Soil and water conservation measures for hill agriculture

SK Pattanaaik *

Soil and water rare the most important natural resources. In North East India these valuable resources are lost due to the hilly terrain topography ad heavy rainfall. The cultivated lands in the hill slopes suffer from nutrient loss and the plants are stressed due to the water scarcity during November to March.

The production of horticultural, agricultural crops is less due to soil erosion. The erosion of soil in the slopes causes flood in the plain. Proper soil and water conservation measures are required for sustainability in the production of crops, which are discussed below.

AGRONOMIC MEASURES

Agronomic measures of soil and water conservation are applicable when the land slope is less than 2% and include measures like contour cultivation, tillage, mulching strip cropping etc. When these methods are used in combination, erosion can be reduced even if the slope is more than 2%.

Contour cultivation

This includes contour ploughing, contour sowing, and other intercultural operations. By ploughing and sowing across the slope, each ridge of plough furrow and each row of the crop act as an obstruction to run off providing more opportunity time for water infiltration.

Tillage

Tillage alters the soil physical characteristics like porosity, bulk density, surface roughness and harness of pans. Conventional tillage includes ploughing twice or thrice followed by harrowing and planking. It leaves no land unploughed and leaves no residues in the field. On the contrary, conservation tillage disturbs the soil to the minimum extent and leaves crop residues on the soil. Conservation tillage reduces soil loss by 50% over conventional tillage.

Mulching

Mulching with plant materials reduces soil loss upto 43 times compared to bare soil and 17 times compared times compared to cropped soil without mulches. Crop residue mulches are also applied in narrow slots (vertical mulching) instead of spreading on the soil surface. They increase infiltration and thus reduce runoff.

Cropping system

Growing a cover crop viz. cowpea, green gram, black gram, soybean etc. reduces runoff and soil loss. In multiple cropping systems, where the soil is covered with crops round the year, there may be run off but soil loss is minimal due to less beating action of falling raindrops. Intercropping of maize and cowpea at 2:4 ratio was found to be best in reducing soil loss considerably in runoff plot having 3% slope.

Strip cropping

This is a system of crop production, in which long and narrow strips of erosion resisting crops (pulses) are alternated with strips of erosion permitting crops (cereals and millets). The strips are laid a ross the slope.

Use of chemicals

Break down of aggregates by the falling rain drops is the main cause of detachment of soil particles causing soil erosion. Aggregate stability can be increased by spraying polyvinyl alcohol @ 450-500 kg/ha depending on the type of soil. Soils treated with bitumen increase water stable aggregates and infiltration capacity of the soil. However practical method of increasing stability of aggregates is by application of organic matter, FYM, crop residues, green manure etc.

ENGINEERING MEASURES

Engineering measure are taken when the average land slope is more than 2%. Various engineering measures are trenching, contour bunding, terracing, vegetative bunding, construction of grassed water-way, revegetation of badly eroded sloppy lands by using geojute, construction of gabion structure, filtered check dam, rainwater harvesting tank, etc.

Trenching

The trenches may be longer across the slope on contour or may be of staggered type. The trenches are constructed across the slope and the excavated materials are placed on the lower side of the trench so as to form an embankment or bund. Planting is usually done on the toe of the embankment with the trench at the upper side. The trenches intercept and break the velocity of runoff water and provide ponding of the intercepted flow.

When the runoff water exceeds the storage capacity of the trenches the excess flow is drained at non-erosive velocity. The stored water percolates and store the moisture in the soil provides the plants as and when required. The cross section of the trench is about 30 x 30cm. The side slopes of the trenches are kept about 1:1 to 5:1.

Contour bund

This is an embankment or bund constructed across the slope to decrease the slope length. It can divert excess runoff during rains to grass waterways and retain eroded soil. Parabolic channel (0.3m top width, 0.2m deep) should be provided along the contour and excavated soil is placed in the form of bund in downside.

Development of bench terraces through slow process within contour bunds is very effective, which subsequently avoids exposure of subsoil as in bench terracing. This takes about 4 to 8 years. Usual cultivation practice is continued between the two consecutive bunds. Bunds require maintenance during first two years. Planting of forage grass strengthens bunds.

Bench terraces

Bench terraces are series of flat beds constructed across the hill slope separated at regular intervals in a step like formation. Bench terraces with inward slopes are adopted in the NEH region due to heavy rainfall. They are normally adopted where soil depth is more than 1.0 m. The terrace risers, which constitute 30-40 per cent of total area, can be utilised for growing perennial fodder grasses and legumes, which not only help in conservation but also provides enough fodder. Row crops such as maize etc. can be grown along the slope in the inter space between the bunds.

