SOIL IS A FOUR PHASE SYSTEM
The meaning of soil varies
depending on the person considering it. To a civil engineer planning a
construction site, soil is whatever unconsolidated material happens to be found
at the surface. To a miner, it is just some worthless material that is in the way
and must be removed. To a farmer, it is the medium that will nourish and supply
water to the crops. Even soil scientists may hold differing definitions,
depending on their area of study.
Soil is the accumulation of
natural bodies on the earth's surface, which containing living organisms and
non living organisms that supporting macro and micro organisms to survive. Its
upper limit is air or shallow water. At its margins it grades to deep water or
to barren areas of rock or ice. Soil includes the horizons near the surface
that differ from the underlying rock material as a result of interactions,
through time, climate, living organisms, parent materials, and relief. In the
few places where it contains thin cemented horizons that are impermeable to
roots, soil is as deep as the deepest horizon (Juma, (1999). Soil constitute four major components,
which are:-
Soil Air
Soil air is a gases occupying the free pore spaces in soil, In
nutrient management soil aeration influences the availability of many
nutrients. Particularly, soil air is needed by many of the microorganisms that
release plant nutrients to the soil. An appropriate balance between soil air
and soil water must be maintained since soil air is displaced by soil water.
Air can fill soil pores as water drains or is
removed from a soil pore by evaporation or root absorption. The network of
pores within the soil aerates, or ventilates the soil. This aeration network
becomes blocked when water enters soil pores.
Soil air is very different than the above-ground
atmosphere. A significant difference is between the levels of carbon dioxide.
Since the soil contains high amounts of carbon dioxide, oxygen levels may
become limited. Since plants must have oxygen to live, it is important to allow
proper aeration in the soil. references
to soil air composition are, N2 in soil air contain 72.2% and 79.0% in
atmosphere, O2 in soil air
contain 20.6% and 20.9 in atmosphere and CO2 in soil air contain
0.25% and 0.03 in atmosphere.
Soil air is very important part in the soil
because without air in the soil plants roots and most soil organisms will not
be able to survive and therefore will die. As a result organic matter cannot
change to humus. Soil air is mixture of various gases and it form 25% of the
total volume of soil constituents.
Soil water
Moisture may be present as adsorbed moisture
at internal surfaces and as capillary condensed water in small pores. At low
relative humidities, moisture consists mainly of adsorbed water. At higher
relative humidities, liquid water becomes more and more important, depending on
the pore size.
According to Leeper and Uren (1993), Soil
Water Retention the
spaces that exist between soil particles, called pores, provide the retention
of gasses and moisture within the soil profile. The soil’s ability to retain water
is strongly related to particle size; water molecules hold more tightly to the
fine particles of a clay soil than to coarser particles of a sandy soil, so
clays generally retain more water.
Charman and Murphy (1977) argued that, conversely sands
provide easier transmission of water through the profile. Clay type, organic
content and soil structure also influence soil water retention. The role of
soil in retaining water is significant in terms of the hydrological cycle;
including the relative ability of soil to hold moisture and changes in soil
moisture over time.
According to Leeper and Uren (1993), The maximum amount of water that a given
soil can retain is called field capacity, whereas a soil so dry that plants
cannot liberate the remaining moisture from the soil particles is said to be at
wilting point. Available water is that which the plants can utilize from the
soil within the range of field capacity and wilting point.
Soil water that is not retained or used by plants may
continue downward through the profile and contribute to the water table, the
permanently saturated zone at the base of the profile this is termed recharge.
Soil that is at field capacity may preclude infiltration so to increase
overland flow. Both effects are associated with ground and surface water
supplies, erosion and salinity.
Soil
water can affect the structural integrity or coherence of a soil saturated soils can become unstable and result
in structural failure and mass movement. Soil water, its changes over time and
management are of interest to geo-technicians and soil conservationists with an
interest in maintaining soil stability.
