Soil Erosion

Introduction

Soil and water are most essential for the growth and sustenance plant life. Soil is important as it provide foothold for plants and majority nutrients needs by them. Alongwith soil, water is another important factor essential for all life and production of food. If rainwater not conserved properly will not only cause scarcity and famine but also wash away the soil, which is a valuable national asset. It is therefore the prime responsibility to conserve soil, which is the main capital of the farmer as well as the nation, at all costs. Soil and water conservation involving collective efforts on the part of farmers, technicians and government. Recognising the seriousness of erosion problem, the central govt. established the Central Board of Soil Conservation to assist the states and River Valley Projects. It takes centuries to form one-inch layer of soil but does not take long to lose it by erosion. In Maharashtra, over 70% of the cultivated land has been affected by erosion in varying degrees and 32% of the land have been highly eroded is no longer cultivable. The denudations of forests and vegetation have resulted in floods, which destroy good agricultural land.

Types

Soil erosion is the wearing away, detachment and transportation of soil from one place to another place and its deposition by moving water, blowing wind or another causes.

• Normal or geologic erosion

Weathering of parent rock and erosion are natural processes by agencies like water and wind. There is always equilibrium between the removal and formation of soil. There is not many harms done unless the equilibrium is disturbed by some outside agency.

• Accelerated soil erosion

The removal of the surface soil from areas denuded of their natural protective cover, grazing of grasses, excessively ploughed the land and expose it to nature accelerates erosion by removing top soil.

• Wind erosion

It is caused by strong wind mainly in arid and desert areas. Wind erosion causes dust storm forms sanddunes and buries localities with deposition of sand. Thus fertile lands are rendered unfit for cultivation. It is more common in Rajasthan.

• Water Erosion

If the rain occurs in torrents there is not enough time for the water to soak through soil and it run off causing erosion. Soil erosion caused by water can be distinguished in following forms:

1. Splash Erosion

It is caused by the falling torrential rain. With this falling the rain drop beat the soil surface into flowing mud which splashes as much as 60 cm. high and 150 cm. away.

2. Sheet Erosion

The uniform removal of a thin layer or ‘sheet’ of soil from entire area, takes place in this type of erosion. Sloping land having a shallow loose topsoil overlying a compact subsoil are the most susceptible to sheet erosion which continuously make the soil shallower and decrease crop yield. It can be detected by the muddy colour of the run-off from the fields.

3. Rill Erosion

It is the removal of soil by run off water through small finger-like channels. It is an intermediary stage between sheet erosion and the gully erosion.

4. Channel or Gully Erosion

As the volume of concentrated run off increases and attains more velocity slopes, it enlarges the rills into gullies are the most spectacular evidence of the destruction of the soil and often starts along bullock cart tracks, cattle trails and burrows of animals. At an advance stage, gullies result into ravine soils and make the soil unfit for cultivation.

• Land Slide or Slip Erosion

This is caused by the pressure of moisture going deep into the soil during heavy rains which being unable to go down further due to hard soil or rocky strata below, move down a big mass of overlying soil on the deep land. Such land slides are more common in ghat areas.

• Stream Bank Erosion

Rivers and streams meander and change their course by cutting one bank and depositing sand and silt on the other. During floods, there is considerable damage and large masses of soil, boulders and plants are carried away and deposited down stream. These deposit in turn reduce the transporting capacity of the torrent resulting in overflows and the meandering of the course and in the erosion of the banks.

Factors Affecting Soil Erosion

• The amount and intensity of rainfall and wind velocity

The rainfall is most forceful factor causing erosion through splash and excessive runoff. Runoff that causes erosion depends upon intensity, duration, amount and frequency of rainfall. Rain occurred in torrents, as usually in monsoon cause runoff resulting in erosion.

• Topography with special reference to slope of land

The speed and the extend of runoff depend of the slope of the land. According to the laws of hydraulics of four-time increase in slope doubles the velocity of flowing water. This doubled velocity can increase the erosive power four times and the carrying capacity by 32 times.

• Physical and chemical properties of soil

The erodibility of the soil is influenced by its texture, structure, organic matter, nature of clay and the amount and kind of salts present. There is less erosion in sandy soil because water is absorbed readily due to high permeability. As organic matter decreases the erodibility of soil increases. Fine textured and alkaline soils are more erodiable. Clay particles are more difficult to detach than sand, but are easily transported on a level land and much more rapidly on slopes.

• Ground cover, its nature and extent

The presence of vegetation, retards erosion as it intercepts the erosive beating action of falling raindrops, retard amount and velocity of surface run off, permits more water flow into the soil and creates more storage capacity in the soil.

