Top 5 Measures for Controlling Wind Erosion

This article throws light upon the top five measures adopted for controlling wind erosion. The measures are:- 1. Control on Dry land Cultivated Soils 2. Control on Irrigated Lands 3. Control on Vegetable and Especiality Croplands 4. Control on Grazing Lands 5. Control on Sand Dunes and Other Problem Areas.

Measure # 1. Control on Dry Land Cultivated Soils:

Practices such as stubble mulching and minimum tillage, herbicides, cover crops, strip cropping, crop rotation, wind barriers and shelterbelts, mulches, emergency tillage, etc. have been successfully used to control wind erosion on dry land cultivated soils.

A Brief discussion of each is given below:

a. Stubble Mulching and Minimum Tillage:

Stubble mulching is one of the effective ways to control wind erosion and con­serve soil moisture. This practice is most appropriate with wheat and other small grains, and sorghum. The stubble mulch is particularly needed on sandy soils where a rough surface cannot be maintained. Tillage practices which result in a rough cloddy surface, preferably with trash cover or stubble mulch help to check wind erosion.

Rough tillage traps the saltating particles, decreases wind velocity at the soil surface, prevents soil particles from breaking up into small particles which will drift, and increases percolation rate and decreases runoff.

The quantity of crop residue required to control wind erosion can be estimated from the analysis of the major factors that affect soil blowing. For example, the more susceptible the soil is to movement by wind, the more residue is required to prevent it from blowing.

Large fields require more residue than narrow fields or fields protected by wind breaks and shelter belts. Vegetables and other crops that are damaged by abrasion require more residue than do field crops. Arid areas need more residue than humid areas, the regions of high winds require more cover than those of low winds.

Table 6.1 shows the amounts of vegetative cover required to hold wind erosion to 12.5 tonnes per ha on field with different soil textures in an average semi-arid region where the climatic factor C’ is about 100 during the windy season. These amounts also vary with weather conditions throughout the growing season, dis­ease, insects, etc.

Table 6.2 shows the approximate amounts of vegetative cover required on sandy soils with about 15% non-erodible fractions ( > 0.84 mm diam) in the more humid areas where the climatic factor C’ is about 18 during the windy season.

When row crops such as corn and sorghum are harvested, either the stubble should be kept about 30 cm high or at-least 2 rows in 20 should be left standing. If these crops are pastured, the stalk should be removed while there are still enough stalks with leaves attached to furnish protection.

The main purpose of minimum tillage is to grow crops with fewer tillage operations and thus reduce costs of crop production. Minimum tillage also conserves residue, reduces soil pulverisation and holds down soil losses by wind. Minimum tillage is often practicable with row crops.

Tillage Equipment:

Tillage machinery and tillage practices can aggravate or alleviate the problem of wind erosion. Machines that pulverize the soil or cut down vegetative cover, increase the soil blowing. On the other hand, the destruction of weeds and preparation of seed bed for raising crops, although reduces vegetation, yet can create a cloddy and rough surface to prevent blowing.

Farmers should choose implements suited to his own condition, operate them at the minimum speeds to avoid pulverizing the soil and reducing residue, and limit tillage operations to as few as are consistent with adequate seed bed preparation and weed con­trol.

In general, tillage machines used for stubble mulching and minimum til­lage are those that stir and mix the soil, and those that cut beneath the surface without stirring or turning the tilled layer.

(i) Machines that Stir and Mix the Soil:

The one-way disc is an excellent im­plement for destroying weeds and often is the only implement that works in extremely heavy residue. Under average residue and soil conditions, however, it buries too much residue and if used more than once in a tillage sequence pulverises the soil which blows readily.

Residue burial increases as depth of tillage is increased. For best conservation, the one-way disc should be operated at a 7.5 to 10 cm depth, about 4.5 km/hr, and at as wide a cutting width consistent with good weed control.

A tool that chops and bury residue and pulverise and loosen the soil, promotes wind erosion. However, if in addition to pulverisation and residue burial the machine leaves a rough, undulating sur­face of depressions and small rounds of soil, it partially compensates for residue and clod destruction. Disc harrows should not be used to cultivate bare soils where soil blowing is a hazard.

Field cultivators are reasonably well suited to stubble mulching because they have flexibility of operating depth, tillage point and spacing between shanks. The tillage points may be narrow sweeps, flat shovels or double or single-end narrow shovels.

The field cultivator cultivates fallow, prepares the seedbed, and to a limited extent roughens and brings clods to the surface to stop soil blowing. For deeper tillage than field cultivators chisel ploughs are designed. If residues are not heavy enough to cause clogging, the chisel plough can be an effective primary stubble mulch tillage machine.

The percentage of residue saved on the surface with the chisel generally decreases with increased amount of residues; it increases with greater height of stubbles. The percentage saved ranges from 50 to 80%. Close spacing of chisels effectively destroys weeds and produces a rough cloddy surface.

