Class 7 science Notes Soil

1. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
SOIL
Soil: Mixture of rocks particles, sand and humus is known as soil. Soil 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 by the action of wind, water and climate. Formation of even one inch of
soil takes many years.
Composition of Soil: Soil contains humus, water, clay, sand and gravel.
Humus: The decaying dead matters in soil are called humus. Humus makes the soil
fertile and provides nutrients to the plants.
Soil Profile: Different vertical layers of soil are known as soil profile. The feel (texture),
colour and chemical compositions of soil are different in each layer.
Horizons: Layers of soil are referred as horizons. Horizons of soil are categorized into
four types: A-horizon, B-horizon, C-horizon and Bedrock.
A horizon: Top layer of soil is called A-horizon. It is also called top-soil. A-horizon is
generally dark in colour. It is rich in humus. A horizon is generally soft and porous. It
retains more water. Roots of small plants are entirely embedded in topsoil. Topsoil also
provides shelter to many living organism, such as worm, insect, moles, rats, snakes,
etc.
B-horizon: The next layer of the soil from top is known as B-horizon. This layer is
harder and more compact than the top soil. This is generally known as the middle layer
of soil. This is less rich in humus but contains most of the minerals present in the soil.
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2. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
SOIL
C-horizon: The third layer of soil is known as C-horizon. C-horizon is made of small
lumps of rocks with cracks and crevices.
Bedrock: The layer below the C-horizon is known as bedrock. Bedrock is composed of
rocks and is hard to dig with spades.
Type of Soil: Size of particles of soil is very important for the properties of soil. So, soils
are categorized mainly in three types based on the proportion of size of particles. These
are Sandy Soil, Clayey Soil and Loamy Soil.
Sandy Soil: Particles of sandy soil are larger in size. Because of larger in size, sand
particles cannot fit close together and hence there is enough space among them. These
spaces are filled with air. Water drains quickly through sandy soil. So, sandy soil is
called well aerated, light and dry. Sandy soil is not fit for vegetation as it does not retain
water. However, millets can be grown on sandy soil.
Clayey Sol: Particles of clayey soil are very small in size. This makes the particles of
clayey soil packed tightly. There is very little space among the particles. Water does not
drain quickly through clayey soil because of less space among particles. So, clayey soil
is not well aerated and retains more water. Clayey soil is used in making of toys. Clayey
soil is good for growing paddy.
Loamy Soil: Particles of loamy soil are smaller than sand and larger than clay. Loamy
soil is the mixture of sandy soil, clayey soil and silt. Silt is the deposit in river beds.
Loamy soil has right water holding capacity and is well aerated. This is considered as
the best soil for the growth of plants.
Properties of Soil
Percolation Rate of Water: Amount of water drained through water in unit time is
known as percolation rate of water in soil. Percolation rate of water can be calculated
using the formula given here.
The percolation rate of water in sandy soil is fastest and in clayey soil is slowest.
Moisture present in soil: 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.
Absorption of water: Different soil absorbs different amount of water. Clayey soil
absorbs the highest amount of water because of its higher water retention capacity. On
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HEAD OFFICE: 131/12 KRISHNA COLONY, STREET NO 4, NEAR SHIV MANDIR, GURGAON

3. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
SOIL
the other hand, sandy soil absorbs the least amount of water because of its lower water
retention capacity.
Soil and Crop: Different types of soil are found 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.
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: Removal of topsoil by weathering agents; such as wind, water and ice is
known as soil erosion. Soil erosion affects the fertility of soil, as humus present in the
soil is eroded along with soil. It is important to prevent the soil erosion.
Preventive measures for soil erosion
 Deforestation should be prevented at all cost to prevent soil erosion.
 Shelter belt plantation of trees is good for preventing soil erosion.
 Terrace farming in hilly areas is effective in preventing soil erosion.
CONTACT: 9654089891, 9811083910, 9953898004
HEAD OFFICE: 131/12 KRISHNA COLONY, STREET NO 4, NEAR SHIV MANDIR, GURGAON