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Soil originates from Latin word “solum” meaning floor. Thus it’s a natural consolidated material that originates from weathered mineral rock and decomposition organic matter which supports plant and animal life.
Importance of soil to agricultural production
• It is a natural medium within which seeds germinate and roots grows
• It supplies plant with the mineral nutrients necessary for crop growth
• It provide anchorage for higher plants
• It provide water, air, and warmth for small small animals, microorganisms and plant roots to sustain life
• It shelters many small animals and microorganisms within the top soil
Soil is a complex body composed of five major components:
• Mineral matter obtained by the disintegration and decomposition of rocks;
• Organic matter, obtained by the decay of plant residues, animal remains and microbial tissues;
• Water, obtained from the atmosphere and the reactions in soil (chemical, physical and microbial);
• Air or gases, from atmosphere, reactions of roots, microbes and chemicals in the soil
• Organisms, both big (worms, insects) and small (microbes)
Mineral matter
According to its size, soil can be separated into various fractions. Two common systems of classification are given in Table I.
Table I.: Classification of soil particles according to two systems (U.S.D.A and International)
Soil separates
U.S.Dept.of Agric. System
International system diameter (mm)
Number of particles per g
Very coarse
Coarse sand

Medium sand
Fine sand
below 0.002
below 0.002
Formation of mineral matter
1. Primary minerals are formed at high temperature during cooling and crystallization of magma, they are inherited from rock materials and have not been altered chemically and they range above 2mm in diameter e.g. quartz.
2. Secondary minerals, formed at ordinary temperatures and chemically alteration of rock and mineral precipitation of the weathering e.g. clay.
The weathering of parent material takes the form of physical weathering (disintegration), chemical weathering (decomposition) and chemical transformation. Generally, minerals that are formed under the high temperatures and pressures at great depths within the earth’s mantle are less resistant to weathering, while minerals formed at low temperature and pressure environment of the surface are more resistant to weathering. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses generally decrease with depth. Physical disintegration begins as rocks that have solidified deep in the earth are exposed to lower pressure near the surface and swell and become unstable. Chemical decomposition is a function of mineral solubility, the rate of which doubles with each 10 °C rise in temperature, but is strongly dependent on water to effect chemical changes. Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts. Structural changes are the result of hydration, oxidation, and reduction.
• Reduction the opposite of oxidation, means the removal of oxygen, hence oxidation number of some part of the mineral is reduced, which occurs when oxygen is scarce. The reduction of minerals leaves them electrically unstable, more soluble and internally stressed and easily decomposed.
1. Rain
The mechanical action of rain consists mainly in the washing of loose particles of soil and rock to lower levels. This phenomenon is known as rain-wash. It is the means by which rivers receive much of the sediments they carry in suspension. The chemical weathering effects of the rain are seen its solvent action on some rocks notably limestone. The process depends on the pressure of feeble acids, derived from gases such CO2 and SO2 which are present in air in small quantities and which enter into solution in rain water.
The denuding effects of heavy showers and rain-storms may be very severing, especially in regions where a covering of vegetation is lacking. It cuts gullies in the surface of the ground, some of considerable size and may cause great damage by the destruction of roads and livestock.
Heavy rains also promote landslides. Vegetation protects the ground from the immediate disintegrating effects of rainfall.
2. Frost
In cold climates the action of the frost is to break off angular fragment from exposed rock surface, a process sometimes referred to as ice-wedge. Water enters rock along pores, cracks and fissures. On freezing it expands and occupies about 10% greater volume exerting a pressure of about 2000 lbs per square inch. This is therefore like a miniature blasting and brings about the disintegration of the rock. The loosened particles fall from the mass and accumulate as heaps of talus at lower levels and this material may later be consolidate into a deposit known as breccia.
3. Wind
It is one of the two natural agents which transport rock material against gravity. Its effect is three-fold. First it removes loose particles of rock decay as it blows over a surface, then charged with these grains the wind act as an abrading sand-blast driving the grains against rock surfaces which becomes worn and polished in course of time. Thirdly the blown grains are accumulated to from sand-dunes.
s of communication may be seriously affected by wind-blown sand in arid countries. It is on record that the telegraphic wire on the trans-Caspian railway was worm down to half of its diameter in eleven years, and renewal was then made. To avoid accumulation of sand alongside railway embankments in Sudan, culverts have been made to allow for easy passage of the wind and its load sediments.
