How to Control Soil Erosion: Agronomical and Engineering Measures

In this article we will discuss about how to control soil erosion using agronomical and engineering measures.

Agronomical Measures for Controlling Soil Erosion:

Soil conservation is a land preservation technique, in which deterioration of soil and its losses are conserved by using within its capability and applying conservation measures for protection as well as improvement. In hilly regions, where land topography has steep slope and is subject to erosion problem, the vegetation cannot get established.

Lack of vegetative cover on sloppy soil surface accelerates the erosion rate and a large amount of soil is transported into the stream/reservoirs through runoff. In addition, the uncovered sloppy land also causes extensive damage to the cultivable land at foothill through deposition of sediments on them. Sediment deposition covers the top fertile soil layer, and thus makes them unsuitable for cultivation.

Under above circumstances, it becomes very essential to treat such lands by adopting appropriate agronomical measures, so that they can be re-clothed with vegetation’s. The vegetation helps in reducing the overland flow/surface runoff and soil erosion both. The agronomical measures include contouring, strip cropping and tillage practices to control the soil erosion. The use of agronomical measures is entirely dependent on the soil types, land slope and rainfall characteristics.

In soil and water conservation programmes, the agronomical practices are counted as first line of defence, the second is mechanical or engineering measures which are employed to check the soil erosion, immediately. The role of agronomical measure is more economical, long lasting and effective. Always, it is advisable to use, but when its use is either inadequate or not sufficient to achieve the goal of erosion control, then use of mechanical measures is recommended.

The agronomical measures are referred by the practices of growing vegetations on mild sloppy lands to cover them, and to control the erosion from there. The lying vegetation on the soil surface dissipates the erosive power of erosion agents, either they are water or wind.

In case of water erosion, it affects the erosion by several ways, such as by enhancing infiltration rate and reducing the runoff volume; by creating surface roughness and thereby reducing runoff velocity to scour the soil particles; screening of the eroded particles to reach them into the channels or reservoirs; dissipating the kinetic energy of falling raindrops and thus reducing the splash erosion.

The effect of vegetation on wind erosion is also very significant, as it directly makes a hinderance in blowing path, and thus deflecting the wind current at some distance away towards down-stream side. The wind-strip cropping is a well-known agronomical practice, used for controlling the soil erosion in wind erosion susceptible areas.

The role of agronomical measures for soil & water conservation has immense importance, perhaps much more than the others. It can be explained by considering the Universal Soil Loss Equation (A = R K L S C P), in which agronomical practices reflect the factor of crop management (C). The other factors such as R & K are the nature dependent factor, we do not have any control on them. The P factors may have value as 1 under worst conditions; however, it can be reduced maximum up to 0.5 by applying an ideal soil and water conservation measures.

Similarly, the factor ‘C’ also has value as 1 for worst conditions, but it can be reduced up to 0.02. At this small value of C, the soil loss can be minimized up to one-fifteenth, which is about 11.25 times more than the other factors. Looking this important effect of agronomical measures on soil loss, its importance is assumed to be more promising in soil and water conservation programmes.

1. Contour Ridging:

As compared to the simple contour cultivation, the effectiveness of contour ridging is very high. In addition, the reduction in soil erosion is up to about 30% on flat-tilled plots with slope ranging from 1 to 8%. However, its effectiveness gets decrease with increase in the slope steepness of the land surface.

On very steep slope, an intense rainfall can break the ridges; and there is also the possibility of development of gullies or landslide. Development of all these effects depends very much on the nature of sub-surface soil, also.

If the surface horizon is sandy and very permeable, and sub-surface horizons are very less, then there is more possi­bility to get occur the above-mentioned effects. Proper solution of them is the formation of ridges at suitable contours. The formed ridges intercept the flowing rainwater and slow down its velocity. This leads to checking of soil erosion, and simultaneously deposition of eroded soil particles (if there) on the soil surface.

