INDIAN SCHOOL AL WADI AL KABIR
DEPARTMENT OF SCIENCE 2016-17
HANDOUT
CLASS: VII
SOIL- A NATURAL RESOURCE
DATE: 16-11-2016
NAME:
Soil is a mixture of rocks particles, sand and humus. It is essential for the existence of life on the
earth. It supports the growth of plants by holding their roots and supplying water to them.
FORMATION OF SOIL – Soil is formed by weathering. Weathering is the process of
breaking down of rocks into smaller particles by the action of wind, water and climate.
Formation of even one inch of soil takes many years.
COMPOSITION OF SOIL: Soil contains-soil particles, humus, water, air and small organisms
(microbes and insects). Humus: The organic matter formed by the decomposition of dead plants
and animals by the microorganisms is called humus. It makes the soil fertile.
SOIL PROFILE: A vertical section through the soil showing the different layers of soil is
known as soil profile. The different layers of soil are called its horizons.
1. O-horizon- mainly contains dead organic matter
which later forms humus.
2. A horizon: is also called top-soil. It is generally
dark in colour, rich in humus and minerals. It is
generally soft porousand retains more water. It also
provides shelter to many decomposers.
3. E-horizon- is the intersection layer between A
horizon and B-horizon.
4. B-horizon: forms the subsoil. This is less rich in
humus but contains most of the minerals present in the
soil. Plant roots generally grow till this layer.
5. C-horizon: is the parent rock layer. The roots can’t
penetrate through it. It lacks humus and is infertile.
The rain water gets collected in this layer in the form
of groundwater.
6. Bedrock: is composed of unweathered rocks. Water can’t penetrate through this layer.
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TYPES OF SOIL: Soils are categorized mainly in three types based on the proportion of size of
particles- sandy, clayey and loamy Soil.
Sandy Soil: Particles of sandy soil are larger in size with enough space among them. These
spaces can hold air but water drains quickly.It is not fit for growing crops but is used for
construction of buildings along with cementing material.
Clayey Soil: Particles of clayey soil are very small in size and packed tightly. It is not well
aerated and retains more water. It naturally forms lumps with water and is therefore used in
making of pottery and bricks.
Loamy Soil: Particles of loamy soil are smaller than sand and larger than clay. It is the mixture
of sand, clay, siltand humus. Space between the particles has right water holding capacity and is
well aerated. Therefore, it is considered as the best soil for farming and gardening.
PROPERTIES OF SOIL:
Texture of soil- Different soils differ in their texture.Sandy soil is gritty and loose, loamy soil is
smooth and clayey soil is smooth and sticky.
Percolation Rate ofSandy, Loamy and Clayey Soils: The process in which water passes down
slowly through the soil is called percolation of water.
The percolation rate of water in a soil is the volume of water in millilitres which passes down the
soil per minute. It can be calculated using the formula-
The percolation rate of water in sandy soil is fastest and in clayey soil is slowest.
Moisture Content in different types of soils: Amount of water present in soil is called moisture
present in soil. The amount of moisture present in clayey soil is highest and that in sandy soil is
lowest.It can be calculated using the formulaMoisture content= Weight of moist soil (g) – Weight of dry soil (g)
Organisms Found in Soil-Soil makes a very good natural habitat for various microorganisms.
These microorganisms form humus and make the soil fertile. Eg. Bacteria, fungi, algae. Some
help in fixing the atmospheric nitrogen into nitrogenous compounds.
Earthworms, commonly called as farmer’s friend, ingest the soil and excrete it after digesting
its organic matter. Thus enriching the soil with nitrogen. It makes burrows into the soil, thus
mixing the soil well.
Soil and Crop: Different types of soil are found at different places. A particular soil type may
not be suitable for certain types of crop. That’s why different types of crop are grown in different
parts of the world.
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Sandy soil: Sandy soil is not fit for any crop as it does not retain water. However, some thorns
and bushes do grow on sandy soil. Millets can be grown on sandy soil.
Clayey Soil: Clayey soil is best suited for paddy, as it can retain water for a longer time. This is
also suitable for wheat and grams.
Loamy Soil: Loamy soil is considered the best for almost all types of crops. It is suitable for
lentils and other pulses.
Sandy Loam: Sandy loam soil is suitable for the growth of cotton plants as it can easily drain
water and is well aerated.
Soil Erosion: Gradual removal of topsoil by the action of wind, flowing water, waves and snow
is known as soil erosion. Soil erosion affects the fertility of soil, as humus present in the soil is
eroded along with soil. Some factors affecting soil erosion are as follows1. Heavy rains and floods which make the soil particles loose.
2. Deforestation loosens the soil particles. It makes the land barren and increase the chances of
flood.
3. Poor farming methods adapted by farmers.
4. Overgrazing by animals.
5. Construction of roads and development of houses.
Soil Pollution- Contamination of soil by presence of man-made chemicals or other non-desirable
substances is called soil pollution. Some of its causes are as follows1. Excessive use of chemical fertilisers, pesticides and insecticides.
2. Excretory products of living organisms.
3. Untreated sewage water.
4. Non-biodegradable waste- plastic, metal
5. Salination degrades the quality of soil.
Soil Conservation- Protection of soil against erosion degradation is called soil conservation. It
can be done by taking the following steps1. Afforestation- The roots of plants and trees bind the soil particle.
2. Terrace farming in hilly areasby farmers. The steps prevent the topsoil to be washed away.
3. Crop rotation.
4. Dams and embankments to control floods and thus preventing soil erosion.
Prepared by- Ms. Leena Chaudhary
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