In NEH region, bench terrace cultivation exists in slopes up to100 per having terrace width as low as 3 m and situations on steep hills normally do not permit a longer length of terrace. Level beds with vertical intervals of about 1 m along with 15 sq. m cross section shoulder bunds and 1:1 batter slope for terrace risers is found. The terrace risers can be protected either through stone pitching or by raising grasses (grass or vetiver).

Half moon terrace

Where complete terracing is not desired or feasible productive, multiple use of steep slopes is possible by planting trees on half moon terraces. These are level circular beds having 1 to 1.5 m diameters, cut into half moon shape on the hill slope. These beds are used mainly for planting of fruit trees like guava, citrus etc. in horticultural land uses.

Vegetative bunds

Barrier hedges substantially reduce runoff and increase infiltration. Some runoff may cross the barrier, while the entrained soil will be partly filtered out and deposited. Pineapple plantation can be successfully used as vegetative bunds in hills.

Grassed waterway

These are trapezoidal or parabolic channels planted with suitable close growing grasses constructed along the slope preferably on natural drainage lines to act as an outlet for the terrace system. These channels along with silting basins serve the purpose of energy dissipation of flowing runoff water.

Geojute

Geojute (also called as “soil saver) is a natural geotextile used as mulch. This is essentially jute matting with an open mesh of 2 to 5 mm thick jute yarn having 10 mm apertures and is bio degradable. It has been successfully tried for stabilization of landslides, mine spoils and steep slopes.

The technique of geojute application included (a) spreading f geojute by overlapping and joining adjacent widths, (b) driving wooden sticks to a depth of 1m to secure matting place, (c) planting rooted slips of local grasses and cuttings of bushes in openings between the geojute strands at close spacing. It is found successful for initial establishment of vegetation on degraded slopes (up to 60-70%).

Gabion structure

Gabion is a material which is very often used for constructing various soil and water conservation control measures like control of stream bank erosion and as filtered check dam, etc. It is defined as “stones filled in wire box”. The wire boxes are generally made with a width and a height of 1.0 and 1.0/ 0.5/ 0.3 metre, respectively. The length is kept from 2 to 4 metre.

The required size of length, width and height is obtained by placing adjacent boxes of the size as referred above and interconnecting faces. Galvanised iron wire of No. 8 or No. 10 gauge is used for fabricating mesh with triple twist.

The wire net has mesh size varies from 7.5-15cm. Fr every 10 m wire mesh, 13 m of wire is used. Then, boxes of wire net are prepared and stones are filled in these boxes. Lastly the opening ends of wire meshes are tied with the help of rods.

Stream bank erosion control

All rivers and streams while flowing through hilly regions have a tendency to erode away their banks. The erosion destroys productive crop fields, human habitation situated on the river margin. A cheaper way of dealing with the problem of riverbed and bank maintenance is through plantation and using locally available stones.

The stream banks can be protected by planting water loving vegetation particularly of out sprouting type. The effectiveness of engineering measures may be enhanced by several vegetative measures like Salix spp., Vitex negundo, Populous spp., Arundo donax, Ipomeacarnea, Tatraphacuras, Pennisetum purpureum etc.

Rain water storage tank

Rain water harvesting is a measure to reduce the splash, sheet and rill erosion while storing the runoff water in hill terrace r valleys. The practice of rain water harvesting in ponds/tanks and reusing the stored water for life saving irrigation of crops and also for pisciculture and other domestic purposes is prevalent in India.

The tank is dug out and the excavated soil is kept as an embankment surrounding the tank. A trench of 1 feet x 1 feet is made surrounding the upper surface of the tank. Construction of such a tank needs some measure to prevent the seepage of stored water is highly porous soil.

The pond or the tank can be lined by 150-250 GSM polyethylene sheets. The tank can be fed by diverting streams or form in-situ high rainfall. The construction work should be completed before monsoon. The stored water can be used for irrigating the horticultural crops during winter by micro-irrigation.

AGROSTOLOGICAL MEASURES

Grasses proven soil erosion by intercepting rainfall, by binding the soil particles and by improving soil structures. A grass legume association is ideal for soil and water conservation. Grasses stabilize the surface of waterways, contour bunds and front faces of bench terraces while legumes build up soil fertility.

Desirable characteristics of grasses are perennial nature, drought resistance, rhizomniferous, and good canopy with deep root system. The useful grasses are Cenchrusciliaris, Chloris Guyana, Cynodondactylon, Dicanthiumannulatum, Panicum antidotale, Festuca arundinaceae, Eragrostiscurvula, vetiveriazizanioides, Centrosemapubescens, Stylosanthessppl and Siratro Spp. growing a mixture of grasses instead of any single grass proved to be better to stabilize newly formed bunds or terraces.