Another solution may be the contour tied ridging, in which a series of pans and ties are constructed perpendicular to the ridges, to prevent the water behind the ridges from falling through the breaches and creating gully, thereby. From experiment, it has been reported that about 30 to 60 mm of water can be trapped in the pans together with heavier sediment deposition. For better result the ties should be constructed at the interval of 1 to 5 m.

The contour ridging has been found very effective to reduce the soil erosion up to about 10% of the normal erosion. But has limitation that it is only suitable to those soils, which are very permeable for a considerable depth.

2. Conservation Tillage:

The term “conservation tillage” includes the reduced tillage, minimum tillage, no-till, direct drill, mulch tillage, stubble-much farming, trash farming etc. In several countries, particularly in USA and Australia, the use of conservation tillage has been recommended to follow on cropland, strictly.

However, due to its significant benefits, this technology is now in wide spread use in several other countries. As for as its use regarding soil conservation it has been found well suitable, mainly for highly mechanizes farming; and alos for controlling wind erosion from the areas where there is large-scale mechanized cereal production in practice. However, it is less suitable to low input based crop production, or subsistence agriculture.

The conservation tillage is also found very effective to develop good cover of crop residues by using vigorous vegetative growth producing crops in the combination of conservation tillage operations. This technique also offers advantages regarding elimination of construction of terraces, bunds or other permanent structures to check the soil erosion from highly sloping lands. However, there are few disadvantages, too, which normally make this technique not favorable for use.

Especially, for semi-arid conditions few main disadvantages of conservation tillage are outlined as under:

(i) The dense plant covers may be incompatible for the situation of low moisture availability in the soil.

(ii) Animals may graze the crop residues, allowed to remain over the soil surface for erosion control.

(iii) The planting under mulched condition is not easy for animal drawn planters.

3. Tillage Practices:

There have been developed a number of modified tillage practices; each of them are related to the specific objectives for providing a better soil and water-plant relations, reducing the runoff as well as soil erosion by enhancing infiltration capacity of the soil.

The important types of soil conservation tillage practices are described below:

i. Mulch Tillage:

It is performed either by making the soil surface cloddy or mulched with the help of crop residues. Mulch tillage becomes to be an effective measure to minimize soil erosion and to conserve the moisture, when it is combined with strip cropping system.

This type of tillage is also practiced to utilize the crop residues as mulch and also performing farming operations, simultaneously. It can also be used as a method, to grow the crops, where all or most of the residues from previous crops are left on the soil surface.

The use of mulch tillage offers following profits:

1. The mulch intercepts the falling raindrops over the land surface and thus dissipating their kinetic energy, which result in reduction/or elimination of their dispersing action on the soil structure.

2. The mulch tillage increases the infiltration capacity of the soil by maintaining the upper soil surface, more permeable.

3. The obstacles caused by leaves, stems and roots over the field, reduce the velocity of surface flow; and thus controlling the sheet erosion.

4. It maintains the soil relatively cool and moist, which are essential for good plant growth.

5. Increases the crop yield by developing several conducive effects in the soil.

ii. Minimum Tillage:

Minimum tillage implies the preparation of seedbed with minimum disturbance of soil. It is a general term, covers several methods. One method is to reduce the number of operations by directly smoothing after ploughing without any intervening cultivation, which is usually carried out for making a fine seed bed. It also involves killing of weeds presents in the field, using chemicals followed by secondary tillage practices. Minimum tillage causes greater erosion control and also reduces the tillage cost very much, as compared to the conventional methods.

Objectives of Minimum Tillage:

The feasibility and success of minimum tillage practices depend on the following two main objectives:

1. The first objective is to provide a proper environment for the seed to germinate, and germinated seedlings. It is basically done by performing tillage activities around the seeds or adjacent to the crop row areas.

