This article throws light upon the top nine biological methods for controlling soil erosion caused by water. The biological methods are: 1. Contouring 2. Strip Cropping 3. Mulching 4. Crop Rotation 5. Fertilization and Maintenance of Soil Fertility 6. Cropping Systems 7. Stream Bank Control 8. Silt Retention Dam 9. Farm Ponds.
Biological Method # 1. Contouring:
The term contouring, also referred to as contour farming, applies to the tillage practices or applying all treatments on the contour in agriculture pasture or range land. Contour farming helps in conservation of rainfall in low rainfall areas because each row acts as a barrier to the flow of water.
In high rainfall areas this reduces soil loss by holding a part of rain water and allowing only small volume of rain water for surface runoff. Since contours make a small embankment which store rain water at or near the point where it falls, they have a great value in controlling floods in small watersheds.
The practice of contouring on 2-4% slopes increases crop yield by 10% and reduces soil loss by 50%. Contour farming on alluvial soils having 2.2% slope at Kanpur has conserved 11.3 kg of N, 11.5 kg of P2O5,44.4 kg of K2O, 398 kg of CaO and 118 kg of MgO in one season only.
Contouring reduces soil loss irrespective of length and degree of slope. On 4-6% slopes with proper rotation, it controls erosion on length up to 100 m. On steeper slopes, the maximum length on which contouring is considered effective is 60 m on 8% slope, 30 m on a 10% slope, 25 m on a 12% slope and 20 m for steeper slope.
Layout of Contours:
The first line of the contour should be laid out at eye level which generally would be 1.5 m vertically below the top of hill. The first line is generally laid out with 0.5% slope towards waterways. Crops should be planted up and down-hill from the line.
Rows should normally be planted on contours but allowance for deviation from contour may not be given more than 3%. If deviation occurs more than 3%, a new line should be established about 1.5 m vertically down-hill. A turn strip should be provided at the end of contoured rows. This should be seeded to a suitable perennial grass as it provides a convenient place for turning farm machinery.
Biological Method # 2. Strip Cropping:
In strip cropping crops are grown in strips or bands at right angles to the slope of the land. Row crops (erosion permitting crops) are alternatively planted with close growing crops (erosion resisting crop).
Thus alternate strips of erosion resisting and permitting crops reduce the speed of runoff water coming from erosion permitting strips, as a consequence, the runoff water enters the next strip with reduced speed and its velocity is further reduced with its movement in farther resulting into less soil loss.
Moreover, a larger proportion of water becomes available to the crop as strip cropping makes it possible for water to remain for longer period of time in the field.
The strip cropping controls erosion in two ways:
(i) By slowing down of runoff water flows through the close growing strip; and
(ii) By increasing infiltration rate, which reduces total runoff volume.
There are 4 types of strip cropping, however the most common types are shown in Figure 5.11.
(a) Contour Strip Cropping:
In this system of cropping fields crops are grown across the sloppy lands, in regular long and narrow strips of variable width, alternating an erosion permitting crop with an erosion resisting crop. Through it is describe to plant the crops on the contour, but sometimes it is difficult to make parallel strip exactly on the contours because of unevenness of slopes.
Experiments conducted at the dry farming research station, Sholapur, Maharashtra, have shown that groundnut, moth bean and horsegram are more suitable crops for cereals such as Jowar and Bajra is 21.6 m and for legumes 7.2 m. The strip width for erosion permitting and resisting crops on different slopes are given in Table 5.11.
(b) Field Strip Cropping:
In field strip cropping, farm crops are planted in more or less parallel strips across fairly uniform slopes, but not exactly on contours. This type of cropping is practiced on the soils of high permeability with uniform slopes or on the undulating land without well-defined slopes.
It is quite effective on regular slopes, however, on irregular slopes, field strip cropping cannot be substituted for contour strip cropping. A good crop rotation and cultural treatments are, therefore, essential for protection to the soil.
(c) Buffer Strip Cropping:
These are the strips of permanent cover crops and usually located on steep and badly eroded areas that do not fit into a regular crop rotation. The strips are generally planted with perennial legumes, grasses or shrubs on a permanent or temporary basis.