4. Insulation
When a rock surface is exposed to a considerable daily range of temperature, as in arid and semi- arid regions, the expansion which occurs during the day and contraction at night, constantly repeated have a weakening effect on the texture of the rock over a period of time . The outer heated layers tend to pull away from the cooler rock underneath a process known as exfoliation. By the unequal expansion and contraction of its mineral constituents the strain is set up in a rock and its texture is loosened. This kind of weathering is prevalent in climates where high day and low night temperatures are prevalent.
5. Weathering by organic materials
Plants retain moisture and any rock surface on which they grow is kept damp, thus aiding the solvent action of the water. The chemical decay of the rock is also promoted by the formation of vegetable humus organic product of the decay of plants. The mechanical break up of rocks is helped by the roots of plants which penetrates into cracks and crevices and tend to wedge apart the rock.
The principal climatic variables influencing soil formation are effective precipitation (i.e., precipitation minus evapo-transpiration) and temperature, both of which affect the rates of chemical, physical, and biological processes. The temperature and moisture both influence the organic matter content of soil through their effects on the balance between plant growth and microbial decomposition. Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the climate zones in which they form. For every 10 °C rise in temperature, the rates of biochemical reactions more than double. Mineral precipitation and temperature are the primary climatic influences on soil formation. If warm temperatures and abundant water are present in the profile at the same time, the processes of weathering, leaching, and plant growth will be maximized. Humid climates favor the growth of trees. In contrast, grasses are the dominant native vegetation in sub humid and semiarid regions, while shrubs and brush of various kinds dominate in arid areas.
Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the regolith. The seasonal rainfall distribution, evaporation losses, site topography, and soil permeability interact to determine how effectively precipitation can influence soil formation. The greater the depth of water penetration, the greater the depth of weathering of the soil and its development. Surplus water percolating through the soil profile transports soluble and suspended materials from the upper to the lower layers. It may also carry away soluble materials in the surface drainage waters. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons. Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions. Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant growth. Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays.
The direct influences of climate include:
o A shallow accumulation of lime in low rainfall areas as caliche
o Formation of acid soils in humid areas
o Erosion of soils on steep hillsides
o Deposition of eroded materials downstream
o Very intense chemical weathering, leaching, and erosion in warm and humid regions where soil does not freeze
Climate directly affects the rate of weathering and leaching. Wind moves sand and smaller particles, especially in arid regions where there is little plant cover. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed. The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations. Cycles of freezing and thawing constitute an effective mechanism which breaks up rocks and other consolidated materials.
Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates of chemical reactions in the soil.
The topography, or relief, is characterized by the inclination (slope), elevation, and orientation of the terrain. Topography determines the rate of precipitation or runoff and rate of formation or erosion of the surface soil profile. The topographical setting may either hasten or retard the work of climatic forces.
Steep slopes encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles. In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation. For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby,
more level sites.
In swales and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered and soil profile development is more advanced. However, in the lowest landscape positions, water may saturate the regolith to such a degree that drainage and aeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated. In such low-lying topography, special profile features characteristic of wetland soils may develop. Depressions allow the accumulation of water, minerals and organic matter and in the extreme; the resulting soils will be saline marshes or peat bogs. Intermediate topography affords the best conditions for the formation of an agriculturally productive soil.
Soil is the most abundant ecosystem on Earth, but the vast majority of organisms in soil are microbes, a great many of which have not been described. There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely. Estimates range from 50,000 per gram to over a million species per gram of soil. The total number of organisms and species can vary widely according to soil type, location, and depth.
Plants, animals, fungi, bacteria and humans affect soil formation. Animals, soil mesofauna and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about. In the same way, plant roots open channels in soils. Plants with deep taproots can penetrate many meters through the different soil layers to bring up nutrients from deeper in the profile. Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding organic matter. Micro-organisms, including fungi and bacteria, affect chemical exchanges between roots and soil and act as a reserve of nutrients.
