Category : 7th Class
Learning Objectives
Soil is a precious gift from nature. Soil supports life on earth. Most people think soil as a layer of dirt and mud. However this layer of mud and dirt is actually filled with life. Food we eat, fibre we use to make fabric, habitat for various organism etc. is provided by soil. If you closely observe a freshly dug pit you may see various creatures like earthworm, ants, beetles etc. Soil provides nutrients to the plants and support their growth. All living organisms depend directly or indirectly on soil. Let’s learn more about soil.
SOIL
The mixture of rock particles and humus is called soil. Soil is an important natural resource. It contains water, dissolved substances, mineral salts and living organisms. Soil forms a very thin layer on the surface of the earth ranging from a few m to 3 to 4 m.
Note: Humus is a brown or black organic substance formed from decaying plant remains or animal matter. It determines the fertility of soil. It is porous in nature and increases the ability of soil to retain water.
SOIL FORMATION
Soil is formed from parent rock material over millions of years by a process of weathering. Weathering is the process of breaking down of rock present on the surface of earth into fine particles.
Weathering Occurs by Two Main Processes
(a) Physical weathering, which is caused by physical phenomena like atmospheric changes (heating, cooling, wetting-drying etc).
(b) Biological weathering, which involves breaking down of rocks by the action of living organisms.
Do you know?
Earthworm’s burrows act as tunnels which allow water to moves quickly and provide pathways for roots to grow. They also decompose dead plants and animal matter. Their castings are valuable as fertilizer.
SOIL PROFILES
Soil profile is a vertical section of different layers of the soil. Various layers are called horizons. Each layer diners in colour, depth, chemical composition. Generally we see the top surface of the soil, not layers below it. Soil profile can be seen while digging a we'l or laying the foundation of a building. Soil profile i.e. various layers of soil can also be observed in a deep cut through the soil. Typically, four distinct soil layers can be seen. It can also be seen at the sides of a mad on a hill or at steep river bank.
A-Horizon
The upper most horizon is dark in colour. It is rich in humus and minerals. The humus makes the soil fertile and provides nutrients to growing plants. It is generally a soft, porous layer and can retain more water. It is also called top-soil or the A-horizon.
Functions of Top-soil or A-horizon
(i) it provides shelter for many living organisms such as worms, rodents, moles and beetles
(ii) The roots of small plants are embedded entirely in the top-soil
B - Horizon
Middle Layer or B-horizon or subsoil is the layer next to the top-most soil or A-horizon. It contains lesser amount of humus but contains more of minerals. It is generally harder and more compact.
C- Horizon
Third layer or C-horizon is the layer below the B-layer and is made up of small lumps of rocks with cracks and crevices. It is difficult to dig beyond this layer.
R- Horizon
Bed rock or R-horizon is a layer below C-horizon. It is hard and difficult to dug with a spade. It mainly consists of parents rock. It undergoes weathering.
COMPOSITION OF SOIL
Main components of soil are:
(i) Soil particles like sand, silt, clay, gravel etc.
(ii) Humus, an organic matter formed by decomposition of dead organisms.
(iii) Air, Water, Soil organisms.
The difference in the proportion of these components leads to the formation of different kinds of soil.
Do you know?
When rainwater sinks underground, it reaches the impervious layer R-horizon and accumulates over it. This water is called groundwater. The upper level of this layer which is saturated with water is the water table. Water table is rarely leveled and follows the general slope and land above it. The level of water table fluctuates from season to season. It rises in the rainy season and falls in the dry season.
TYPES OF SOIL
On the basis of proportion of particles of various sizes soil can be classified as.
(i) Sandy soil: If soil contains greater proportion of big particles it is called sandy soil.
(ii) Clayey soil: In such a soil the proportion of fine particles is relatively higher.
(iii) Loamy Soil: In such a soil the amount of large and fine particles is about the same.
Properties of Various Types of Soil
(i) Sandy Soil: Contains sand particles of large size and they can't fit close together. Large spaces are available between them. The spaces are filled with air and thus such a soil is well aerated, water can drain through the spaces and so sandy soil is light, well aerated and dry.
(ii) Clayey Soil: The smaller particles present in it can pack tightly together, leaving little space for air. These tiny gaps can hold water so clayey soil has little air. They are heavy as they hold more water as compared to sandy soil.
(iii) Loamy Soil: Best top soil for growing plants is loam. Loamy soil is a mixture of sand, clay and silt (a type of soil particles). The loamy soil also contains humus. Such a soil has the right water holding capacity for the growth of plants
Note: The properties of soil are greatly influenced by the size of particles present in it.
TYPES OF INDIAN SOIL
PROPERTIES OF SOIL
Adsorption of Water in the Soil Plants need water to grow. If the soil does not hold water the plants would need frequent watering or they will die. The amount of water a particular type of soil can absorb is called its water absorption tendency.
Note: Silt occurs as deposit in river beds. The size of silt particles is between those of sand and clay.
Moisture in Soil
Soil holds water in it which is called soil moisture. The capacity of soil to hold water is important for various crops.
Percolation Rate of Water
The rate at which water exits in the soil is known as its percolation rate. Different soils have different percolation rate of water. To calculate percolation rate we use the following formula
\[\text{Percolation rate (mL/min)=}\frac{\text{Amount of water percolated (mL)}}{\text{Percolation time (min)}}\]
Do you know?
Percolation rate is highest for sandy soil and least in case of clayey soil.
SOIL EROSION
The removal of top soil by water and wind is known as soil erosion.
The top soil contains humus and mineral salts, which are vital for the growth of plants. So, removal of top soil by water and wind leaves the underneath subsoil and rocky base exposed.
Thus, erosion causes a significant loss of humus and nutrients and hence, decreases the fertility of soil.
Soil Erosion
Causes of Soil Erosion
There are several causes of soil erosion, which can be divided into two categories.
(i) Natural causes: It involves natural agents like wind and water.
(a) High wind velocity over lands, which have no vegetation, carries away the loose top soil.
(b) Pouring raindrops, over areas with no or very little vegetation, also carries away the top soil.
(ii) Man-made causes: Besides natural agents, there are certain man-made activities, which cause soil erosion. For example:
(a) Deforestation: Deforestation is the cutting or removal of trees or other vegetation for timber or for farming purposes. It increases soil erosion. Roots of plants hold soil particles together. In the absence of plants, the top layer of soil is easily removed by the action of high speed winds or water flow, thereby increasing the chances of soil erosion.
Deforestation
(b) Overgrazing: Overgrazing by flocks of cattle, buffaloes, goats and sheeps leaves a very little plant cover on the soil. The hooves of the animals make the soil dry, which reduces its porosity and percolation.
(c) Improper agricultural practices: Improper tillage and burning of stubble of weeds reduces the water-holding capacity of the soil. As a result, soil become dry and hence, can be easily blown away as dust.
(d) Heavy rainfall and strong winds: Uncovered soil is eroded quickly by heavy rain and strong winds.
(e) Slope: Run off water passing along the slope gathers speed and develops high cutting and carrying capacity.
Overgrazing
Effects of Soil Erosion
Decertification of Land
Control of Soil Erosion
Terrace Farming
SOIL POLLUTION
Soil pollution occurs either by direct wastes or indirectly by air pollution. The main source of air pollution are:
Dumping Of Garbage
Acid Waste and Spilling
Do you know?
Over 80% of items in landfills can be recycled, but they’re not.
Control of Soil Pollution
CONCEPT MAP
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