Soils are products of a complex mixture
of weathered and eroded rock materials
on the one hand and organic residues
on the other. Weathering changes consolidated
mass (rock) to an unconsolidated (loose)
material.
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The decomposition of plants and
microorganisms releases organic chemicals
(humus) that interact with the loose material,
giving rise to various kinds of soils. These
changes involve addition, loss, transformation
and translocation. Additions are made by
water (rainfall, irrigation), nitrogen from
bacterial fixation, energy as sunlight, sediment
from wind and water, salts and organic
residues. Losses result from chemicals soluble
in soil water, eroded small-sized fractions,
nutrients removed in grazed and harvested
plants, water losses, carbon losses as carbon
dioxide and denitrification loss of N2.
Transformations happen because of many
chemical and biological reactions that
decompose organic matter. Water and
organisms move within the soil causing
translocation of substances at different
depths. Organic soils develop from an
accumulation of plant residues that are
preserved by the low oxygen environment of
shallow and stagnant waters. Non-soils are
surface material that do not support plant life,
such as the salt flats of Great Salt Lake and
rock barrens.
SOIL FORMING FACTORS
All soil forming processes involve weathering,
There are, however, several other factors that
influence the end product of weathering. Five
of them are primary factors. They singly or
jointly are responsible for the development of
various types of soils. These factors are:
Parent Material
Soils from weakly cemented sandstone will be
sandy and soils from shales will be shallow
and fine-textured. Similarly clay formation is
favoured more by a high percentage of
decomposable dark minerals and less by
quartz. The parent material, thus, influences
the soil formation by their different rates of
weathering.
Climate
Acidic soils are formed in humid areas due to
intense weathering and leaching. Alkaline
soils are formed in areas of low rainfall due to
the accumulation or retention of lime. The
climate is an increasingly dominant factor
in forming varied type of soils especially
because of the effects of temperature and
precipitation. It also plays an indirect role in
the formation of soil by way of its influence on
vegetation.
Biota
The decomposition of organic wastes and
residues and the activities of living plants and
animals have marked influence on the soil
development. Burrowing animals, such as
moles, prairie dogs, earthworms, ants and
termites help soil development slowly by
decomposing organic matters and forming
weak acids that dissolve mineral faster. The
roots of living plants and decomposed plant
material releases weak organic acids that help
in weathering and soil development.
Topography
Steep hillsides have thin soil cover because of
surface runoff that results in the erosion of
surface. On the other hand, gentle hillsides
preserve appreciable soil cover due to the
luxuriant vegetation and sufficient water
passing in vertically to deeper levels. The
landlocked depressions receive high amount
of runoff water that favours appreciable
vegetation cover but slower decomposition
because of oxidation deficiency. This results
in the formation of soil that is rich in organic
matter. The topography influences the soil
formation through its relationship with water
and temperature.
Time
Under ideal conditions, a recognisable soil
profile may develop in 200 years and under
less favourable circumstances, it may extend
to several thousand years. The rate of soil
development is determined by the effects of
time and other distinct factors � climate,
parent material, topography and biota.