2. The second objective refers to increase the water intake capacity of soil, and thus to control the erosion. This is applicable relatively for larger areas; the smaller areas are not suitable. To achieve fast absorption of water, reduction in erosion and discourage of weed growth, the minimum tillage has been proved to be very effective, because it creates favourable conditions for water intake, reduction in erosion and removing the weeds.

iii. Conventional Tillage:

The tillage operations such as ploughing, secondary cultivation with one or more disc harrowing and planking as conventional tillage have been found suitable for wide range of soils to control the erosion. The ploughing at inappropriate moisture content produces a rough cloddy surface with variations in the soil surface height ranging from 120 to 160 mm.

The secondary cultivation produces the surface roughness 30 to 40 mm in height, which forms a favourable condition to control wind erosion, especially. The tillage roughness gets reduce over time by raindrop impacts, water flow and wind erosion. The soil loss due to water erosion, gets decrease with increasing surface roughness, according to the following relationship, given by Cogo, Moldenhauer and Foster (1984) –

SL α e^{–0.5 R} … (11.9)

In which, SL is the soil loss by water erosion and R is the surface roughness caused by tillage practices. This equation demonstrates that, how a tillage operation is useful for erosion control. The use of this equation can be extended for studies of effect of different conventional tillage operations on wind erosion.

The equipments such as mouldboard plough and disc plough for ploughing operations and disc cultivator for secondary cultivations are the conventional tillage implements. The most common type cultivator is the chisel type, which can cut the soil up to the width ranging from 50 to 75 mm. Sweep or blade type cultivators are also suitable implements, which consist ‘V’-shaped blade of 50 to 200 cm width.

iv. Listing:

Listing is also a tillage technique used for controlling the soil erosion. It is preferred in the low rainfall areas where large amount of rainfall is received from the short and intense storms, and field permits the use of contour farming along with erosion control measures. Listing is done with the help of lister/planter (2-row or 4-row). Sometimes, the planters are modified by using furrow openers, to act as mouldboard plough, called mouldboard listers.

Hard Ground Listing:

When listing is done without ploughing is called hard listing.

Loose-Ground Listing:

The practice in which soil is ploughed first and then listing is done is called loose-ground listing.

4. Crop Rotation:

Crop rotation greatly affects the soil erosion/soil loss by water and wind, both. In the areas, where soils are highly susceptible to erosion, the land management practices such as zero tillage and reduced tillage can be supplemented to the specific crop rotation to reduce the raindrop impact, particles detachment, transport, and generation of surface runoff and soil loss.

The protection against soil loss can be made more effective with the adoption of suitable crop rotation. In general, that crop rotation is found more effective, in which a large amount of crop stubbles (plant residue left after harvest) are left on the top soil surface.

The stubble cover over the soil surface is very effective to minimize the soil erosion from water by reducing the raindrop impact on soil particle detachment and lowering of overland flow velocity. In other words, it reduces the ability of rain water/wind to detach the soil particles and transport the eroded soil mass or sediments. For example – in case of wheat crop harvesting a part of its plant stem is left uncut, is called stubble. These stubbles remain over entire field, and provide very conducive effect on soil erosion control from turbulent wind blowing.

The crops producing very little stubble mass should not be preferred for growing, because they are in­capable to hinder the soil erosion. For example, the sunflower typically produces less than 40% soil cover after harvest in the form of stubble, as result there is significant percentage of soil surface is left for soil erosion. But if at the place of sunflower the wheat crop is taken, then soil surface gets covered with large percentage of stubbles, as result there is less soil erosion.

A corn-soybean crop rotation in no till system results good control on soil erosion. The corn plants produce substantial residue mass after harvest, while soyabean is less residue producing plant, but when it is planted after corn, then a sufficient cover from the previous crops (corn) is found available on the soil surface, which is very helpful in checking the soil erosion/soil loss. For reducing soil loss, the mono-cropping low-stubble producing crop should not be grown.

The crop residues added through a particular crop rotation with substantial biomass also improve the soil structure. The stubble cover prevents the disruption and detachment of soil aggregates, as result the macro-pores are remained unblock; and thus there is fast infiltration rate and reduced sheet/overland flow. A significant reduction in sheet/overland flow causes appreciable reduction in soil erosion.

