Mechanical measures of erosion control.   Mechanical measures (also called engineering measures) play a very vital role in controlling erosion on agricultural land. They are adopted to supplement the agronomical practices when the latter alone are not adequately effective. These measures include basin-listing, subsoiling, contour-bunding (also called level terraces, ridge terraces, absorptive-type terraces), graded bunding and bench-terracing on steep slopes.

The main objectives of the mechanical measures for controlling erosion are : (i) to increase the time of concentration by intercepting the run-off and thereby providing an oppurtunity for the infiltration of water, and (ii) to divide a long slope into several short ones so as to reduce the velocity of the run-off and thus prevent erosion.

BASIN-LISTING.   Basin-listing consists in making of small interrupted basins along the counter with a special implement, called a basin-lister. Basin-listing helps to retain rain-water as it falls and is specially effective on retentive soils having mild slopes.

SUBSOILING.   This method consists in breaking with a subsoiler the hard and impermeable subsoil to conserve more rain-water by improving the physical conditions of a soil. This operation, which does not involve soil inversion and promotes greater moisture penetration into th soil, reduces both run-off and soil erosion. The subsoiler is worked through the soil at a depth of 30-60 cm at a spacing of 90-180 cm.

CONTOUR-BUNDING.   This practice consists in making a comparatively narrow-based embankment at intervals across the slope of the land on a level that is along the contour. It is an important measure that conserves soil and water in arid and semi-arid areas with high infiltration and permeability, and is commonly adopted on agricultural land up to a slope of about 6 per cent. The cross-sections of bunds adopted in the dry areas of Maharashtra are given below :

These cross-sections of the bunds can be used as a guide in similar situations. It may be noted here that, as no specifications for bunding in deep black soil are available so far, no large-scale bunding on them can be recommended, pending further research.

As regards spacing between bunds it should be not exceed 150 cm vertical drop or 67.5 cm horizontal spacing, whichever is less. The following schedule , which is used in Maharashtra, can be used as a fair guide :

Where the rainfall is more than 62.5 mm and moisture conservation is important, bunds may be constructed at shorter intervals.

In the alluvial soils of Gujarat, a vertical interval of 1.83 m and a cross-section of 1.3 sq m were found to be suitable for land with slopes ranging from 6-12% and for slopes less than 6% contour bunds with cross-sections of 0.9 to 1.3 sq m spaced at 0.9 to 1.2 m vertical interval were found to be effective.

Observations and experiments at Sholapur, Bellary and Kota have shown that even in semi-arid climate, contour-bunding in deep black soils, with montmorillonitic type of clay is not suitable. The low rates of permeability and infiltration in these soils cause a prolonged impounding of water on the upstream side, and the crops are consequently damaged.

Graded bunding or channel terraces. Graded bunding has been used in areas receiving rainfall of more than 80 cm per year, irrespective of the soil texture. In clay soils, graded bunding has to be used even for areas having less than 80 cm of annual rainfall. Graded bunds may be narrow-based or broad-based. A broad-based graded terrace consists of a wide-low embankment constructed on the lower edge of the channel from which the soil is excavated. The channel is excavated at suitable intervals on a falling contour with a suitable longitudinal grade.

At Dehra Dun, studies on the channel terraces in the alluvial soils of the valley with 3-4 per cent slope have shown that the two vertical intervals (V.I.)* tried, viz. 0.3 (S + 2) and 0.3 (S + 3) did not show much difference in run-off, whereas the soil loss per mm of run-off was more from the V.I. = 0.3 (S + 3). However, the annual soil loss was within the permissable limits and, therefore, V.I. of 0.3 (S + 3) can be adopted safely. *V.I. = Vertical interval in metres. S = Slope %

The channel grades of 0.4 per cent and 0.6 per cent did not show any signs of scouring.

Observations on cross-section revealed that the broad-based and narrow-based cross-sections functioned well without any special maintenance. Broad-based terracing is recommended where farming is practiced with tractors.

Studies on broad-based channel terraces constructed on black soils (land slope, 1%) in the semi-arid climate at Bellary showed that a V.I. of 0.6 m was better than that of 0.75 m and that the channel terrace with a variable grade showed less of run-off and soil loss than the channel terrace with a uniform grade.

At Kota, graded bunds, 0.7 to 1.0 sq m cross-section and with 0.1 to 0.5% channel grade were found suitable for clay loam to clay black soils.

Bench- terracing. This consists of a series of platforms having suitable vertical drops along contours or on suitably graded lines across the general slope of the land. The vertical drop may vary from 60 to 180 cm, depending upon the slope andsoil conditions, as also on the economic width required for easy cultural operations. The material excavated from the upper part of the terrace is used in filling the lower part. A small 'shoulder' bund of about 30 cm in height is also constructed along the outer edge of the terrace.

Bench terraces may be 'table top' or sloping outward or inward with or without a slight longitudinal grade, according to the rainfall of the tract - medium, poor or heavy, and the soil and the subsoil are fairly absorptive or poorly permeable.

On steeply sloping and undulating land, intensive farming can be practiced only with bench-terracing. The initial cost of bench -terracing is more than that of bunding. However. Bench-terracing helps to retain the soil, moisture, manure and fertilizer better and facilitates the application of irrigation, if available. In rainfed areas, terracing is usually practiced on slopes ranging from 6 to 33 per cent. It may have to be used on gentle slopes, if irrigation is to be applied to the crop.

At Ootacamund, studies have been conducted on the optimum length of bench terraces, longitudinal and inward grades of bench terraces.

The difference in the length of bench-terraces did not influence significantly the run-off, soil loss and the total yield of potato growth on them. However, from the analysis of the fortnightly soil-moisture data, it was observed that on benches longer than 91.5 m, the farther ends of the terraces were significantly drier than those near the cross disposal drains. On this consideration, it was recommended to have bench terraces, of about 100 m in length in the Nilgiris.

Five longitudinal grades of bench terraces (viz. 0.15, 0.33, 0.50, 0.60 and 0.84 per cent) constructed on land having 25 per cent slope and with 2.5 per cent inward slope were compared to Ootacamund. The run-off and the soil loss increased significantly with the increase in the grade. However, the maximum run-off was only 0.9 per cent of the total rainfall and the soil loss was only 0.3 tonne/ha under the maximum grade of 0.84 per cent, both of which are negligible. The differences among the yields of the potato crop from bench-terraces of various grades were also not significant. Similarly, the effect of the inward grade of the bench terraces (viz. 1, 2, 4 ,6 ,8 and 10 per cent) on the yield of potato was not found to be significant.