Soil refers to the top layer of the earth
comprising of loose, unbound material overlying the crustal rocks. It results
from weathering of the crustal rock and forms the substrate for plant growth. Soil
comprises of inorganic and organic matter as well as living organisms. It is
formed when rocks break down through the process of denudation to produce
parent material which in turn weathers to produce soil. The process of soil
formation involves organic accumulation, eluviation, leaching, illuviation,
precipitation and organic sorting (Queensland
Soil has a vertical profile consisting of
different layers from the surface to the bed rock. The properties of the
different horizons in the profile are used to identify different soils and how
the land can be utilized to attain its full potential from the type of soil
present. For example, if a piece of land exhibits the top layer rich in humus,
it is suitable for farming whereas if the top most layer is eroded, the land
can be used for mining gravel and rock particles (Van Breemen & Buurman,
2002). The soil profile mostly exhibits three main horizons:
soil – this layer is rich in humus, nutrients, organic
matter and biological activities. This layer always forms at the surface and
due to the rich organic matter present, gives the soil a dark color.
– this layer holds more moisture and is less fertile compared to the top soil.
It mostly comprises of clay soil and as a result there is less biological
activity due to the soil texture of the type of soil present here.
parent material – This layer comprises of rock particles
that have broken down from the bed rock. The rock particles here are relatively
small and it’s from them that the first two layers eventually form (USDA, n.d).
soil a renewable or non-renewable resource?
Soil is a renewable
resource since it forms from decomposition of plant materials as well as
weathering of the bed rock. The Food and
Agriculture Organization of the United Nations (however views it as a
non-renewable resource due to the long amount of time it requires to regenerate
and form the lost top soil FAO, 2015).
Soil formation is
influenced by the following factors:
of parent rock- the type of parent rock largely
determines the mineral composition as well as the structure of the soil.
Weathering occurs in three major ways. It can be a physical, chemical or
heat and moisture resulting from climatic events influence the speed of
weathering and leaching of parent rock. Where climate appears as cold and dry,
weathering can be slow whereas regions with high temperatures differentials and
high precipitation experience accelerated physical weathering (USDA, n.d).
– the shape, length and grade of a slope affects drainage and also determines
the type of vegetation growing in the area as well as the amount of rainfall
received. All this aspects of topography result to a change in the way the soil
forms. Steep slopes are easily eroded and lose their top soil to the lower
regions in the slope where the soil is deposited (USDA, n.d).
refers to biotic factors which include plants, bacteria, insects, burrowing
animals and humans. When plant leaves dry and drop on the ground, with time
they decompose and make part of the top soil. Tilling of land by farmers before
planting helps to breakdown and loosens the soil. Some farming methods they use
to improve fertility of land such as mulching using dry twigs or plant material
and animal waste eventually form the organic matter which forms humus (Queensland
materials – Most physical and chemical characteristics of the
soil are derived from the bedrocks which mainly are the parent material for
soil formation. However, where soils form from materials eroded from elsewhere;
there is little or no change in them once they are deposited.
– Refers to the age of soil determined by its development and not chronological
age hence properties tend to differ depending on the period the soil has been weathered.
Time however cannot be a factor by itself as it relies on the other above
Despite soil being a
renewable resource, it should be conserved and preserved since its formation is
a long process that takes years. Some of the ways soil can be conserved
existing forests since trees prevent erosion.
planning of land use.
and growth of soil-binding plants.
cultivation of water-catchment areas and river banks