The effect of crop rotation on erosion control also varies with the climatic condition of the area. In regions of relatively consistent climate, a particular crop rotation can produce sufficient plant growth, soil cover; and thus reduction in the soil erosion. On the other hand, in the regions where climate condition is less predictable, and there is occurrence of unexpected rain/drought, a flexible crop rotation is very promising producing good soil cover.

In erratic climatic condition the opportunity cropping system provides adequate soil cover to check the soil erosion. In opportunity cropping system the crops are grown, when adequate moisture content is available in the soil. This type of cropping system produces better soil cover, as compared to the rigid crop rotation.

Apart from above, the crop rotation also affects the timing and length of ground cover. When a field is under fallow condition, and there takes place intense rainfall, then a cropping system which produces long lasting soil cover, is very effective to protect the soil from erosion.

The effectiveness of crop rotation for soil erosion control can be made better by using soil management practices such as zero tillage etc. In zero tillage the crop stubbles remain for longer duration on the soil surface; or in other words the crop stubbles remains till the period when crops cannot be grown. This ulti­mately causes protection of soil from erosion.

5. Supporting Practices:

It involves application of fertilizers to the soil, either to make more fertile or to recover the fertility loss because of different physical actions. Sometimes, the erosion control practices, like cover crops, cropping system etc. are not able to provide best result on erosion control due to poor vegetative growth because of less nutrients in the soil. In such situation, the application of fertilizer plays very significant role to develop abundance vegetative growth.

This point can be made more clear by the example – the grassed water ways and terrace outlets are generally established on low-fertile sub-soils. At such places extra plant nutrients are required to apply for establishment of good vegetative cover. During bench terrace construction the low-fertile sub-soil is often appeared at the terrace bottoms, which makes the bench unsuitable for cultivation.

In this condition, the application of fertilizer helps to bring the yield back to a high level. Similarly, sometimes the lands are so-pronounced by erosion, that the top fertile layer is removed, as result the crop production gets reduced, drastically. In this critical condition, the application of fertilizer also becomes very important to achieve good crop yield.

In the same view, sometimes, the badly eroded and steep land areas are retired from crop production. However, by application of adequate amount of lime and fertilizer, these lands can also be cultivated successfully with permanent grasses/legume crops or other suitable crops. The eroded or steep lands are thus become under erosion control.

The type and quantity of fertilizer application for increasing the yield as well as achieving good cover for better erosion control, depend on the soil types and crops grown. A good kind of fertilizer and its appropriate application rate also has great meaning in this regard, should be taken care very sincerely. The application rate and fertilizer types to be used can be determined by conducting soil tests. The fertilizers like, nitrogen, phosphorus and potassium are primary elements, are most likely to be needed by the majority of crops and soils.

The minor elements such as sulphur, boron, manganese, copper, zinc and molybdenum are also required to apply, occasionally. Application of lime is an oldest practice for soil management. It not only rectify the acidity problem, but also adds calcium content into the soil and thus helps to make calcium availability in the soil. Few crops very well respond to the lime; among them the legumes are especially Lime-Loving crops. This is the reason that, when legume crops or grasses are grown to erosion control, the application of lime is necessarily done for assuring good vegetal stand and growth.

The use of organic manure is also very important to maintain an appropriate level of organic matter in the soil, particularly where low-residue producing crops are grown or where residues are largely removed from the land during crop harvesting. The nutrients availability in the manure varies widely; it depends on its source and amount of straw or other materials mixed in it.

Inter Planting:

It refers to the seeding of grasses/legume crops in combination of maize or other crops, to achieve better result on erosion control. Apart from erosion control, the creation of source of establishing the hay or pasture crop is also done by inter planting.

The use of this practice (i.e. inter planting) is observed as important, when lands are grown with the maize crop to check the erosion problem, because growing of maize crop solves the purpose of erosion control and also provides hay for cattle feed. In this cropping system, after harvesting of maize the land does not become clean, but in the meantime the grass/legume develops vegetative growth and covers the soil, and so forth the erosion is also checked.