On crop land, where wind erosion is severe, the spring-tooth harrow is somewhat better than the spike-tooth harrow. The spring took harrow penetrates deeper, brings more clods to the surface, causes more ridging, and destroys more small weeds than the spike-tooth and spade-tooth harrow. Mulch treaders are used to distribute and anchor heavy residues and to destroy small weeds just before drilling.

The residues are punched into the soil and anchored. Rotary tillers, specifically the side-winder tilther, provide good wind erosion control for row crops such as corn grown on sandy soils. The tilther leaves the soil and residue between rows undisturbed protecting seed­lings from sandblasting. When the seeding mechanism is attached to the machine, the tillage and planting is done in one operation.

(ii) Machines that Cut below the Surface:

Sub-surface sweeps and blades are the most effective implements available for conserving residues on the soil sur­face. The implements consist of one or more rigid blades (v-shaped or straight) mounted on a heavy tool bar or frame. The frame may be rigid or flexible to better fit the shape of the land. Blades vary in size from about 75 to 210 cm.

Rodweeders are often used for weed control and final seed bed preparation after initial operation with sub-surface sweeps or one-way discs. Rodweeders also maintain residues on the soil surface. Their main dis­advantages are clogging in heavy residues and a tendency to compact the soil through repeated operation at the same depth.

The tendency to form a tillage pan can be alleviated by placing narrow duck-foot shovels or chisel points ahead of the rod to loosen and fracture the soil below the normal tillage depths. All the implements described above will bury some surface residue depend­ing upon the speed and depth of operation, angle and concavity of discs and width pitch and angle or sweep blades.

Height and length of stubble, amount of pretillage residue and previous positioning or orientation of residue also in­fluences the amount of residue buried. Several investigations indicate that the average amount of residue buried by indicated machines is as given in Table 6.3.

These approximate values may also be used to estimate the amount of residue remaining at the end of a tillage season. Proper planting equipment is most important in controlling wind erosion.

Often the most erosive conditions occur after planting and before the crop is large enough to provide protection. Planting equipment should preserve as much residue as possible, keep the surface rough and cloddy, and at the same time, place seed in moist, firm soil.

Observations indicate that semi-deep furrow, single-disc drills, and deep furrows drills with shovel—shoe or hoe type openers are most satisfactory for planting small grains in mulch.

Deep furrow drills also provide a roughness in the 5 to 12.5 cm range, which is most effective for wind erosion control. Row crops have been most successfully planted in mulch with surface planters equipped with furrow openers, with listers operated at shallow depth, and with till planters.

b. Herbicides:

More residue is conserved for wind erosion control if tillage operations could be avoided. The ultimate aim in stubble mulch or minimum tillage is no tillage where crops are planted directly into the residue of the previous crop. As ef­fective chemicals for weed control are developed, no tillage systems are in­creasingly used.

Herbicides (such as atrazine) are also combined with limited tillage to provide better weed control and, at the same time, conserve as much residue as possible for effective wind erosion control. The effectiveness of her­bicides and the techniques for applying them vary with climatic and soil condi­tions.

c. Cover Crops:

A cover crop is any crop planted solely to control erosion. It is usually planted for protection when regular crops are off the land, but also may be planted in strips or between rows to provide protection for vegetables or other crops high­ly susceptible to abrasive injury in the seedling stage. Wheat, rye, oats, sor­ghums and peas have been used successfully as cover crops.

Cover crops are more suited to humid areas because in drier areas they compete for moisture. In drier areas, the cover crops are used only for control on erosion-susceptible knolls, on land without protective cover, or on cultivated land.

In the first two cases, pasturing should be avoided or extremely limited because the aim is to produce vegetative cover for the land. In the third case, overgrazing must be avoided to retain enough residue to protect the land. Planting beans or sorghum in summer can provide a protective cover for the critical wind period in field where former crops have not left adequate residue and severe soil blowing in imminent.

d. Strip Cropping:

It is generally true that wind with a wide, unbroken sweep carries more drifting material than it does over narrow strip, and, therefore, its sand-blast action is greater on un-stripped land. This seems to be at-least one reason why strip-crop farming assists in control in most of the localities.

In clay soils, drifting fre­quently piles the soil in the stubble at the edges of each strip to as great an extent as on the edge of a large field. Strip cropping on this type of soil, there­fore, has definite limitations.

Strip cropping aids in the control of soil blowing by shortening the distance that loose soil can move. Strips laid out on contour lines also conserve moisture which in turn helps to prevent blowing. To control wind erosion, crop strips are run straight and at right angles to the prevailing winds.

To control water erosion, crop strips follow the contour of the land whenever possible. The rela­tive severity of the two types of erosion determine which way to place the strips. In controlling soil blowing, however, even contour strip cropping is better than planting large areas of a single crop.