Organic
matter
Soil organic matter
(SOM) is the organic
matter component of soil,
consisting of plant and animal residues at various stages of decomposition,
cells and tissues of soil organisms, and substances synthesized by soil
organisms. Soil organic matter exerts numerous positive effects on soil
physical and chemical properties, as well as the soil’s capacity to provide
regulatory ecosystem services. Particularly, the presence of
Soil organic matter is regarded as being critical for soil functions and soil
quality.
The positive impacts of SOM result from a number of
complex, interactive edaphic factors; a non-exhaustive list of SOM's effects on soil
functioning includes improvements related to soil structure,
aggregation, water retention, soil biodiversity,
absorption and retention of pollutants,
buffering capacity, and the cycling and
storage of plant nutrients. SOM increases soil fertility
by providing cation exchange sites and acting as reserve of
essential nutrients, especially nitrogen, phosphorus, and
sulfur, along
with micronutrients, which are slowly released upon SOM mineralization.
As such, there is a significant correlation between SOM content and soil
fertility.
SOM also acts the major sink and source of soil carbon.
Given that SOM is typically estimated to contain 58% C, the terms 'soil organic
carbon' (SOC) and SOM are often used interchangeably, with measured SOC content
often serving as a proxy for SOM.
The mass of SOM in soils as a percent generally ranges
from 1 to 6% of the total topsoil mass for most upland soils. Soils whose upper horizons
consist of less than 1% organic matter are mostly limited to desert areas, while the SOM content of soils
in low-lying, wet areas can be as high as 90%. Soils containing 12-18% SOC are
generally classified as organic soils.
Inorganic
Materials In Soil
The
inorganic materials found in soils account for about half of the total mass of
most soil. These inorganic materials take the form of sand, silt and clay, and
are referred to commonly as dirt. All these are high in
nutrients that plants need and are natural fertilizers. Good topsoil is rich in
organic material. Inorganic material composes the rest of the soil and the
subsoil, and comes initially from the particular type of subsoil underneath the
topsoil area. It's basically ground down rocky material, which may appear as
clay, sand, gravel, stones or or chalky particles. Plants need the air spaces
between both the organic and inorganic matter in soil, in order for their roots
to get oxygen and grow. So a range of sizes of inorganic particles to provide
drainage, mixed with organic matter for nourishment, is good for them. A very
heavy clay soil may be hard to cultivate because of its lack of drainage - the
clay particles stick closely together and plants find it hard to penetrate it,
or for their roots to breathe. It needs mixing with organic material, or a
lighter inorganic type such as sand, to make it workable. A very light sandy
soil may drain too fast to retain water, but again, the answer is more organic
material, which in this case will increase the soil's ability to retain
moisture Some examples of the inorganic matter plants require are magnesium,
boron, and manganese. Organic matter is more complicated. Humus is the
breakdown product of plant matter. It is composed of carbon and nitrogen forms
as well as bacteria and fungi along with insects. Without knowing why you need
the information it is impossible to answer it well and is inorganic and can be
added to soil to increase draining. Perlite and vermiculite are also helpful to
add to potting mixes since they contain minerals and retain water well.(Troeh
et al, 2005)
Soils usually contain inorganic
material of three sizes: sand, which ranges in diameter from 0.2 to 2
millimetres; silt, between 0.02 and 0.002 millimetres in diameter; and clay,
which is less than 0.002 millimetres. Most soils have some of each. Soils with
a large proportion of each component are called loams.
REFERENCES
Brady,N. C., and Weil, R.R. (1999). The nature and properties of soils. Upper
Saddle River, NJ Prentice Hall,In.
Juma, N. G. (1999). Introduction to Soil Science and Soil Resources. Volume I in the Series
"The Pedosphere and its Dynamics: A Systems Approach to Soil
Science." Salman Productions, Sherwood Park.
Troeh, Frederick R., and Louis M. Thompson(2005). Soils and Soil Fertility. 6th ed. Ames, Iowa: Blackwell Pub.
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