Humans impact soil formation by removing vegetation cover with erosion as the result. Their tillage also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed up
per layers.
Earthworms, ants and termites mix the soil as they burrow, significantly affecting soil formation. Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies. They aerate and stir the soil and increase the stability of soil aggregates, thereby assuring ready infiltration of water. As they build mounds, some organisms might transport soil materials from one horizon to another.
In general, the mixing activities of animals, sometimes called perturbation, tend to undo or counteract the tendency of other soil-forming processes that create distinct horizons. Termites and
ants may also retard soil profile development by denuding large areas of soil around their nests, leading to increased loss of soil by erosion. Large animals such as gophers, moles, and prairie dogs bore into the lower soil horizons, bringing materials to the surface. Their tunnels are often open to the surface, encouraging the movement of water and air into the subsurface layers. In localized areas, they enhance mixing of the lower and upper horizons by creating, and later refilling, underground tunnels. Old animal burrows in the lower horizons often become filled with soil material from the overlying A horizon, creating profile features known as crotovinas.
Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result from surface runoff. Plants shade soils, keeping them cooler and slow evaporation of soil moisture, or conversely, by way of transpiration, plants can cause soils to lose moisture. Plants can form new chemicals that can break down minerals and improve the soil structure. The type and amount of vegetation depends on climate, topography, soil characteristics, and biological factors. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.
Human activities widely influence soil formation. For example, it is believed that Native Americans regularly set fires to maintain several large areas of prairie grasslands in Indiana and Michigan. In more recent times, human destruction of natural vegetation and subsequent tillage of the soil for crop production has abruptly modified soil formation. Likewise, irrigating an arid region of soil drastically influences the soil-forming factors, as does adding fertilizer and lime to soils of low fertility.
Soil water
Further information: Water content and Water potential
Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons:
It constitutes 80%-95% of the plant’s protoplasm. It is essential for photosynthesis.
It is the solvent in which nutrients are carried to, into and throughout the plant. It provides the turgidity by which the plant keeps itself in proper position.
In addition, water alters the soil profile by dissolving and re-depositing minerals, often at lower levels, and possibly leaving the soil sterile in the case of extreme rainfall and drainage. In a loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume of which only half will be available to most plants.
A flooded field will drain the gravitational water under the influence of gravity until water’s adhesive and cohesive forces resist further drainage at which point it is said to have reached field capacity. At that point, plants must apply suction to draw water from a soil. When soil becomes too dry, the available water is used up and the remaining moisture is unavailable water as the plant cannot produce sufficient suction to draw in the water. A plant must produce suction that increases from zero for a flooded field to 1/3 bar at field dry condition (one bar is a little less than one atmosphere pressure). At 15 bar suction, wilting percent, seeds will not germinate, plants begin to wilt and then die. Water moves in soil under the influence of gravity, osmosis and capillarity. When water enters the soil, it displaces air from some of the pores, since air content of a soil is inversely related to its water content.
The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores. The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away. Consequently, not all soil water is available to plants. When saturated, the soil may lose nutrients as the water drains. Water moves in a drained field under the influence of pressure where the soil is locally saturated and by capillarity pull to dryer parts of the soil. Most plant water needs are supplied from the suction caused by evaporation from plant leaves and 10% is supplied by “suction” created by osmotic pressure differences between the plant interior and the soil water. Plant roots must seek out water. Insufficient water will damage the yield of a crop. Most of the available water is used in transpiration to pull nutrients into the plant.
Water retention forces; Water is retained in a soil when the adhesive force of attraction that water’s hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water’s hydrogen feels for other water oxygen atoms. When a field is flooded, the soil pore space is completely filled by water. The field will drain under the force of gravity until it reaches what is called field capacity, at which point the smallest pores are filled with water and the largest with water and gases. The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles. As a result, high clay and high organic soils have higher field capacities. The total force required to pull or push water out of soil is termed suct
ion and usually expressed in units of bars (105 Pascal, about one atmosphere) which is just a little less than one-atmosphere pressure. Alternatively, the terms “tension” or “moisture potential” may be used.

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