Strip cropping does not require any change in cropping practices, and it does not remove any land from cultivation. The field is simply sub-divided into alternate strips of erosion resistant crops and erosion susceptible crops or fal­low.

Strip cropping also requires adequate quantities of crop residues as an additional protection against wind erosion. Strips of crops like pulses (urd, moong, lentil, grain etc.) or sorgham or jowar may be alternated with groundnut or ragi.

Erosion-resistant crops include small grains and other crops seeded closely to cover the ground rapidly. Erosion susceptible crops are tobacco, sugar beets, peas, beans, potatoes, peanuts, asparagus, etc. Cotton and sorghum are intermediate in resistance to wind erosion.

Strip cropping controls soil blowing by reducing soil avalanching. The rate of soil avalanching varies directly with the erodibility of the soil and the width of the eroding field. So one factor that determines the width of the strips is the kind of soil and another the land use.

In the vegetable-growing areas, for ex­ample, buffer strips consisting of very narrow strips of rye, wheat, or grass may be used with wider strips on erosion susceptible land.

A common practice is to make the buffers one-tenth as wide as the erosion susceptible strips. For ex­ample, buffers would be 2.45 m wide if erosion susceptible strips are 24.5 m wide. In drier areas with wheat and sorghum, erosion resistant and erosion susceptible strips are generally equal in width.

Table 6.4 shows average width of strips on different soil textures based on wind velocity of 64.4 km/hr at a height of 15.24 m, wind blowing at right angles to the strips and stubble 30.5 cm high upwind from the erodible strips.

As shown in Table 6.4, width increases as soil texture becomes finer, except for clays subject to granulation. Granular clay aggregates act much as sand particles and, therefore, susceptible to wind erosion. Topographic features, such as irregularity, length, degree and exposure of slope in relation to prevailing winds, influence the effectiveness of crop strips.

Standard farm machinery does not work efficiently on strips narrower than 15 metres. On field that requires strips narrower than 15 m, erosion resistant crops should be grown continuously or field should be under permanent cover.

e. Crop Rotations:

A system of crop rotations is commonly used on dry land subject to soil blowing. In areas, where moisture is available, legumes and grasses should also be included in the rotations. Crop rotations especially those that in­clude legumes and grasses often improve soil tilth and productivity, but their major function is to provide year round cover that prevent soil blow­ing.

A good crop rotation that will effectively maintain soil structure and con­serve moisture should be followed. In general, close growing crops are more effective for erosion control than intertilled crops. Intertilled crops such as corn, cotton, and vegetables offer some protection.

The effectiveness of crops, however, is dependent upon stage of growth, density of cover, row direction, width of rows, kind of crop and climatic conditions.

f. Windbreaks and Shelterbelts:

Windbreaks and shelterbelts are important soil conservation practices im­proving the microclimate for production and protection of crop and range-lands against destructive wind erosion.

A windbreak is a barrier for protection from winds commonly associated with homestead garden, or­chard, etc., while a shelterbelt is usually a longer barrier than a windbreak consisting of a combination of shrubs and trees intends for protection of field crops and conservation of soil.

The factors influencing the effective­ness of windbreaks are density, height and length. The three systems of windbreaks recognised usually are permanent, intermediate and temporary windbreaks.

The spacing of windbreaks depends upon the density and angle of windbreaks to the direction of the hazardous wind. The species suitable for a site much depends upon the soil type, climate including rain­fall, elevation moisture supply, etc.

The selection of species should be so made that the tallest may be in the middle row and smallest trees or shrubs are planted only in the end rows of the windbreaks. A more or less conical cross-section of windbreaks will provide the best protection from wind ef­fects and also increase the zone of protection.

The tall growing and shrubs like species should, therefore, be intermingled to obtain such a cross section to the windbreak.

Protection of shelterbelts are carried out by effectively fencing the area, fill­ing up of casualties so that there are no gaps, measures to protect from fire and carrying out cleanings and thinning of shelterbelts according to the needs of the belts.

Following species are recommended, especially for dry regions of India.

Five Row Belt:

(i) Central Row:

Acacia arabica (babul), Albizzia lebbek (siris), Azadirachta indica (neem), Dalbergia sissoo (shisham), Eucalyptus rostrata, Ficus spp., Holop-telea integrifolia, Kigelia pinnata (sausage tree), Prosopis specigera (khejri), Tamarindus indica (imli).

(ii) Flank Rows:

Acacia Senegal, Acacia leucophloea, Anacardium occidentale, Casuarina equisetifolia, Cassia siamea, Inga dulcis (Madras thorn), Moringa pterygosperma (Sainjna), Prosopis juliflora, Tamarix articulata (Farash).

