After reading this essay you will learn about:- 1. Formation of Soil 2. Soil Profile 3. Classification 4. Conservation Methods.
Essay on the Formation of Soil:
According to the size of the particles, which consists of the broken and disintegrated rock (Mechanical weathering), the following gradations are recognized.
Pickman of rock from the size of a small melon and up are termed as ‘Boulders’. Those larger than peas are called ‘Gravel’. Those smaller than peas that do not cohere when wet Sand, and the finest material, which can be carried by wind, is Dust and the last which generally cohere when well is termed as Silt, Mud or Clay according to its character.
Ordinary Soils composed of variable mixtures of sand and these finer materials may roughly be classified as the following groups:
Composed of sand grains mostly without clay.
Mixture of sand and clay.
The finest material, mostly kaoline without sand.
The red and yellow colors, which may possess soil are due to oxide of iron produced by decay of minerals. The black colors seen in soil are due to the carbonaceous material resulting from the decomposition of vegetation. This substance is known as ‘Humus’.
Weathering is of vital importance to human race because it produces soil, the basis of all life on land.
Thus the surface of earth soil profile may be divided into four horizons, they are:
1. Horizon A:
Top soil, organic layer.
2. Horizon B:
Rich in mineral matter, zone of accumulation of clays. Colloids, Iron and Aluminium oxides.
3. Horizon C:
4. Horizon D:
Solid rock (fresh parent rock).
Now that is very clearly the process of weathering irrespective of the kind result in the ultimate breaking down or decay of original hard and coherent rocks. Soil is, therefore, the ultimate end product of all weathering process.
Essay on Soil Profile:
A vertical trough A, B, C and D horizons (Fig. 5.4) in sequence are termed as a soil profile.
Essay on Classification of Soils:
Soils have been classified into two types:
1. Geological Classification of Soils and
2. Engineering Classification of Soils.
1. Geological classification of soils:
Depending upon the mode of formation, soil deposits have been broadly grouped into two classes:
(a) Transported soil deposits:
The weathered and broken ROCK material are eroded and transported from one place to another by the natural agencies such as wind, running water, ice or glaciers or gravity. The deposits of soil formed in this manner are called transported soil deposits.
Such soil generally has no relation with the underlying rock mass. The transported soil has been classified according to the nature of transporting agency responsible for the formation as given in the Table 5.1.
(b) Residual soil deposits:
In plain regions the rock weathering continues to accumulate in place over the parent rock masses and gives rise to residual deposits. As the weathering action decreases with depths such soil deposits gradually change from soil and the surface of broken rock fragments and merge with fresh rock beneath. The most common examples are lateritic deposit of West Coast of India, Terra-Rosa, Peatboghs, Bauxite deposits, etc.
2. Engineering classification of soils:
For general civil engineering purposes soil may be classified based on particle size and plasticity properties. The sizes of the soil particles are extremely variable. They range from big boulders to fine clay particles. The nomenclature of the soil particle accordingly to the size is given in the following table. The soil containing mixture of clay and sand is called Loam and the clayey soil having appreciable LIME content is called ‘MARL’.
Unified soil classification is one in which the basis is both grain size plasticity properties of soil and hence it is applicable to any use.
Soil Conservation Methods:
Soil is one of the reusable natural materials of the Earth’s crust. Excess soil erosion converts fertile land into barren land. Methods by which soil is protected from erosion are termed ‘soil conservation’.
Soil conservation in the catchment region is very essential as the transported soil is deposited in reservoirs, dams, tanks, fertile lands in towns and villages and, etc. Mainly, there are two soil conservation methods.
1. Engineering soil conservation method, and
2. Agronomic soil conservation method.
1. Engineering soil conservation method:
In this method the following practices have to be carried out to regulate and minimize the rate of runoff water.
i. Contour trenching:
Excavation of trenches is along a particular contour level across the slope of the uncultivated wasteland in the top portion of the catchment area. This method is mainly adopted in catchment area consisting of hills and valleys, forest wasteland, etc., contour trenching mainly done to control the surface run off and to protect the contour bunds in the lower regions.
ii. Contour bunding:
Construction of small bunds across the slope of land along the contour level is called contour bunding. These bunds split the area into small strips and precipitation falling on the soil between the two contour bunds is retained, eliminating run off.
This helps in filtering of water and consequently increases in the rise in water table. The height of contour bunds depends on the slope of the land, the space between the two contour bunds and maximum intensity of rainfall at any particular time.
Terracing is essentially a process of constructing a series of drainage channels across the slope of the hill side, so that the runoff water may collect before it attains harmful velocity or volume and it is conducted gradually to an erosion-proof outlet.
iv. Steep slopes:
A ridge or an earth embankment is constructed across the slope of suitable field locations to intercept run off and minimize the rate of soil erosion. This is practiced in Assam and Niligiri Hills tea estates.
v. Gentle slope areas:
Gentle sloping land is divided into a series of parts and horizontal terraces are made generally to a height of about 50 to 60 cms. Flat land is utilized for cultivation of paddy. Helps in regulating the soil erosion.
vi. Gully plugging:
Gully plugging is undertaken generally in unconsolidated rock formation and alluvium sediment areas. The gully are caused due to improperly located roads, poorly maintained terraces, lands, etc. stream velocity has to be controlled by erection log dams, boulders dams, bamboo dams, etc.
vii. Nullah bunding:
Precipitated water flows from higher altitude to lower altitude in the hilly region. The run-off water carries eroded material from the sides of the river valley. Soil erosion depends upon the velocity of the stream water, gradient and nature of the rock type. The soil cover banks of rivers should have bunding on either side to protect the river system. This reduces soil erosion.
2. Agronomic soil conservation method:
The following measures are adopted to protect topsoil erosion:
i. Contour farming:
In this measure, crops are cultivated along the contour planes, so that it controls erosion of soil by holding or rather arresting run off due to increase of absorption.
ii. Rotation of crops:
Various crops are grown in the same agricultural field with definite scheme of rotation depending upon soil conditions. First, a cultivated crop is grown followed by small grain variety crop, then grass and again a cultivated crop. The sequential orders of agriculture minimize soil erosion.
Afforestation result in depletion of trees and consequently the rate of soil erosion increases. Afforestation measures help plant roots penetrate into sub soil zone and firmly fix soil particles in the ground. Fallen leaves from timber yielding trees on the ground minimize the rate of run off.
iv. Cover cropping:
Cover cropping on the soil area reduces soil erosion because of the fact precipitation intercepted, minimizing the force of rain water before they reach the soil. This method mechanically abstracts the flow of run-off water and increases filtration.