Pedologists define soil as a collection of natural bodies on the earth’s surface containing living matter supporting or capable of supporting plants.

Soil is a dynamic medium in which many chemical, physical and biological activities go on constantly. Soil is a result of decay, it is also the medium for growth. It is a changing and developing body. It has many characteristics that fluctuate with the seasons. It may be alternatively cold and warm or dry and moist. Biological activity is slowed or stopped if the soil becomes too cold or too dry.

Factors leading to the formation of Soil:

Two types of factors lead to the development of soils.

1. Passive Factors – Parent material, topography and time are factors that constantly impact the soil.
2. Active factors – Climate and biological activity constantly vary with time.

Five basic factors control the formation of soils:

1.    Parent Material:

Parent material is a passive control factor in soil formation. Parent materials can be any in-situ or on-site weathered rock debris (residual soils) or transported deposits (transported soils).

• Soil formation depends upon the texture (sizes of debris) and structure (disposition of individual grains/particles of debris) as well as the mineral and chemical composition of the rock debris/deposits.
  • Nature rate and depth of the weathering mantle are important considerations under parent materials.
  • When soils are very young and have not matured these show strong links with the type of parent rock. However, there may be differences in soil and the bedrock below it due to prolonged erosion or the level of maturity.
  • Also, in the case of some limestone areas, where the weathering processes are specific and peculiar, soils will show a clear relation with the parent rock.

2.    Topography:

Topography Influences the amount of exposure suffered by the parent materials to the sunlight and the amount of surface and sub-surface drainage through the parent materials.

• Soils will be thin on steep slopes and thick over flat upland areas.  Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favourable. Soils over flat areas may develop a thick layer of clay with a good accumulation of organic matter giving the soil a dark colour.
• Hilly Area: In middle latitudes, the south-facing slopes exposed to sunlight have different conditions of vegetation and soils and the north-facing slopes with cool, moist conditions have some other soils and vegetation.

3.    Time:

The third most important factor. Determines maturation of soil and profile development. Soil becomes mature when all soil-forming processes act for a sufficiently long time developing a profile.  Soils developing from recently deposited alluvium or glacial till are considered young – exhibit no horizons or only poorly developed horizons.

4.    Climate:

There are a number of elements involved in soil development, that vary with the local climatic conditions, such as:

1. Moisture in terms of its intensity, frequency and duration of precipitation, evaporation and humidity, varies according to region.
2. Temperature in terms of seasonal and diurnal variations. It acts in two ways — increasing or reducing chemical and biological activity.

Chemical activity is increased in higher temperatures, reduced in cooler temperatures (with the exception of carbonation) and stops in freezing conditions. That is why, tropical soils with higher temperatures show deeper profiles and in the frozen tundra regions soils contain largely mechanically broken materials.

• Precipitation gives moisture content which makes the chemical and biological activities possible. Excess water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation). Wet equatorial rainy areas with high rainfall, not only Ca, Na, Mg, k etc. Silica is also removed from the soil. Removal of silica from the soil is called Desilication.
• Dry climates: In dry areas (where evaporation exceeds precipitation), groundwater is brought up to the surface by capillary action and in the process the water evaporates leaving behind salts in the soil. Such salts form into a crust in the soil known as hardpans. In tropical climates and in areas with intermediate precipitation conditions Calcium carbonate nodules – Kankers formed.

5.    Biological Activity:

Vegetative cover, organisms and parent materials help in adding organic matter, moisture retention, nitrogen etc.

1. Vegetation: Dead plants provide humus, the finely divided organic matter of the soil. Some organic acids which are formed during humification, aid in decomposing the minerals of the soil’s parent materials.
   • Humus accumulates in cold climates as bacterial growth is slow. Thus, layers of peat develop in sub-arctic and tundra climates, with un-decomposed organic matter.
   1. In humid tropical and equatorial climates, bacterial growth and action are intense and dead vegetation is rapidly oxidised leaving very low humus content in the soil.
2. Bacteria and other soil organisms: For Example:
   • The influence of large animals like ants, termites, earthworms, rodents etc., is mechanical. This is important in soil formation as they rework the soil up and down. In the case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.
   1. Rhizobium Bacteria live in the root nodules of leguminous plants and take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants, in a process called nitrogen fixation. Their action leaves acidic residue in the soil.

Process of Soil Formation:

Soil formation or pedogenesis depends first on weathering. It is this weathering mantle (depth of the weathered material) that is the basic input for soil to form.

• First, the weathered material or transported deposits are colonised by bacteria and other inferior plant bodies like mosses and lichens. Also, several minor organisms may take shelter within the mantle and deposits.
• The dead remains of organisms and plants help in humus accumulation. Minor grasses and ferns may grow; later, bushes and trees will start growing through seeds brought in by birds and wind.
• Plant roots penetrate down, burrowing animals bring up particles, a mass of material becomes porous and sponge-like with a capacity to retain water and to permit the passage of airand finally a mature soil, a complex mixture of mineral and organic products forms.