(iii) Outer Rows:

Acacia jacquemontii, Agave spp., Capparis aphylla, Calligonum polygonoides (phog), Cassia aurticulata, Dodonaea viscosa, Glyricida maculata, parkinsonia aculeata, Prosopis julifora, Saccharum munja (sarkanda), Sav- ladora persica, S. oleoides, Vitex negundo, Ziziphus spp.

Three Row Belt:

(i) Central Row:

Same species as in the central row and flank row for a 5-row belt.

(ii) Outer Rows:

Same species as in the outer rows for a five row belt.

Single Row Belt:

Tamarix articulata, Acacia spp, Albizzia amara, casuarina equisetifolia, Cypres­ses spp., Inga dulcis, Parkinsonia aculeata (Jerusalem thorn) Prosopis juliflora. The effectiveness of any barrier would in fact depend on the wind velocity and direction, and the shape, width and porosity of the barrier.

When the wind blows at right angles to the average tree shelterbelts, wind velocity is reduced 70 to 80% near the belt. Velocity is reduced by 20% at a distance equal to 20 times the height of the belt. But no reduction in velocity occurs at a distance equal to 30 to 40 times the belt height.

The higher the average wind velocity, the closer shelterbelts or other barriers should be spaced to protect the soil from blowing. If the wind velocity is 64.4 km/hr at a 15.25 m height, the follow­ing distances may normally be protected from soil erosion by the barriers indi­cated.

Other wind erosion control practices, such as stubble mulching, are applied to the land in combination with the windbreaks. Annual crop and grass bar­riers are also generally planted at wider intervals than required for full protec­tion. For example, one or two rows of sorghum or grass planted at 18 metre intervals is a common supplement to other wind erosion control measures.

It is generally observed that narrower belts of medium porosity are equally or more effective and take less land out of production. The ideal is a one row belt. When trees die in a row in the more arid areas, three rows should be planted to ensure protection.

g. Mulching:

The maintenance of crop residues on or near the surface reduces evaporation and constitutes an essential phase of water management in potentially wind- erodible arable soils, which is the greatest single factor in wind erosion control.

The threshold velocity for movement of un-decomposed crop residues, even if scattered on the surface of the ground, is higher than most of the erodible soil particles. If the wind is not too strong, some of its force is absorbed by residue and soil erosion is considerably reduced.

To be fully effective in reducing wind erosion, part of the mulch material should be anchored) in soil. The portion of residue protruding above the surface forms an effective trap for soil particles moving by saltation or surface creep.

The principal use of mulches in dry land farming is to treat highly erosive knolls and blowouts particularly in sandy soils. Generally, cotton gin trash, straw, manure and corn cobs are used as mulches.

It has been observed that use of 1 to 2 tonnes straw or hay, 4 to 5 tonnes corn cobs, 6 to 8 tonnes manure, or about 5 tonnes cotton gin trash per acre effectively controls erosion on vul­nerable spots and prevents their spreading to other parts of the field.

Materials may be spread by hand or with a manure spreader. They should be anchored with a disc packer or ordinary disc operated at a very small angle so that discs do not burry residue.

h. Emergency tillage:

Once vegetative cover is depleted, it is necessary to do emergency tillage (Fig. 6.3) for immediate protection. Emergency tillage, however, should be used only after such methods as stubble mulching, cover crops, strip cropping, crop rotations, regular tillage, and windbreaks and other barriers have failed.

Emergency tillage creates a rough, cloddy soil surface to resist the force of the wind. It is only a temporary measure because clods readily disintegrate.

Emer­gency tillage should, therefore, be done before soil blowing starts. Soil erodes more rapidly under abrasion of moving soil particles and requires more drastic measures to prevent it from further erosion. If soil blowing has started, the emergency tillage should be done on windward edge of the field.

Fig. 6.3 Emergency tillage for wind erosion control

Sandy soils are by far the most difficult to hold with emergency tillage Generally, clods are not obtained, regardless of the depth of tillage At best any emergency measure in sand will be rather short lived, it is far better to keep such soils permanently covered with vegetation. However if tillage is required, it is better to till the entire area with a lister at sufficient depth to produce a rough surface (Fig 6.4).

As far as possible natural vegetation should be en­couraged because there is more danger of erosion from too much tillage of sandy soil than from no tillage. Fine- and medium-textured soils, however, respond more readily to emergency tillage than sand. In these soils, chiseling may be done to produce a rough, cloddy surface (Fig. 6.5).

Effective implements for emergency tillage are: listers with 20 or 35 cm bot­toms (Fig. 6.4), narrow and heavy duty chisels (Fig. 6.5), duck foot and wide spade shovel cultivators, one-way with two or three discs removed at intervals to give lister effect, the sand fighter, and pitting machines. The choice of im­plement and the method used depend on the seriousness of possible erosion, soil texture and the cropping system.