The strips are meant to take care of critical slopes (steep or highly eroded) in fields under contour strip cropping. This type of strip cropping is not as much effective as contour strip cropping but will serve as temporary measures until a more effective system is devised.
(d) Wind Strip Cropping:
It consists of planting tall growing crops such as jowar, bajra or maize and low growing crops in alternatively arranged straight and long but relatively narrow, parallel strips laid right across the direction of prevailing wind.
The following points should be kept in mind while laying out a strip cropping system of cultivation:
(i) It must fit into the cropping pattern prevalent in the region.
(ii) It should work as an effective cover to control soil and water erosion.
(iii) Strips should be as nearly on contour as practicable.
(iv) Soil, slope, rainfall and degree of erosion should be taken into consideration in determining the width of strips.
Biological Method # 3. Mulching:
The value of mulch in erosion control is well recognised and is of special value for the periods when the crop itself does not adequately protect the soil. Mulches are more effective on land capability classes II, III and IV. Covering soil surface with crop residue can reduce soil loss by 50% or more depending upon the amount of rainfall and mulch, soil type and other characteristics.
Various types of material used for mulches are straw, stubble, Stover, sawdust, woodchips and shaving paper. Plastic films are also occasionally used as mulch. However, it would be desirable and economically feasible to grow mulch in place where it is to be used as mulch. Leaving crop residues on the soil surface protects the soil.
It dissipates the force of falling rain drops and reduces splash erosion, reduces surface sealing and gives water a better chance to be soaked into the soil. The dead residues are absorptive and help to hold considerable water at soil surface. Furthermore, the mulches tend to retard the soil surface flow which permits more time for water to infiltrate the soil.
The amount of mulch added to the soil should not suppress the growth of most plants. This quantity corresponds to 1.5 to 2.0 tonnes of straw or 4 to 5 tonnes of farm yard manure. Besides the chemical consideration of decomposition of mulches, the physical conditions also favour for an early application of mulch. If soil surface is bare at the time of seeding a crop, mulch should be applied immediately to protect the soil from erosion.
Early mulching proved advantageous, since the structure of soil improves progressively after the mulch is applied. Mulches increase soil moisture because they improve infiltration of rainfall and slow down evaporation from soil surface. It helps to improve yield, if moisture becomes limiting in dry years or dry farming areas.
Biological Method # 4. Crop Rotation:
The crops in a rotation may be cereals, legumes, a grass or legume or grass- legume mixture. Row crops (cereals) expose the soil to maximum erosion, small grain crops allow less erosion and grass legume mixtures effectively control erosion during the period of their use on a given area. Crop rotations also help to maintain soil productivity.
The benefits of rotations in soil conservation include:
(i) Crop rotation provides high degree of protection to the soil thereby reducing soil erosion.
(ii) Legumes include in the rotation leave behind crop residues which add organic matter and nitrogen and improve soil structure.
Crops differ a great deal in their root activity and amount of crop residues added to the soil. The dense and fibrous root system of grasses holds soil aggregates together and creates good soil structure. After the plants die they leave channel through which water can move to deeper layers.
The row crops (intertilled) do not have such effective root system so as to improve soil structure. Repeated cultivation of row crops destroys soil aggregates and reduces the proportion of larger pores. Row crops leave very little residue after harvest and thus are likely to permit serious soil erosion.
Despite the advantages of cereal-legume based crop rotations, it is possible and desirable, to grow the same crop year after year on the same piece of land or to use rotations which are cereal based such as maize-wheat or rice-wheat rotation.
Using land more intensively and maintaining high yield and preventing soil deterioration have been possible due to new technology available to the farmer such as greater use of fertilizer, particularly N, improved tillage practices, increased use of mechanical measures of land erosion control (such as contouring, terracing, strip cropping, etc.).
Biological Method # 5. Fertilization and Maintenance of Soil Fertility:
Soil fertility is kingpin in soil and water conservation which helps to reduce runoff and consequently erosion is automatically minimised. Therefore, use of manures and fertilizers should find important place in any soil conservation programme. A high fertile soil guards it against erosion as it permits quick establishment of crop on such soils than on depleted soils.
