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Soil is a natural body consisting of layers (soil horizons) of primarily mineral constituents of variable thicknesses, which differ from the parent materials in their texture, structure, consistence, color, chemical, biological and other physical characteristics
Soil is a natural body of animal, mineral and organic constituents differentiated into horizons of variable depth which differ from the material below in morphology, physical makeup, chemical properties and composition, and biological characteristics (J. Joffe, 1949)
The study of soil, its origins and characteristics is referred to as Pedology.
Factors that affect Soil Formation Parent material The material from which soil forms is called parent material. It includes: Weathered primary bedrock; Secondary material transported from other locations, namely colluvium and alluvium; Deposits that are already present but mixed or altered in other ways—old soil formations, organic material; and anthropogenic materials, such as landfill or mine waste.
Factors that affect Soil Formation Climate • Soil formation greatly depends on the climate, and soils show the distinctive characteristics of the climate zones in which they form. • 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. • Temperature and moisture affect the rate of weathering and leaching. Leaching is the downward transport of mineral salts via water movement through the soil. • Wind moves sand and smaller particles, especially in arid regions where there is little plant cover. • Water is an important Soil Formation Factor
Factors that affect Soil Formation Topography • The topography or relief characterized by the inclination of the surface determines the rate of precipitation runoff and rate of formation and erosion of the surface soil profiles. Seep slopes allow rapid runoff and erosion of the top soil profiles and little mineral deposition in lower profiles.
Factors that affect Soil Formation Organisms • Plants, animals, fungi, bacteria and humans affect soil formation (see soil bio mantle and stonelayer). • Animals 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.
Factors that affect Soil Formation Time • Time is a factor in the interactions of all the above. Over time, soils evolve features dependent on the other forming factors. • Soil formation is a time-responsive process that is dependent on how the other factors interplay with each other. • Soil is always changing. It takes about 800 to 1000 years for a 2.5 cm thick layer of fertile soil to be formed in nature. • For example, recently deposited material from a flood exhibits no soil development because there has not been enough time for soil-forming activities. The original soil surface is buried, and the formation process must begin anew for this deposit.
Factors that affect Soil Formation Weathering Weathering is the degradation and disintegration of rock in situ. Weathering is an important factor in forming soil, as it involves the changing in the form and nature of soil. Chemical and physical weathering contribute to the change from igneous rocks to metamorphic and sedimentary rocks.
Soil Characteristics Soil Profile • The soil profile is a vertical section through the soil showing its different horizons. • It is a product of the balance between soil system inputs and outputs, and the redistribution of, and chemical changes in, the various soil constituents. • A soil horizon is a layer parallel to the soil surface, whose physical characteristics differ from the layers above and beneath. Each soil type has at least one, usually three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly colour and texture.
Soil Characteristics O) Organic matter: Litter layer of plant residues in relatively undecomposed form. A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored "E" subsurface soil horizon is apparent at the base of the "A" horizon. A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A-horizon is regarded as a "biomantle".
Soil Characteristics • B) Subsoil: This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation. The materials removed from the A horizon is deposited in this layer. • C) Parent rock: This layer consists of recently weathered parent material (regolith) resting on bedrock.
Soil Texture • The mineral components of soil consist of a mixture of sand, silt and clay. In the illustrated textural classification triangle the only soil that does not exhibit one of those predominately is called "loam." • While even pure sand, silt or clay may be considered a soil, from the perspective of food production a loam soil with a small amount of organic material is considered ideal. • The mineral constituents of a loam soil might be 40% sand, 40% silt and the balance 20% clay by weight. Soil texture affects soil behaviour, including the retention capacity for nutrients and water
Soil Texture • Coarse sand : Between 2.0 and 0.6mm • Medium sand: Between 0.6 and 0.2mm • Fine sand: Between 0.2 and 0.06mm • Silt: Between 0.06 and 0.002mm • Clay: Less than 0.002mm
Soil Structure • Soil structure is the aggregation of the sand, silt, and clay soil components into larger units. • It is caused by the adhesion of those particles into larger aggregates (peds) by organic substances, iron oxides, carbonates, clays and silica and by the breakage of the aggregates due to expansion and contraction from freezing and thawing, wetting and drying cycles of the soil. • These peds evolve into units that may have various shapes, sizes and degrees of development
Organic Matter Content • Organic matter, which includes humus, is derived mainly from decaying plants and animals or from secretions of living organisms. Fallen leaves and decaying grasses and roots are the main source of organic matter. • Three distinct layers of organic matter: • 1. L or leaf litter layer • 2. F or fermentation layer • 3. H or Humus layer • Organic matter interacts with the A horizon through the work of organisms such as earthworm. Humus gives the soil a black or dark brown colour.
Soil Moisture • Soil moisture is important because it affects the upward and downward movement of water and nutrients. • It helps in the development of horizons • It supplies water for living plants and organisms • It provides solvent for plant nutrients • It influences soil temperature • It determines the incidence of erosion
Soil Air Spaces • Pore space is that part of the bulk volume not occupied by either mineral or organic matter but is open space occupied by either air or water. The air space is needed to supply oxygen to organisms decomposing organic matter, humus and plant roots. Pore space allows the movement and storage of water and dissolved nutrients. • When pore space is less than 30 microns the forces of attraction that hold water in place are greater than those acting to drain the water. At that point soil becomes water logged and it cannot breathe. For a growing plant, pore size is of greater importance than total pore space. A medium textured loam provides the ideal balance of pore sizes. Having large pore spaces that allow rapid air and water movement is superior to total percentage pore space.
Temperate vs. Tropical • The richest soil with large amounts of nutrients can be found in the temperate climate zone • In the tropical climate zone, the soil is very thick, comprised mainly of the A-horizon from which minerals are leached out. The B-horizon is missing. The humus layer is extremely thin. The reason is that in the warm humid environment, organic material decays so fast that humus cannot accumulate. The rainforest adapted by taking up nutrients through the roots faster than plants in temperate climates.