Emergency tillage must be at right angles to prevailing winds. Tillage must be done deep enough to bring compact clods, usually 7.5 to 15 cm to the sur­face. If serious soil blowing is expected, tillage should be done on larger area with at-least a 35 cm lister spaced 105 cm or a 20 cm lister spaced 50 to 60 cm.

If moderate soil blowing is expected, individual fields may be tilled with chisels or cultivators. For most types of chisels, 60 cm spacing gives good protection. Intermediate speeds of cultivation 3.5 to 4.0 m.p.h. provide the most effective surfaces.

The crop also indicates the choice of emergency tillage methods. Often a wheat crop may be too sparse to hold against erosive winds, yet a partial crop may be salvaged. In such cases, the entire field should be tilled with a chisel with the points spaced 135 cm apart.

i. Deep Ploughing:

Deep ploughing provides wind erosion control on sandy soils if adequate amounts of clod forming clay sub-soil can be brought to the surface, this is done with large moldboard or disc ploughs to a depth of about 40 to 120 cm.

Generally, the increased cloddiness and roughness that results from deep ploughing of sandy soils is only temporary. Therefore, the practice must be supplemented with other wind erosion control measures.

j. Land forming and Benching:

Land modifications for irrigation, water erosion control, and moisture conser­vation sometimes changes exposed knolls, tops and slopes of hills and field lengths that affect the rate and amount of erosion of soil by wind.

Research data on the effect of land modification on wind erosion are meager, but calcula­tions using the wind erosion equation for average climate, soil and residue con­ditions indicate that shortening field lengths from 305 to 30.5 m could reduce potential soil loss by 50%.

Similarly, in a 366 m long and 4% slopy land benched with a series of 73 m wide level benches showed 60% reduction in soil loss by wind erosion. While it seems unlikely that land will be extensively modified to control wind erosion, all indications are that it will be increasingly modified for irrigation and to control water erosion. These same land modifications also may provide substantial wind erosion control.

Measure # 2. Control on Irrigated Lands:

Wind erosion occurs on irrigated land because:

(1) A high percentage of the crops grown are row crops that are harvested and leave very little or no residue; and

(2) Land leveling and other intensive field preparations for irrigation pul­verise the soil and destroy all vegetation.

Wind erosion control on irrigated land can be attained by applying most of the control measures practiced on dry-lands. Some special procedures, however, are used depending on whether the land is a new or an established irrigated field.

a. Control on New Fields:

For successful soil conservation, it is necessary to decide the capacity of the management system to handle the area before clearing the vegetation for cul­tivation. The operations such as leveling and ditching should be carried out during the growing season when wind velocities are usually low and a cover crop can be established, if needed.

If wind erosion is a threat, a winter hardy small grain crop having good root system should be planted to tie down the soil mass. Crops must be taken regularly in the following seasons if the soil is not highly susceptible to blowing. If it is highly susceptible, the field must be kept under cover crop or pasture.

b. Control on Established Fields:

The time between seed bed preparation and planting should be as short as possible. Tilling of old residues should be delayed and the succeeding crop must be planted in time. As far as possible, the soil should not be pulverised.

Also all ploughing, ridging, discing or other tillage operations should be done when the soil is moist enough to produce clods, but not wet enough to cause compaction. If soil is so dry that deep tillage does not form clods, post-tilling irrigation should be given.

The ridges need not be destroyed unnecessarily. For pasture crops, residues need not be ploughed but sub-surface seeps may be used to kill weeds. To ensure minimum soil loss, row crops, such as corn, beans, sugar-beets and soybean, should be planted on ridges that have rough, cloddy surfaces.

The ridges will control wind erosion, shelter seedlings and provide furrows for irrigation. Conventional planters can be adapted to ridge planting by attaching shovels in front of planters. Chisels, listers and other implements suggested for dry land farming may be used to provide rough and cloddy surface. The ridges should be at right angle to the prevailing winds.

If there is no residue from the previous crop, a cover crop should be planted to protect the land. Winter wheat, rye and oats provide effective covers in adapted areas. On large fields, the field length may be reduced by leaving strips of stubble or cover crop when preparing seedbeds.

After the crop produces sufficient growth for protection, the strips can be tilled. Strips should be about 100 ft apart on soils that blow easily. Mulches such as straw or hay may be used on small, highly erosive spots.

Row crops should be planted in rows running at right angles to the direction of the prevailing winds or on the contour. Wheat planted in rows parallel to the direction of the wind causes about 2.5 times more soil loss than when planted in rows at right angles to it.

Tree windbreaks should be used to protect farmsteads and gardens on irrigated lands. They are not recommended for irrigated fields because they use water and take valuable land out of production.

