Home News CHEMISTRY FORM FOUR STUDY NOTES TOPIC 3: SOIL CHEMISTRY

CHEMISTRY FORM FOUR STUDY NOTES TOPIC 3: SOIL CHEMISTRY

Soil Formation
Soil Formation
Describe soil formation
Soil
is formed by the process of weathering. All types of weathering
(physical, chemical or biological) result to disintegration of rocks
into smaller particles. Air and water enter the space between these
particles and chemical changes take place, which lead to the production
of chemical substances. Bacteria and plant life soon appear. When plants
and animals die, they decay and produce humus. Bacteria and other
decomposers play a vital role in the decomposition of plant and animal
substrata. The end product of these mechanical, chemical and biological
processes is soil.
Therefore,
soil can be defined as unconsolidated mineral (inorganic) and organic
material on the immediate surface of the earth‟s crust that serves as
the medium for plant growth.
All
soils contain mineral matter, organic matter, water, air and living
organisms, especially bacteria. If any one of these is substantially
reduced in amount or is removed from the soil, then the soil
deteriorates. There are many types of soil and each has specific
characteristics related to the climate, the vegetation and the rock of
the region in which it forms. The weathering processes of a region also
play an important part in determining soil characteristics. The
relationship of these factors is as shown in figure 3.1.
The Factors Influencing Soil Formation
Describe the factors influencing soil formation
Information
about soil formation can lead to better soilclassification and more
accurate interpretation of soil properties.There are several factors
responsible for soil formation. Thefactors include climate, living
organisms, relief (topography),parent material and temperature. All the
factors, except time,depend to a greater or lesser extent upon each
other, upon the soilitself or upon some other factor. None of the
factors can beconsidered more important than any other, but locally one
factormay exert a particular strong influence. These factors
areexplained in details below.
Parent material
Parent
materials are made up of mineral material or organic matter or a
mixture of both. The organic matter is usually composed predominantly of
unconsolidated, dead and decaying plant remains. The mineral material,
which is the most widespread type of parent material, contains a large
number of different rock– to form Climate which decays to form results
in weathering of influences the type of climate rocks vegetation humus
Climate Soil mineral soil Climate forming minerals and can be in either
consolidated or unconsolidated state.
Some
rocks are more easily weathered than others. Acidic rocks are more
resistant to weathering than basic rocks. The parent rock affects soil
texture and water permeability.
Parent
rock with fine particles is more resistant to chemical weathering than
mechanical weathering. Very compact parent rocks like sandstone are very
much resistant to weathering. Porous rocks weather easily by chemical
processes. This is because they have large surface areas for weathering
agents to act upon.
Climate
Climate
is the principal factor governing the rate and type of soil formation
as well as being the main agent determining the distribution of
vegetation. The dead vegetations decay to form humus as one of the
components of the soil.
To
understand well the influence of climate on soil formation let us have a
look at its components and how each of these components affects soil
formation.
Temperature
The
main effect of temperature on soil is to influence the rate of
reactions; for every 10°C rise in temperature, the speed of a chemical
reaction increases by a factor of 2 or 3 (twice or thrice). Temperature,
therefore, influences the speed of disintegration and decomposition of
the parent materials and its consolidation to form the soil.
Rainfall (water)
The
water in soils includes all forms of water that enter the soil system
and is derived mainly from precipitation as rain. The water entering
soils contains appreciable amounts of dissolved carbodioxide, forming a
weak carbonic acid. This dilute, weak acid solution is more reactive
than pure water. It thus reacts with unconsolidated minerals and organic
matter, breaking them down into mineral (clay, sand) and organic debris
(humus) respectively.
Organisms
The
organisms influencing the development of soils range from microscopic
bacteria to large mammals including man. In fact, nearly every organism
which lives on the surface of the earth or in the soil affects the
development of soils in one way or another. More important soil
organisms of interest to soil formation are as follows:
Higher plants.
Higher
plants (particularly grasses) extend their roots into the soil and act
as binders. So they prevent soil erosion. The roots also assist in
binding together small groups of particles hence developing a crumby or
granular structure. Large roots are agents of physical weathering as
they open and widen cracks in rocks and stones. When plants die they
contribute organic matter to the soil, which acts as a binder of the
soil particles. Higher plants intercept rain and they shelter the soil
from the impact of raindrops. They also shade the soil and hence reduce
evaporation.
Vertebrates
Mammals
such as moles, ground squirrels and mice burrow deeply into the soil
and cause considerable mixing up of the soil, often by bringing up
subsoil to the surface, and creating burrows through which the top soil
can fall and accumulate within the subsoil.
Microogarnisms
These
include bacteria, fungi, actinomycetes, algae and protozoa. These
organisms act as decomposers of organic and even mineral matter.
Mesofauna
These
include earthworms, nematodes, millipedes, centipedes and many insects,
particularly termites and ants. Activities of mesofauna include:
  • ingesting organic mineral materials e.g. earthworms and millipedes;
  • transportation of materials e.g. earthworms, millipedes, termites, beetles, etc; and
  • improvement of soil structure and aeration.
Man
Activities of man are too many and too diverse. Man‟s roles include:
  • Cultivation
    of soils for production of food and tree crops, which in many cases has
    negative effects causing impoverishment of the soil and erosion.
  • Indiscrimate grazing, casual burning, cutting of trees, manure and fertilizer use, all of which alter the soil characteristics.
Relief (Topography)
This
refers to the outline of the earth‟s surface. All land surfaces are
constantly changing through weathering and erosion. It may take millions
of years, in the case of Himalayas and the Andes, to be worn down to
flat undulating surfaces. The soils on steep mountain slopes are shallow
and often stony and contain many primary minerals. In areas where the
difference in elevation between the highest and the lowest point is
great, then climatic changes are introduced. These differences in
elevation, slope, slope direction, moisture and soil characteristics
lead to the formation of a number of interesting soil sequences.
Time
Soil
formation is a very slow process requiring thousands and even millions
of years. Hence, it is impossible to make definite statements about the
various stages in the development of soils.This is because it takes a
considerable period of time for a particular soil type to be formed and
categorized.
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