Damage Caused by Erosion

• Washing away of fine soil

The top 18 cm of soil is most important from the point of plant growth. If the top soil is washed away by erosion, the water holding capacity of the soil is decreased and productivity goes down.

• Deposition of coarse material in low lying areas

Low lying areas are exposed to the danger of deposition of coarser particles which are washed from higher hilly areas, this makes the soil less productive.

• Silting of tanks

Tanks get filled every year during monsoon season by water from catchment area. This water also brings with it large quantities of silt and clay. If the proper care is not taken, reservoirs get silted and their storage capacity is considerably reduced.

• Lowering of the underground water table

If surface run off is allowed to go on unchecked, the quantity of water that should infiltrate into the soil is very much decreased.

Soil and Water Conservation Methods

To minimize loss of soil and water and to cultivate land without much harm to the soil, following agronomic and mechanical measures are followed:

1. Agronomic Measure

1. Strip cropping

This consists of growing erosion permitting crops and erosion resisting crops in alternate strip. The soil which flows from the strips growing erosion permitting crops is caught by the alternating strips of erosion resisting crops. Erosion permitting crops- cotton jowar, bajra, etc. The erosion resisting crops- groundnut, matki, hulga (Dolichos biflorus), soybean.

2. Mulching

A mulch is natural or artificially applied layer of plant residues or other materials on the surface of the soil with the object of moisture conservation, reduction of runoff and erosion and soil losses e.g. jowar or bajra stubbles, paddystraw or husk, sawdust etc. The quantity of mulch use @ 5 tonnes per/ha.

3. Crop rotation

Rotation means growing a set off crops in a regular succession over the same field within a specified period of time. Continuous growing jowar or bajra crop causes more erosion, but if followed by a legume crop namely Hulga, Matki or Gram which covers the soil it causes less erosion.

4. Contour cultivation

Tillage operations viz. ploughing, harrowing, sowing and Inter Cultural should be done across the slope of land this will help creating obstruction to the flow of water at every furrow, which acts like a small bund and result in uniform distribution of water, less runoff and erosion.

5. Planting of grasses for stabilizing bunds

Grasses prevent soil erosion and improved soil structure. Several grasses as well as legumes were tried on bunds should give maximum root growth and canopy coverage and stabilize bunds effectively e.g. anjan, marvel-8, rhodes, thin napier, blue panic, kusal.

6. Planting of trees and afforestation

Forests conserve soil and water quite effectively. They not only obstruct the flow of water, but the falling leaves provide organic matter, which increases the water holding capacity of soil.

7. Cashewnut plantation

In coastal districts of Maharashtra, which receive more than 1250mm. Rainfall, cashewnut plantation has been undertaken on hills having slope between 10 and 20 p.c.

1. Mechanical Measures

These measures requiring engineering techniques and structures.

1. Bunding

(i) Block bunding: It was not uncommon to found tals i.e. big bunds across large blocks of sloping lands. These bunds are constructed of earth or stone or both to impound water and arrest soil washed from the fields lying above.

1. Contour bunding

It consists of construction of a series of earthen bunds of suitable sizes along contours at a lateral distance of every 60m, or a fall of 1 to 1.5m.The slope of land is thus broken into smaller and more level compartments which hold soil as well as rain water. The size, cross-section and interbund spacing depends upon the nature of rainfall, soil and slope of the area.

2. Graded bunding

In high rainfall areas, drainage of surplus water has to be attended to, for avoiding waterlogged conditions of soil. The bunds are therefore slightly graded longitudinally about 7.5 cm, per running 33 m to prevent safe disposal of water into the nala. For safe removal of excess runoff water it is essential to provide suitable waste weirs or outlet structures at proper places so that no damage is done to bunds e.g. Stone outlets, Channel weirs or pipe outlets in low rainfall area,

Grass outlets in heavy soil.

b. Terracing

It is suitable on bigger slope upto 10 p.c. and rainfall is higher than 1250mm. Terrace bunds consist of comparatively narrow embankments constructed at intervals across the slope and the vertical spacing between bunds may varied from 1 to 2 m., depending upon the slope, types of soil, rainfall etc. Bench terracing is done when gradient is stripper than 10 p.c. as in hilly ranges of Himalayas, Sahyadri etc. These terraces are like table tops slopping outwards and provided with stone waste weirs to drain away surface water.

c. Gully or nala control

The slopping sides are planted with grass and trees to prevent its extension and further destruction of cultivated lands and grassland. Small gully can be stabilized by converting them into paddy fields e.g. Check dams, Overflow dams and drop structures.

d. Control of stream and river banks

This should be protected by providing spurs, jetties, rivets and retaining walls. Adjoining areas should be stabilized under permanent vegetation.