However, annual crops or grass barriers suitable for dry land conditions can be planted at 18 to 30.5 m intervals. Irrigation is also recommended as a practice in case where a high value crop could be severely damaged due to wind erosion.

Measure # 3. Control on Vegetable and Especiality Croplands:

Vegetable and other speciality crops are often damaged by moving soil par­ticles, burial of plants, exposure of roots and by the drying and twisting effects of wind on the plants. The problem is more severe on mineral soils, but also occurs on organic mucks. Erosion tolerance for most vegetables is extremely low. Beets, carrots, onions, spinach, and squash are damaged by soil move­ment of 1/10 to 1/2 tonnes/acre.

Potatoes, asparagus, cabbage and eggplant will tolerate up to 1.0 tonne/acre of soil blowing and sweet corn and peanuts will not suffer severe damage with soil losses up to 2.0 tonnes/acre.

Damage to sub- crops generally occurs in the springs, most of the wind erosion control practices used for dry land agriculture also apply to vegetable and speciality crops. How­ever, the method of use and designing may be slightly different.

a. Buffer Strips and Annual Crop Rows:

Buffer strips have proved effective for wind erosion control of the crops. A suggested width of strip and the buffer crops are given in Table 6.5.

Where single row cereal buffers are used to protect carrots, onions or beets, the best practice is to seed the buffer with the vegetable. After the critical blow season is past, the buffers may be destroyed by cultivation or by the use of chemicals.

b. Cover Crops:

Planting a cover crop is an effective way to control wind erosion on vegetable land. Rye, wheat, barley and oats have been found as effective covers for vegetable growing areas. Tree shelterbelts can provide long time protection for cropped fields.

Generally two rows of tall growing, adapted evergreen or deciduous trees, should be planted around farm boundaries. Kinds of trees for this purpose include pine, spruce, mulberry and poplar.

Multi-flora rose, privet and other adapted species of shrub also provide effective protection for vegetable crops. On most sandy soils, they may be spaced about 91 m. Objections to tree and shrub windbreaks are that they compete with the crop for water and plant nutrients.

Solid board fences, snow fences, burlap fences, crate walls and earthen banks can also be used to protect vegetable crops. Fences should be 0.9 to 1.5 m high and spaced at intervals of about 15 times their height. Earthen banks should be 60 cm high and spaced at 12 to 15 times their height. However, high cost of earthen banks limits their use to small fields and high value crops.

c. Adhesives:

Several adhesives of petroleum, chemical and organic origin are available for temporary wind erosion control of vegetable seedlings on mineral soils. Some of the adhesives are relatively expensive, but a few are economical to use on high value crops threatened by serious blowing that cannot be controlled by other methods. A list of materials used to effectively control erosion at reasonable cost are given in Table 6.6.

When applied as a pre-emergence spray, the adhesives should provide protection until the crop has established an adequate canopy. A herbicide must be applied with the stabilising material because any mechanical stirring of the soil to destroy weeds would also reduce the effectiveness of the stabilisers.

d. Other Measures:

Irrigation, use of heavy rollers and maintaining high water tables will provide some protection from soil blowing in areas where these controls can be applied. Irrigating vegetables to control wind erosion is expensive and often difficult to justify. However, if a sprinkler system is available, one can irrigate highly erodible spots in fields to protect tender crops and prevent spread to other parts of the field.

Compaction with a roller is an effective method of reducing wind erosion of muck soils. Rollers pack the fluffy light weight soil material together and smooth the land so that wind forces cannot attack the particles to initiate move­ment.

The capacity of the roller should be such that it provides from 9.1 to 12.1 kg per linear centimeter pressure to the soil. Maintaining high water tables is a possibility in areas that are provided with tile drainage. High water tables are best used with muck soils. The tile outlets may be blocked to allow the water table rise.

Measure # 4. Control on Grazing Lands:

Wind erosion occurs on overgrazed lands and around water sites, saltboxes, lanes and shaded spots where animal traffic concentrates. The problem is serious in arid and semi-arid regions.

Erosion control on grazing lands can be attained by adapting the following measures:

(1) Limiting livestock number on any given area.

(2) Providing reserves of harvested forage to feed during drought periods.

(3) Dividing pastures and using rotational grazing.

(4) Supplying several watering and saltbox sites and moving the boxes from time to time.

(5) Fencing animals away and planting grasses (Fig. 6.7).

(6) Avoiding gates or lanes on erosive sites and moving them if an erosion problem develops.

(7) Providing wind barriers to protect permanent water sites and lanes.

(8) Applying mulches and brush to blowout spots caused by heavy traf­fic, utility poles or gas line installations.

The increased vegetation resulting from any crop improvement practice also helps to control wind erosion. Practices that have proved beneficial are contour furrowing or staggered trenching to reduce runoff and conserve moisture, inter-seeding of more productive species of grass, spraying to control weeds and brush, and applying fertilizers when economically feasible.