A vigorous crop reduces the hazards of erosion by dissipating the impact of falling raindrops, reducing splash erosion, retarding the velocity of runoff water and holding of soil by root anchorage. Better crop improves the absorption capacity of the soil and consequently reduces the amount of runoff and soil loss.
Though close growing crops provide greatest protection to soil, however, regardless of kind of crop, the greatest protection is offered by fast growing, vigorous crop and high yielding crop. And such crops are possible only on fertile soils where adequate manures and fertilizers are added.
The value of organic matter has been long recognised in increasing the soil aggregation. It increases the porosity of soil thereby increased infiltration and percolation capacity of soil which, in turn, reduces the runoff and erosion hazards.
Therefore, the first and the foremost thing in soil and water conservation is to produce large amounts of high quality organic matter. Selection of crops, varieties and ample fertilisation are pre-requisites for providing high quality organic matter for maintaining organic matter content of the soil.
Biological Method # 6. Cropping Systems:
Cropping system is one of the most important means of prevention of erosion, preservation of good soil structure, maintenance of organic matter and soil fertility. Crops vary a great deal in their response to soil erodibility and building up soil fertility due to variation in growth characteristics (root system, height and development of crop canopy).
Based on growth habits, capacity to control soil erosion and conservation of soil moisture, three general groups of cropping systems are recognised:
(a) Soil Building (Soil Conserving-Cum-Soil Building System):
A system of growing crops which not only conserve the soil but also build up the soil fertility by addition of organic matter and nitrogen is termed soil building system. The crops produce dense foliage cover and prevent soil against the action of agencies of water and wind erosion. The crops under this system include green manure crops and grasses.
The following are the characteristics of soil building crops:
(i) These are legumes or grasses which add to the fertility of soil by adding organic matter and nitrogen.
(ii) These crops cover ground quickly and protect the soil even in early stages of crop growth.
(iii) They have less tillage requirement.
(iv) They are close and thick growing crops.
(v) The peak growth period coincides with critical period of rains.
(vi) They have fairly well developed root system.
(b) Soil Protecting System (Soil Conserving or Semi-Soil Building System):
A system of growing crops which require a minimum of stirring the soil is termed soil protecting system. Generally, small grained crops such as wheat, rye, oat and barley are put under this category.
These crops are not turned down into the soil and, therefore, do not enrich the soil. These crops allow some degree of erosion to occur and, therefore, are not true soils conserving crops. There is, however, less soil depletion than where maize, sorghum or cotton is grown.
(c) Soil Depleting System (Soil Exposing, Soil Exhausting, intertilled or Clean Tilled Cropping System):
A crop which causes rapid removal of the essential nutrients and encourages the soil losses due to water and wind for want of adequate soil cover is called soil depleting crop. The crops under this category are jowar, cotton, maize, tobacco, sugarcane and vegetables. These crops leave the soil surface bare during the rainy season.
The characteristics of crops under this group are as follows:
(i) The plants are mostly erect and foliage do not cover the ground fully at any time of the year.
(ii) Plants possess shallow and fibrous root system and do not hold soil together.
(iii) Crops require frequent tillage operations which make the soil more prone to erosion either by wind or water.
(iv) The cultivation in the crop coincides with the peak period of erosion.
(v) The crops are generally grown in wider rows and this favours more erosion than close growing crops.
Any type of cropping system can be used on a soil building rotation on a gentle slope but as the slope of land increases, cropping system for soil conservation becomes exceedingly important, particularly in high rainfall areas favourable for clean cultivation crops.
Page and Willard (1946) observed that land planted with corn every year lead to decrease the productive capacity of the land rapidly when sufficient amount of organic matter was not returned to maintain a favourable physical structure for aeration and moisture movement. The role of green manure and cover needs no elaboration for soil conservation purposes.
The difference between cover and green manure crop from the stand point of soil conservation must be noted here with caution. Crops do serve as cover crops that are not used as green manure. Green manure crops are always cover crops.
