Briefly discuss various factors that control the formation of soil.

Soil is a relatively thin layer of unconsolidated mineral and organic material on the immediate
surface of the earth. Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that
together support life. Earth's body of soil, called the Pedosphere. Fertile soil contains approximately
25% of both air and water, about 5% organic matter and about 45% mineral matter.
Soil formation process (paedogenesis)
In general, soil formation starts with rocks that are pushed to the surface of the earth by geological
or climactic forces. These rocks then undergo weathering - the chemical alteration and physical
breakdown of rock during exposure to the atmosphere, hydrosphere, and biosphere. Through the
weathering process, eventually enough essential elements become available to support lichens and
other lower forms of plant life. As continuing generations of lichens grow, die, and decay, they leave
increasing amounts of organic matter. Naturally-occurring organic acids further hasten decay of the
rock. An increasing build-up of organic matter and formation of fine rock fragments result in more
water retention in the soil and more water available for use by larger numbers of plants and animals.
Five soil forming factors
Active factors:
Which represent agents that supply energy that act upon the mass for process of soil formation?
They are climate and biosphere.
Passive factors:
Which represent the source of the soil-forming mass and conditions affecting it. They are parent
material, relief, and time.
1. Climate:
Climate influences soil formation largely through precipitation and temperature.
(a) precipitation or rainfall:
Water percolates and moves from one part of parent material to another it carries with it substance
in solution as well as in suspension. Rainfall also affects profile development through erosion,
producing these soils on slopes and deposition of soil material downhill. In general, with increasing
moisture, nitrogen and carbon content, clay content, aggregation, saturation capacity and
exchangeable hydrogen tend to increase. On the other hand, exchangeable bases and ph values
show a decrease with increasing moisture. The depth of the calcium carbonate horizon in pedocal
(lime accumulating) soils increases with increasing moisture.
(b) temperature:
It influences the organic matter decomposition and microbiological activities in the soil. In general,
with increasing temperature, the depth of weathering and clay content show an increase. Under
tropical and sub-tropical conditions, with high rainfall highly leached soils have developed such as
laterite soils. Tropical – soils, especially those developed from igneous and metamorphic rocks, have bright yellow or dark red colours because of intense hydration and oxidation respectively, during
their weathering.
2. Organism
(a) flora:
It exerts main influence on soil formation through the amount and nature of organic matter it adds
to the soil. Vegetation also aids in the control of erosion. They facilitate percolation and drainage
and bring about a greater dissolution of minerals through the action of carbon dioxide and other
acidic substances.
(b)fauna:
Burrowing animals, rodents, earthworms, ants etc., are highly important in soil formation. Burrowing
animals cause constant mixing within the soil profile. The role of microorganisms as soil formers is
intimately related to humification and mineralizations.
3. Relief (topography)
Relief influences soil formation primarily as a factor affecting erosion and as a modifier of climate
and water-air relationship in the soil. Topography largely determines the drainage condition and the
ground water level in the soil. With the same kind of climate and parent material, soils that have
developed on steep hill sides are thinner. This is because the surface erodes quite rapidly and less
water moves downward within the profile. Soil materials on gently sloping topography have more
water passing through them and the profile is generally deeper, the vegetation more luxuriant and
the organic matter level higher, than in soils on steep topography
4. Parent material:
Parent material refers to the primary material from which the soil is formed. The type of soil that
forms depends on the type of rocks available, the minerals in rocks, and how minerals react to
temperature, pressure, and erosive forces. Soil parent material could be bedrock, organic material,
an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a
slope.
5. Time:
The length of time required for a soil to develop horizons depends upon many interrelated factors,
such as climate, nature of the parent material, burrowing animals and relief. Certain soils are termed
mature or immature which give some idea of the time factor.