Measure # 5. Control on Sand Dunes and Other Problem Areas:

The sand dune problem develops mostly along coastal areas and affects summer beach cottages, military installations farmsteads and highways in addition to agricultural lands in the areas of Bikaner. Barmer, Churu, Jaisalmer and Jhunjhunu districts of Rajasthan and Mahcndragarh district of Haryana.

The beach cottage, highway and military situations can be treated with costly quick control measures, but sand drift on agricultural and rangelands must be treated taking economics into consideration.

The programme of stabilisation of drifting sand dunes calls for afforestation of these areas, moisture conservation and use, selection of suitable crop species and efficient planting methods. The drifting sands bury adjoining fertile lands, pastures, villages, canals, irrigation channels, wells, threaten the adjoining lawns, disrupts communication.

a. Stabilisation of Sand Dunes:

Stabilisation of sand dunes depends on reducing wind velocity at the surface through windbreaks or establishing a vegetative cover. Most attempts to con­trol sand dunes have aimed at complete vegetation with trees or grasses. Any plant which is successful in binding sand must be one that is well anchored, able to hold sand and catch the windborne load.

It must also withstand abrasion and keep its aerial portion above the accumulating sand. Such plants usually have either deeply buried rhizomes or prostrate growth with stem anchored by abundant adventitious, fibrous roots for rehabilitation of sand dunes.

There are 3 distinct processes in the stabilisation of sand dunes:

(i) Protection against biotic influences;

(ii) Treatment of the shifting sands and dunes by fixing barriers from the crest down to the heel of the dune; and

(iii) Re-vegetating by afforestation and establishment of grasses.

(i) Protection of Sand Dune:

Protection of sand dune the area to be afforested for fixation of sand should be taken up one year in advance; and fenced, preferably with five strands of barbed wire fixed on angle iron posts spaced 6 m apart.

Brush wood (Thorn) fencing can also be done by Zizyphus or Prosopis cineria as they are cheaper. Amongst the various types of fences, angle iron post and barbed wire fencing have proved to be the most efficacious and economical.

(ii) Creation of Barriers:

Stabilisation of dunes is based on the principle of reducing the wind velocity at the dune surface by providing planned obstruc­tion as suggested by Central Arid Zone Research Institute, Jodhpur. The first step in dune fixation operation consists of placing closely spaced vertical brush wood barriers starting from the crest to the heel on the windward side of the dune.

These barriers may either be erected in 5 m parallel strips or in 5 m² chess board pattern depending upon the wind velocity and wind direction. The planting could be still closer in more wind prone areas.

Species such as Crotalaria burhia (senia); Leptadenia pyrotechnica (khimp); Ziziphus num- mularia (bordi); Aerva tomentosa (buida); Calligonum polygonoides (phog); Lasiurus hirsutus (sewan); and Panicum turgidum (munt) have been found economical for creating such barriers. The fixing of brush wood should be completed before monsoon.

(iii) Establishment of Vegetation:

Vegetation resists wind velocity and reduce scouring of soil surface against the wind. In many coastal areas, the sand washed ashore by wave action is blown inland which on drying gets deposited in dunes of various types. The only permanent solution to this type of problem is the establishment of vegetal cover over the entire surface of the dune.

The windward side being the active part of the dune, should be stabilized first. After that, the rest of the dune can be brought under control. Three principal steps are usually required to establish a permanent vegetal cover on coastal dunes. The initial step consists of planting beach grass and dune grasses to arrest sand movement.

Then intermediate seeding or plantings of leguminous species are necessary as soon as the beach grass has stilled the moving sand. This step may be combined with the seeding or planting of per­manent grasses and legumes or shrubs and trees, if the initial sand stilling plant­ings are sufficiently vigorous to have completely stilled the sand and eliminated the possibility of movement.

The most successful method of plantation is ‘planting bricks’ developed at CAZRI, Jodhpur. These bricks are prepared with the help of a wooden or steel mould (30 cm high, and 10 cm² and 15 cm² at the top and bottom, respec­tively).

Planting bricks are made out of a mixture of equal proportion of clay, sand and manures in the moulds and sun dried with a cavity of 2.5 cm diam and 15 cm deep from the top to place the seed or stump.

During planting season, these bricks along with seedlings in them are taken out of the nursery beds and planted about 60 cm below the dune surface with the bricks intact. The plant­ing is carried out at a spacing of 5 x 5 m. Species which are found successful by this method are Acacia Senegal, Acacia tortilis, Dalbergia sissoo, Parkinsonia aculeata, Prosopis juliflora, cineraria and Ziziphus jujuba.

In general, the following types of vegetation can be established in low rain­fall areas:

Trees and Shrubs:

For successful planting in sand dunes, well developed, about 1.0 m long, seedlings should be planted about 60 cm deep and keeping 40 cm above the ground.

