Soil can be defined as the solid material on the Earth’s surface that results from the interaction of weathering and biological activity on the parent material or underlying hard rock.
The salient characteristic features of soil are:
• Textures range from clay, sand, and silt at the extremes, to a loam which has all three sized fractions present. The main influence of texture is on permeability which generally decreases with decreasing particle size.
• A certain amount of air is contained between the individual particles except for the waterlogged soils. The air in the soil helps in the process of oxidation which converts part of the organic material into nitrogen in a form readily available to the plants.
• In damp climates, especially in high latitudes where the evaporation rate is low, water tends to move predominantly downward, particularly in coarse-grained sandy soils. This dissolves the soluble minerals in the soil, together with soluble humus material and carries both downward, a process called leaching or eluviations.
• In a hot, arid climate, evaporation exceeds precipitation for greater part of the year, so the water tends to move upward and the soil dries out.
• Generally soil colour is determined by the amount of organic matter and the state of the iron.
• Soil colour is also related to soil drainage, with free draining, well AERATED soils (with pore space dominated by oxygen) having rich brown colours.
• In contrast, poorly draining soils, often referred to as gleys, develop under ANAEROBIC conditions (the pore space dominated by water) and have grey or blue-grey colours.
• The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration of H ions (acidity) or OH ions present in it. Acidity is due to the excess of H ions over OH ions, and alkalinity is due to the excess of OH ions over H ions. A neutral reaction is produced by an equal activity of H and OH ions.
a) Acidic: It is common in region where precipitation is high. The high precipitation leaches appreciable amounts of exchangeable bases from the surface layers of the soils so that the exchange complex is dominated by H ions. Acid soils, therefore, occur widely in humid regions and affect the growth of plants markedly.
b) Alkaline: Alkali soils occur when there is comparatively high degree of base saturation. Salts like carbonates of calcium, magnesium and sodium also give a preponderance of OH ions over H ions in the soil solution. When salts of strong base such as sodium carbonate go into soil solution and hydrolyze, consequently they give rise to alkalinity.
FACTORS RESPONSIBLE FOR SOIL FORMATION
Soil formation is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material.
a) Parent material
• This is the material from which the soil has developed.
• The parent material can influence the soil in a number of ways: colour; texture; structure; mineral composition and permeability/drainage.
• The absorption of the solar radiation at the soil surface is affected by many variables such as soil color, vegetation cover, and aspect. In general, the darker the soil color, the more radiation is absorbed and the lower the albedo. The absorption differs in areas with deciduous trees (soil surface is shaded by trees most of the year) and arable land (soil surface is not shaded throughout the year).
• Biological processes are intensified by rising temperatures. Reaction rates are roughly doubled for each 10°C rise in temperature, although enzyme-catalyzed reactions are sensitive to high temperatures and usually attain a maximum between 30 and 35°C.
c) Biological Factors
• The soil and the organisms living on and in it comprise an ecosystem. The active components of the soil ecosystem are the vegetation, fauna, including microorganisms, and man.
• Earthworms are the most important of the soil forming fauna in temperate regions, being supported to a variable extent by the small arthropods and the larger burrowing animals (rabbits, moles).
• Time is a factor in the interactions of all the above factors as they develop soil.
• Relief is not static; it is a dynamic system (its study is called geomorphology). Relief influences soil formation in several ways:
I. It influences soil profile thickness i.e. as angle of slope increases so does the erosion hazard.
II. It has an effect on climate which is also a soil forming factor.
III. Gradient affects run-off, percolation and mass movement.
IV. It influences aspect which creates microclimatic conditions.
SOIL FORMING PROCESSES
The four major processes that change parent material into soil are additions, losses, translocations, and transformations.
• Leaching – leaching is the removal of soluable components of the soil column. As water washes down through the soil it can carry away bases such as calcium, held as exchangeable ions in clay-humus complexes, as well as acidification through the substitution of hydrogen ions.
• Eluviation – here soil particles held in suspension, such as clay, are removed (eg. washed away).
• Illuviation – here soil particles held in suspension, such as clay, are accumulated (eg. deposited).
• Podsolisation – podsolisation occurs when strongly acid soil solutions cause the breakdown of clay minerals. As a result silica, aluminium and iron form complexes with organic substances in the soil. These minerals are removed from the surface zone of the soil and can accumulate in distinct dark sub-surface layers – very evident on inspection. Upland heaths and moors often contain podsols.
• Gleying – gleying occurs in waterlogged, anaerobic conditions when iron compounds are reduced and either removed from the soil, or segregated out as mottles or concretions in the soil. Marshy wetlands often contain gleyed soils.
Soil is a natural body of mineral and organic constituents differentiated into horizons usually unconsolidated, of variable depth which differs among themselves as well as from the underlying parent material in morphology, physical makeup, chemical properties and composition and biological characteristics.
Formation of Soil Profile
• Soil formation begins with the breakdown of rock into regolith. Continued weathering and soil horizon development process leads to the development of a soil profile, the vertical display of soil horizons.
• The top layer of the profile is the O horizon which is composed of organic matter. Decomposition of organic matter enriches the soil with nutrients (nitrogen, potassium, etc.), aids soil structure (acts to bind particles), and enhances soil moisture retention.
• Next layer is the A horizon in which organic material mixes with inorganic products of weathering. The A horizon typically is dark coloured horizon due to the presence organic matter.
• Eluviations, the removal of inorganic and organic substances from a horizon by leaching occurs in the A horizon due to the downward movement of soil water.
• The E horizon generally is a light-colored horizon with eluviation being the dominant process. Leaching or the removal of clay particles, organic matter, and/or oxides of iron and aluminum is also active in E horizon.
• The E horizon has a high concentration of quartz under coniferous forests, giving the horizon an ash-gray appearance.
• The B horizon is a zone of illuviation where downward moving, especially fine material, is accumulated. The accumulation of fine material leads to the creation of a dense layer in the soil.
• Eluviation is significant in humid climates where ample precipitation leads to the downward movement of minerals in the soil.
• Illuviation are found closer to the surface in semiarid and arid climates where precipitation is scarce because due to capillary action cations like calcium and sodium dissolved in soil moves upward.
• The C horizon represents the soil parent material, either created in situ or transported into its present location. Beneath the C horizon lays bedrock.
Soils are divided into: (i) zonal, (ii) intrazonal, (iii) azonal categories.
– A soil whose characteristics are dominated by the influence of climate and vegetation is known as a zonal soil. These soils occur on gently undulating land where drainage is free and where the parent material is of neither extreme texture nor chemical composition. They occur in latitudinal zones.
– Zonal soils are further divided into: (i) pedalfers, and (ii) pedocals.
(ii) Intrazonal Soils
– A soil which has been influenced in its development less by climate and vegetation than by other local factors, such as defective drainage, excessive evaporation or an unusual parent material (such as lime stone), terrain or age.
– They can be sub-divided into : (i) calcimorphic soils, (ii) hallomorphic soils, and (iii) hydromorphic soils (bog-peat, fen-pear or meadow soil.)
(iii) Azonal Soils
– A soil which has not been sufficiently subjected to soil -forming processes for the development of a mature profile and so is little changed from the parent rock material.
– Azonal soils do not have B horizon because it is too immature. Thus, the A horizon lies immediately above the C horizon.
– Examples are soil forming on scress, recently deposited alluvium, sand dunes, and newly deposited glacial draft, wind-blown sand, marine mud flats and volcanic soils.
– Azonal soils are further divided into: (i) lithosol, (ii) regosol and (iii) entisol (alluvial).