. The spaces present between soil particles in a given volume of soil are called pore spaces. The percentage of soil volume occupied by pore space or by the interstitial space is called porosity of the soil. Porosity of soil depends upon the texture and structure compactness and organic contents of soil. Porosity of the soil increases with the increase in the percentage of organic matter in the soil.

Further humus is black coloured. coagulation. 7. Soil solution. chlorides. (iv) Combined water. . texture. Colloids for example exhibit absorption. as at below pH 5. This is because they contain many large pores which remain gas-filled. Soil water. Co. waxes. water content. etc. Complex mixture of minerals such as carbonates. (ii) Capillary water (iii) Hygroscopic water. Soil also contains smaller amounts of compounds of following inorganic elements—B. CO2. Zn. Soil atmosphere. the pH value of soils lies between 2. chemical nature of organic matter and climatic factors and other factors involved in pedogenesis. The chief inorganic constituents of soils are the compounds of following elements—A1. whereas those with suboptimal concentration of these nutrient salts are called oligotrophic. and hail) and irrigation.. 6. Mg. This soil solution contains almost all the essential minerals. therefore. Many chemical properties of soils centre around soil reaction. dialysis. resins. normal growth of roots and biological activity of soil-inhabiting micro-and macro-organisms which require proper and specific temperature. The acidity. .0 bacterial as well as fungal activities are reduced.6.0 indicates neutrality. Tyndal phenomenon. homogeneous complex substance. all of which can be discussed as follows : (a) Inorganic elements and compounds of soil. as the fine textured soils have a greater total pore space and provide a larger gas-filled cross sectional area for diffusion. Poor soil aeration suppresses root hair development. 5. altitude of the land and also by climate and vegetation cover of the soil. odourless. (v) Water vapour.3. (c) Colloidal properties. snow. Soil pH strongly affects the microbial activities. the germination of seeds. Permeability of soil. It comes in soil mainly through infiltration of precipitated water (dew. therefore. Soil aeration is very important in growth of roots. slope. sleet. A pH value of 7. remains best for the growth of majority of plants. the situation is reversed. Loam soils with humus contain a normal proportion of air and water (about 34% air and 66% water) and therefore are good for majority of crops. etc. etc. Soils with optimal concentration of various nutrient solutes are called eutrophic. etc. 4. Soil air differs from atmosphericair in having more of moisture and CO2 and less of O2. and heat formed in the interior of earth. Na. however. Fe. aromatic compounds. microorganisms. and organic salts of Ca. electrical properties. alongwith other liquids and gases in the soil mass. F.. O2 diffuses ten thousand times more slowly in water than in gas. nitrates. The soil atmosphere contains three main gases namely O2. The soil atmosphere is affected by temperature. arrangement and compactness of soil particles and makes soil habitat livable for plants and animals. Ca. (i) Gravitational water. The chemical nature of soil solution depends on the nature of parent matter. The temperature of soil is affected by its colour. Mo. There exists a weak solution of various salts. The soil solution is the primary source of inorganic nutrients for plant roots.00) indicates alkaline condition and a value below (0–6. some are acidic and some basic. rainfall. Course-textured or well-structured soils contain higher gaseous diffusive transfer rates than fine-textured or poorly-structured soils under wet conditions. The chief organic component of soil is humus which chemically contains amino acids. lignin and some pigments. sugar alcohols. tannins. purines. it exhibits all the physico-chemical properties which are related with these two soil particles. Chemical Properties of Soil Soil is a mixture of various inorganic and organic chemical compounds and exhibits certain significant chemical properties. Gases found in pore spaces of soil profiles form the soil atmosphere. The water-filled pores of the fine-textured soils form potent barriers of gaseous diffusion.9) indicates acid conditions. (b) Organic matter of soil. In dry soil. are found as dissolved in water. decomposing organic matter. Mg. it is higher for the loose soil with large number of macro-pore spaces than it is for compact soil with numerous micro-pore spaces. The characteristic of soil that determines the movement of water through pore spaces is known as soil permeability. As regards their nature. sulphates. The soil temperature greatly affects the physico-chemical and biological processes in the soil. Cu. Si. a value above this figure (7. and may reduce rate of absorption of water and nutrients. seed germination and microbial activity. pyrimidines. oils. etc. rain. proteins. Soil permeability is directly dependent on the pore size. and N2. For example. methyl sugars. hexose sugars. wind. I. alkalinity and neutrality of soils are described in terms of hydrogen-ion concentrations or pH values. K and Na. Mn. Neutral or slightly acidic soil. fats. water is not only important as a solvent and transporting agent but in various ways it maintains soil texture. Soil gets heat energy from different sources such as solar radiation. Highly acidic and highly saline or alkaline soils often remain injurious for plant growth. some soils are neutral. In soil. Brownian movement.2 and 9. (d) Soil pH. atmospheric pressure. As soil is composed of crystalloids and colloids. Soil temperature. Normally.1–14. K.