Soil is made up of four main constituents, which are: those resulting from the breakdown of rocks, organic matter, water and air. In addition, soil contains microbes or micro-organisms.
Soil is made up of inorganic particles of different sizes such as
stones, sand, silt and clay. Each of these is surrounded by microscopic film of
moisture in which nutrients are dissolved. These particles aggregate together
to form larger lumps of soil crumbs about l-5cm diameter. The presence of
organic matter facilitates the formation of soil crumbs. The presence of these
crumb structures indicates the ideal soil structure for cultivation and
planting. If the soil texture is too friable, then the roots will not be held
firmly and crops will fall over, be blown or knocked out of the soil.
Water and Air
Water comes down from the soil surface trickling between the soil
particles and the soil crumbs. Some water will be held but there are some
spaces filled with air (soil contains water and air). Roots of crops penetrate
the soil always by growing downwards (the direction of gravity). The roots
touch the nutrient solution; roots take in water and they take the nutrient
solution as food. Scattered about the soil are the soil organisms and dead
plant and animal remains.
Air is not solid
or liquid, but a combination of gaseous elements that are found naturally in
Earth's atmosphere. In soil, air pockets allow water to pass through the soil
and into the plants growing above and below the soil line. Water in soil
usually contains dissolved salts and other chemicals. Water is an essential
part of soil; plants cannot survive without it. Some soils, like clay, retain
water much better than others. When water lingers in soil instead of passing
through it easily, the soil becomes more densely packed. Some plants cannot grow
in heavy, moisture-rich clay soils.
Minerals
All soil is composed of sand, silt and
clay, though some types of soil have heavier concentrations of these minerals
than others. Rocks and minerals make up the largest chunk of soil's
composition. The rocks and minerals found in soil come from non-living,
inorganic materials. Sand comes from small fragments of quartz and other
minerals, and by itself is not rich in the nutrients that plants need. Sand is
the largest and coarsest of soil particles; water passes through sand more
easily than other soil types. Silt is a combination of quartz and other rocks.
Silt particles are smaller than sand but
larger than clay. Clay is the richest of soil minerals, containing nutrients
like iron, potassium and calcium. The smallest soil particles come from clay,
which can become very dense and hard to work.
Organic
Decayed plants and animals provide the organic
materials found in soil. Through decomposition, organic materials are broken
down and turned into nutrients that plants can use. Mineralisation also occurs
through decomposition, and through this process organic materials become
inorganic.
Biological Materials
The fifth component of soil, which is not always
recognised, is the living world that exists under the ground -- the biological
component. Bacteria and fungi
are microscopic organisms; those living in the soil decompose and are
beneficial (but there are many others which cause diseases).
The biological world, provides these important organic
elements that are so essential. Plants and animals, when they die, become part
of the ground once more, and so the cycle continues. Soil gives life; life
gives itself back to the soil.
All parts of soil are essential to plant development,
and each is necessary for plants to survive. The composition of the components inside
soil varies to create different soil types. Soil that is classified as clay,
for example, contains more water than light, sandy soils.
Experiment:
Carry out experiments to show that the soil contains organic
matter, air, water, and rock particles.
You may find the following materials useful in your
experiment. These are:
Samples of garden soil, scale, water, clear plastic or glass bottle,
cans, Bunsen burner/charcoal brazier, cotton wool, funnels, stirring rod, etc.
After carrying out the experiment, now answer the
question below:
Describe the importance of the following soil
components to a growing plant: -
(i)
Rock particles
(ii)
Soil water
(iii)
Soil air
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