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 and rate of weathering and depth of weathering mantle are important considerations under parent materials.
• There may be differences in soil over similar bedrock and dissimilar bedrocks may have similar soils above them.
• But when soils are very young and have not matured these show strong links with the type of parent rock.
• Also, in case of some limestone areas, where the weathering processes are specific and peculiar, soils will show clear relation with the parent rock.

Topography

• Topography like parent materials is another passive control factor.
• The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and 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 good accumulation of organic matter giving the soil dark colour.
• 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.

Climate

• The climatic elements involved in soil development are :
• Moisture in terms of its intensity, frequency and duration of precipitation – evaporation and humidity;
• Temperature in terms of seasonal and diurnal variations.
• Precipitation gives soil its moisture content which makes the chemical and biological activities possible.
• Excess of water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation).
• In climates like wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium, potassium etc. but also a major part of silica is removed from the soil.
• Removal of silica from the soil is known as desilication.
• In dry climates, because of high temperature, evaporation exceeds precipitation and hence ground water 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 (kanker) are formed.
• Temperature acts in two ways — increasing or reducing chemical and biological activity.
• Chemical activity is increased in higher temperatures, reduced in cooler temperatures (with an 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.

Biological Activity-

• The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages help in adding organic matter, moisture retention, nitrogen etc.
• Dead plants provide humus, the finely divided organic matter of the soil.
• Some organic acids which form during humification aid in decomposing the minerals of the soil parent materials.
• Intensity of bacterial activity shows up differences between soils of cold and warm climates.
• Humus accumulates in cold climates as bacterial growth is slow.
• With un- decomposed organic matter because of low bacterial activity, layers of peat develop in sub-arctic and k6tundra climates.
• In humid tropical and equatorial climates, bacterial growth and action is intense and dead vegetation is rapidly oxidised leaving very low humus content in the soil.
• Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants.
• This process is known as nitrogen fixation.
• Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant.
• The influence of large animals like ants, termites, earthworms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as they rework the soil up and down.
• In case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.

Time

• Time is the third important controlling factor in soil formation.
• The length of time the soil forming processes operate, determines maturation of soils and profile development.
• A 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 and they exhibit no horizons or only poorly developed horizons.
• No specific length of time in absolute terms can be fixed for soils to develop and mature