In addition to protecting the soil from erosion cover crops offer several advantages such as addition of organic matter and enriching the soil by addition of nitrogen to soil. Also they improve physical conditions of the soil.
The cropping system selected for any region should meet the following requirements:
(i) The chosen cropping system minimizes the erosion, maintains the organic matter and improves the physical conditions of soil, particularly permeability of soil.
(ii) It meets the needs of the farmer.
(iii) Maintains and improves the productivity of the farm.
(iv) It effectively supports the farm business by providing cash need, food and livestock feed and fodder.
Biological Method # 7. Stream Bank Control:
Stream bank is a problem in our country along chaos, small streams, farm brooks and rivers. This problem is further accentuated due to increase runoff because of improper land use and cultivation methods in the catchment.
Cultivation down to the bank of the river or stream makes it more susceptible to erosion. The extent of damage is the greatest along the rivers flowing through alluvial plains. The damage is less in the peninsular India except in deltaic tracts.
To principal types of stream bank protection structures are common in India:
(1) Structures that deflect the river current from the affected bank and thus are helpful in siltation. Such structures are called spurs.
(2) Structures covering the surface and thus reduce further damage like revetment and retaining walls.
The stream size should be trained by tree planting on the flood plain parallel to the banks. Low bunds may be built parallel to the stream flow and planted with suitable grasses. Flood can be confined by plantations such as Nara (Arundo donax) and Banha (Vitex negundo) along the torrent direction. The Kans grass (Saccharum munja) should also be planted in the back of these plantations so as to encourage siltation.
Construction of check dams and spurs may also be taken up along with the bank protection work.
The following points may be considered while constructing a check dam:
(1) The top of the check dams should be concave.
(2) The length, width, cross-section and interval between check dams should properly be designed.
(3) Foundation of the dam should be sound.
(4) The dam should be away from the erodible banks.
(5) It should have provision of maximum discharge without outflowing.
Spurs are the projections to deflect the flow of water. It may be permeable and impermeable. Most obviously the permeable spurs are trees and brushwood. The impermeable spurs are generally made of boulders which should be protected against being washed away by using galvanized wire.
Biological Method # 8. Silt Retention Dam:
The silt retention dams are constructed below the watershed which have small percentage of row crops and protected by terrace or other suitable means of erosion control. The site of the dam would depend upon the purpose it is built. The dams are constructed close to pasture areas and farm buildings if it is to be used as water for live-stock. If it is meant for irrigation it should be near the fields to be irrigated.
The failure of siltation dams may be due to either or a combination of the following:
(1) Large siltation than expected in the dam,
(2) Inadequate provision of spillways,
(3) Organic matter in the fill,
(4) Inadequate storage capacity,
(5) Insufficient compaction of fill materials,
(6) Inadequate drainage area,
(8) Side slope too steep, and
(9) Inadequate fencing.
The life of reservoir would depend upon the rate of erosion from this area. Maintenance of the dam is the most important for long life of the dam. Therefore, structure should be inspected periodically. Spillways should be sodded or reseeded if necessary. A watch on rodent damage is necessary.
Biological Method # 9. Farm Ponds:
The farm pond is a multipurpose conservation structure meant for water storage (for irrigation, livestock, fish production), for protection and recreation. Current interest in irrigation and soil conservation for environmental protection has further added to the importance of farm-ponds.
While constructing ponds for irrigation purpose consideration should be given not only to the capacity of the pond but also to the amount of water that will be available for irrigation excluding the allowances for seepage, evaporation and other losses. As a means of erosion control the pond is effective in reducing the amount and rate of runoff below the structure and this helps in stabilising the channel.
The following points should be kept in mind while selecting the site of a pond:
(1) Enough water should be available.
(2) The size of watershed should be known for collecting known volume of water.
(3) Pond should be located at a site so that adequate storage capacity will be provided with a dam of reasonable size.
(4) The type of soil and its permeability should be considered. Soils of well graded texture is most desirable from the point of view both of reducing seepage losses and providing good material for dam construction.
(5) A natural spillway would facilitate the construction of pond.