The inter planting should be based on following points:

i. Prepare the seed bed for maize sowing in usual way, using minimum or mulch tillage method.

ii. Plant the maize seed in the row at the interval of at least 150 cm.

iii. Use secondary tillage operation once or twice for removing the weeds from the crop. During this operation keep the land smooth; do not make ridge around the maize plant.

iv. Make inter-planting, when height of maize plant becomes 25 to 75 cm. Broadcast or drill the grass/legume seeds and cover them using suitable means.

v. Apply fertilizer of appropriate type and quantity, both, to the maize and inter-planted grass or legume crops.

vi. Remove the weeds from the field using weeding equipments. It is generally done when weed population is so high, that the inter-planted grass or legume crops are not suppressed; and thus not to affect the vegetative growth.

Engineering Measures for Controlling Soil Erosion:

1. Bunding:

It is an engineering soil conservation measure, used for retaining the water, creating obstruction, and thus controlling the soil erosion/soil loss. Bunds are the embankment like structure, constructed across the land slope. When they are constructed on the contour of the area, are called contour bund; and when grade is provided to them, then they are known as graded bund.

In bunding practice the entire area is divided into several small parts, thereby the effective slope length of the area is reduced. The reduction in slope length causes not only to reduce the soil erosion but also retain the runoff water in the surrounded area of the bund. Contour bunds are used in low rainfall areas for the purpose to control the soil erosion and to store the rain water, while graded bunds are constructed in relatively medium to high rainfall areas for the same purpose as the contour bunds.

Generally, bunding practice is found most suitable for the lands having the slope from 2 to 10 percent. However, they can also be used beyond 10 percent land slope but there must be a close spacing of bund, which results into high cost of construction.

Several experiences on bunding practice in our country revealed, that the bunding is reasonably successful in the shallow and medium-depth soils. Black soils are not suitable for bund construction. The deep black soils are generally cracked in hot weather seasons, is one of the main demerits regarding bund construction. Table 12.1 enlists the soil classes, suitable for bund construction. Various soil groups are also shown in Table 12.2.

In the state of Karnataka for the purpose of bunding the soils are classified on the basis of their depth suitable to bund construction, except for some red soils, is shown in Table 12.2. The shallow, medium and medium-deep black soils are quite suitable for bunding project, but deep-black soils are not being suitable. In addition, very shallow soils which depths are less than 7.5 cm, are also not found suitable for bund cons­truction.

In Tamil Nadu the contour bunding is practiced in all types of soil, except clay and black cotton soils. The red and brown soils are found suitable, especially for the areas receiving less and erratic rainfall.

The suitability of soil for contour bunding in Maharashtra has been recommended for the land having average slope greater than 0.5% and maximum up to 6% for scare zone of cultivated areas with well drained soils and high rainfall. Likewise, in Gujarat the contour bunding is followed in dry areas with the slope ranging up to 6%.

In U.P. the lands having slope up to 6% and receiving the annual rainfall less than 100 cm are recommended for bunding works. In addition, in U.P. the contour bunding is also done for the purpose of water spreading on gentle cultivated slopes, with favourable soil depth and permeability. In Kerala the contour bundings are practiced on the lands which have slope up to 15%, with the vertical interval of 1.5 to 2.0 m.

2. Contour Trenching:

The land capability classification revealed that the class V to VIII has severe limitations, by virtue of which they are not suitable for cultivation. The limitations may be in terms of steep slope, severe erosion, stoniness, rockiness, shallow soil depth, wetness, flooding etc. Such lands are referred as non- agricultural lands, denuded land or waste lands, which cannot be easily converted into cultivable form.

However, the practices like contour or staggered trenching on hill slopes, nalla plugging, installation of drop structures in narrow and deep gullies, contour stone walls on steep slopes of hilly areas, retaining walls for stabilizing precipitous hilly slopes etc., can be adopted to achieve some returns from them.

