The word soil is derived from Latin solum meaning soil or land,
Soil is the upper and biochemically weathered portion of the regolith.
Soil is a collection of natural bodies of Earth that is composed of mineral organic matter and is capable of supporting plant growth.
Soil is the weathered outer layer of Earth’s crust, which ranges front a thin film to thick layers composed of weathered rock materials and organic matter interspersed with pores filled with air and water, and which support plant life.
The soil is the product of both destructive and synthetic forces. The former includes weathering and decomposition of organic matter by the microbes and the later Involves formation of new minerals, organic compounds and development of characteristics layer patterns called horizons.
Regolith and Parent Material
The unconsolidated geological material present below the soil and above the rocks is called regolith. It may be in form of a thin layer or several meters in thickness, and maybe weathered from the underlying rock or transported elsewhere by the action of wind, water or ice.
The upper part of the regolith is called parent materials It is the basic inorganic framework of the soil into which organic matter. living organisms, water and gases are incororporated. The upper 1 -2 meters of regolith is rich in organic matter because plant concentrate there and is subject to more weathering. The products of this weathering give rise to characteristics layering called horizons.
Pedology and Edaphology
Pedotogy the study of soil as a natural body. It includes the origin of soil, classification and its description. One who studies, examines and classifies soils as they occur their natural environment is known as a pedoloist.
Soil is natural habitat for plants, therefore the study of soil from standpoint of higher plants is Edaphology. one who considers the various properties of soils in relation to plant production is an edaphologist.
A three-dimensional soil body large enough that we can study all Its physical and properties and its horizons is called pedon. Thus, it ranges in area from one to 100 square meters.
Land and Earth
Land is the physical environment consisting of soil, water, climate and vegetation.
Earth is a general name used by engineers for unconsolidated land masses that can be dug, moved or formed by equipment. The scientific study of earth’s crust, rock strata and history of Its development is called geology.
Topsoil & Subsoil
The upper 12-18 cm surface layer of the soil is topsoil. It can be ploughed and cultivated. It is also called furrow slice. Topsoil is the major zone of root development for crop plants. It contains many nutrients available to plants and supply much of the water needed for their growth.
The soil layers underneath the topsoil are subsoil. It is not affected by tillage; however, it affects crop production. The roots of the plants penetrate the subsoil and it provides moisture and nutrients to the plants.
Major Phases of Soil
The soil consists of three major phases:
Solid Phase: The solid phase is major reservoir of plant nutrients. It is composed primary and secondary minerals, amorphous substances and fragments of parent rocks; and organic components including soil fauna (animals) and flora (plants), plant residues and humus.
Liquid Phase: The liquid phase of the soil consists of water held among spaces and minerals dissolved in it. It is also called water solution. The liquid phase is responsible for nutrient transport in the soil and is immediate source of most nutrients and water absorbed by the plant roots.
Gaseous Phase: Soil gases occupying the soil pores make gaseous phase of the soil. The gases phase provides oxygen to plant roots and soil micro-organisms for respiration. The carbon dioxide concentration in soil atmosphere is higher than in air due to root respiration and decomposition of soil organic matter
Any vertical cut through a body of soil or pedon is the soil profile.
The individual layers are called horizons. The horizons develop largely through the action of rainwater which leaches materials from surface and deposits most or all of them at a slightly greater depth. Each horizon has a characteristic set of features. particularly color, that distinguishes it from other horizons. Each horizon has its own thickness, texture, structure, consistency, porosity, chemistry and composition. The horizons differ in their Characteristics from Soil to soil; therefore, the soils can be differentiated on the basis of characteristics of their horizons.
In general soil has five major horizons. These are designated using the capital O, A, E, B and C. 0 is an organic layer and A, E, B and C are the mineral layers. Below the four may the R (regolith) or non-soil horizon. Subordinate layers are found in horizons These are designated by lowercase letters, for example Oi, Oe and Oa. Horizons A constitute true soil or solum.
O Horizon (Organic): The O horizon is the surface layer composed of fresh or partially decomposed organic matter that has not been mixed into mineral soil. It usually found in forest regions and absent in grasslands and cultivated soils. The O horizon is further subdivided into:
Oi, the litter layer composed of plant and animal residues. This layer fluctuates with seasons. In temperate regions, it is thickest in fall, when new litter is added, and thinnest in the summer after the decomposition has taken place,
Oe, the humus layer in which organic matter is partially decayed,
Oa, the humus layer in which organic matter is highly decomposed.
A horizon: It is the upper layer of mineral soil with a high content of organic matter. it is darker in color and consists of humus derived from O horizon.
E Horizon (Eluviations): E horizon is the zone of maximum leaching or eluviations. Suspended and dissolved materials such as clay, iron, aluminum oxides, etc., move out of it through weathering and leaching. It is lighter in color, composed of resistant minerals as quartz and silt particles, and poor in organic matter.
B Horizon (Illuviation): It is the zone of illuviation. The leached materials from horizons above accumulate in this layer. It accumulates silicates, clay, iron, aluminum and humus from E horizon.
C Horizon: It consists of unconsolidated material either like or unlike the material from, which the soil has developed. This layer is without biological activity and weathering process
Soil Genesis or Soil Formation
Soil formation is the creation of soil from a non-soil parent material. The process involves breakdown of parent material into smaller particles (weathering), rearranging and changing their mineral structure, adding organic matter (humification), producing clays, and creating horizons.