Soil and water conservation measures for hill agriculture
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 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%.
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 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 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.
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.
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 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.
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.
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 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.
(To be contd)