The contour trenching is one of the practices, used as a soil conservation measure to intercept the runoff. In this system, several trenches are excavated along the contour in a uniform level across the land slope at the top portion of the field. The excavated earth materials are placed at down-stream side of the trench, to make bund like/Structure along that. The function of contour trench is to break the runoff velocity and intercept the runoff.

The rain water collected into these trenches is slowly and slowly percolated into the lower soil strata which spreads horizontally or laterally, as result the soil of the area gets enriched with moisture content. In addition, if the lower portion of catchment is under bunding system, then contour trenches also protect the bund against breaching by the runoff generated from the upper part of the catchment.

A contour trench involves following details:

1. The cross-sectional area should rarely exceed 0.3 x 0.3 m². It should be designed to collect and convey the runoff, expected from the inter-space area of two successive trenches. This space acts as catchment area to determine the trench size.

2. The trenches are arranged in staggered form to make the system more effective.

3. Trenches should perfectly be level to use their full capacity.

4. The side slopes of the trench are used in the range of 1:1 to 0.5:1, depending on the nature of soil.

5. The contour trenches are excavated at suitable intervals, as per suitability of the land slope.

6. The height of bund is fixed, using the same concept.

3. Terracing:

It is engineering or mechanical measures, used to control the soil loss at highly sloping lands. Terracing involves the construction of embankment or ridge and steps like structure, across the land slope to check the flow of surface runoff, and to reduce the soil loss. In this system, the effective length of land slope is reduced to a large extent.

From experimental evidence it has been found that the soil loss is proportional to the slope length of power 0.5. According to this statement, if the length of slope is increased twice then the soil loss gets increase in proportion of 1.4 times. In addition, terraces also play an additional role in trapping the splashed soil particles and depositing them over the bench.

Terracing practice is not feasible in flat and cropped lands. On such lands, if there is any chance to occur the soil loss, then simply agronomical measures are adopted to control that.

The agronomical measures includes the practice of contour cropping, crop rotation, strip cropping etc., but for the land which is more erodible, has steep slope and prevail high rainfall intensity, the use of agronomical measures is not justified; only terracing is an alternative measure.

In brief, the terracing involves following main features:

i. These are constructed across the slope to intercept the surface runoff and convey it to a suitable outlet with non-erosive velocity.

ii. They reduce the length of slope by splitting the slope length in different parts.

iii. Terracing practice is adopted for soil and water conservation in that area, where land slope is greater than 10%; soil is more erodible and prevails high rainfall intensity.

iv. Terraces, not only control the soil loss caused by sheet flow, but also trap the splashed soil particles.

v. This practice is not feasible, particularly on those hill sloped lands, where soil depth is not sufficient.

vi. Since, it involves huge earthwork, therefore it proves a costly work.

4. Grassed Waterways:

“The grassed waterways and diversions are the means to drain/divert the runoff from the catchment.” Grassed waterways are the natural or man-made water courses, lined with erosion resistant grasses, used to dispose the surface runoff water from the bunded/terraced area. The use of grasses in the waterway section acts as a lining material to control the problem of soil erosion caused by turbulent runoff flow through the section. The grassed waterways are constructed along the slope of the area.

Apart from disposing the runoff from area, they also act as an outlet for the terraces or graded bunds. Waterways are very important means for removing surplus water from the terraced field, and for erosion control.

In addition, they are also used for other purposes such as to handle the natural runoff or to carry the discharge from general fields, contour furrows, diversion channels, or used as emergency spillway in farm ponds or other water storage structures. The grassed waterways should be fully established with grasses before water is turned into them. In other words, the waterways should be ready to hold the water before the bunds, terraces, or diversions etc. are constructed.

Site Selection Criteria:

The important criterion followed for selecting the sites of waterways or grassed waterways construction are outlined as under:

i. The land slope should be less than 20%.

ii. Where flow velocity does not exceed 1.8 m/s.

iii. The other types of waterways can also be constructed on steeper slopes; and also where flow velocities are very high.