Direct planting with branch cutting of Tamarix articulata (Pharash) and Calligonumpolygonoides (Phog) is successful provided it is done with the onset of rains or soon after good showers. Pre-sprouted stumps of Prosopis juliflora (Vilayati babul); Albizzia lebbek (sires) and Dalbergia sissoo (shisham) have also done well.

The success of sand dune afforestation depends on choosing right species.

The following tree and shrubs are recommended for afforestation on sand dunes in different rainfall areas:

(a) In areas receiving more than 40 cm rainfall, the trees species like Prosopis cineraria, P. juliflora, Acacia nilotica, Ziziphus jujuba, Acacia Senegal, Tacomella undulata, Parkinsonia aculeata and Acacia tortilis are suitable. The suitable shrubs for such rainfall areas are: Calligonum polygonoides, Cassia auriculata, Ziziphus rotundifolia and Ricinus communis.

(b) Areas receiving annual precipitation between 30-40 cm should be planted with tree species like Acacia Senegal, Prosopis cineraria, P. Juliflora, Tacomella undulata, Ziziphus jujuba, Parkinsonia aculeata and Acacia tortilis. The suitable shrubs are Ricinus communis and Calligonum polygonoides.

(c) Prosopis juliflora, Parkinsonia aculeata and Acacia tortilis species of trees are suitable for areas receiving less than 30 cm annual rainfall. Calligonum polygonoides and Ricinus communis are the suitable shrubs for such areas.

b. Establishment of Grass Cover:

Grasses grow more quickly than shrubs and trees and bind the drifting sands, utilizing soil moisture from upper strata. Saccharum munja is deep rooted and quite efficient in binding loose sand. Hardy grasses like Panicum turgidum and Panicum antidotale may be sown with the onset of monsoon.

To improve forage quality legumes like Atylosia scarabaeoides, Rhynchosia minima, Clitoria ternatea, and Crotalaria burhia should also be sown before the onset of rains for further self-seeding. Mechanical structures that stop sand movement only temporarily are costly and, if not properly placed, may cause turbulence that makes dune difficult to stabilise.

Stabilisation of sand dunes affecting military bases, and highways may be accomplished by one or a combination of the following methods:

(1) Sand dune crests should be levelled to reduce areas exposed to ma­ximum winds and the sides falling prevailing winds should be stabilized.

(2) Sand fences may be used to trap and hold moving material. Fences should be installed perpendicular to prevailing winds. Severe cases may require paddocks, sometimes as small as 7.6 x 7.6 m. Sand dikes should be 45 cm high and spaced not more than 30.5 m apart.

(3) Crushed rock, stones, gravel or shale blankets may also be used. To provide adequate protection about 20 tonnes/acre of fine gravel, 50 tonnes of medium gravel or crushed stone or 100 tonnes of coarse crushed rock or sale would be required.

(4) Straw mulches at the rate of 2 to 3 tonnes per acre, anchored with disc harrow, disc packers or asphalt adhesives, may be used. Blower type mulch spreader or beater type manure spreader may be used to apply mulches on large scale.

(5) Brush, chicken wire or snow fence may be used flat to the surface on highly erosive spots. Wire should be anchored with stakes.

(6) Petroleum and chemical spray on adhesives may be used. Resin-in- water emulsions (coherex), asphalt emulsions, cutback asphalt, and oil latex emulsions (Unisol) are highly effective.

(7) Sand should be tested for nutrient requirements. Application of up to 560 kg/ha of 16-20-0 fertiliser (N-P-K mixture) have been required on some dune sands to establish vegetation.

(8) Seeds of culms of adapted grasses should be planted with mulch, ad­hesive, or other protection. Effective grasses where they will grow include American and European beach, bamboo, pampas, maram, and perennial rye. When planting beach grasses, about 3 to 5 culms per hill, spaced 45 cm apart should be placed.

(9) Adapted trees and shrubs should be planted on some dune areas. For example, pine trees and strawberry could be planted successfully.

A low cost method of stabilising dunes on low value, sandy range lands in­clude:

(i) Excluding livestock completely, and

(ii) Stimulating growth of existing vegetation by applying fertiliser after testing the sand for nutrient require­ments. If fertilisation alone is not sufficient to stimulate native vegetation, sand binding grasses should be planted and mulches and fertilisers should be ap­plied.

The stabilized sand dunes should be managed carefully. The trials and roads leading through loose sand should be covered with non-erodible material such as gravel or concrete. Grasslands, woodlands, or scrublands should be protected from fire, overgrazing, or excessive tree cutting. Stabilized dunes should be used for recreational purposes and for limited pasture or woodland.

Central Soil Conservation Board at Research Station, Jodhpur has recom­mended the following flora for reclamation of sand dunes.