Land reclamation is restoring the productivity and the fertility of the land which has become unculturable or has suffered a reduction in the crop yield because of waterlogging, salinity, etc Before starting the reclamation process, it is necessary to properly level the land surface so that uniform reclamation is achieved. Land reclamation is usually done by adopting one or more of the following methods:
1) Drainage and lowering of the water table:
In general, the waterlogged land improved by adopting various drainage methods and lowering the water table. The source of water which is causing the waterlogging should be located, and suitable measures should be adopted to check the flow from that source.
If the waterlogging is caused by surface water, suitable surface drainage systems should be adopted. If the waterlogging is caused by underground water, the source should be cutoff by constructing intercepting drains.
Measures recommended for prevention of waterlogging are also useful for reclamation of saline land. A good drainage system helps in avoiding efflorescence if the water table skept below the land surface at a safe limit so that water does not rise upto the land surface by capillary is action. The safe limits are different for different soils.
Leaching is the process in which the land is flooded with an abundant quantity of water depth shing in the of 15 to 25 cm over the surface.
The excess salts are washed down from the land surface to the ground water, provided the water table had already been lowered to a safe limit. The process is continued till the salts in the surface layer are reduced to a safe limit.
Before starting the leaching process, small dikes (or bunds) are huilt to enclose the fields of the suitable size. The dikes are usually 20 to 30 cm high so as to contain the water without spilling.
The land is flooded between the dikes and the water is maintained for several days. The water gradually percolates through the soil and the surface becomes dry. The flooding operation is then repeated. The process is continued till most of the salts have been washed to the depth below the root zone of plants.
Drying of the land surface between the flooding operations permits the formation of surface cracks in the soil. These cracks increase the infiltration during subsequent floodings.
The rate of infiltration during leaching operation depends upon a number of factors such as soil texture, the degree of dispersion of clayey soil and the depth of the water table.
Alkali soils usually disperse during leaching process and become more permeable as the soluble salts are washed down. Sometimes soil amendments are made for reclaiming black alkali soils in which sodium ions are replaced by calcium ions.
The resultant sodium compounds are then removed by leaching. Thus the sodium soils are converted to calcium soils which have better permeability, more favourable conditions for acration, root development and plant growth.
Leaching operation can be repeated after 4 to 5 years, if necessary. However, excessive leaching is not desirable, as it may remove the salts from the soil which are essential for the plant growth.
Moreover, over-use of flooding water creates the drail problem During the reclamation process, weeds and grass are allowed to grow to consume some of the salts. On completion of the leaching process, salt-resistant crops, such as coarse rice and berseem, are grown for a year or two to reduce the salinity further.
3) Use of chemicals:
Chemicals are sometimes added for the reclamation of land. Gypsum is the most commonly used chemical for alkali soils. Sodium carbonate in the soil is easily removed by spreading gypsum on the land at the rate of 2 tonne/ha before leaching.
Calcium chloride is also sometimes used. However, it is less effective than gypsum. The low solubility of gypsum results in better activity than that in calcium chloride. But addition of gypsum is less effective if the soil has excess sodium salts (as in Thur soils). For such soils, gypsum should be added after the excess salts have been reduced by leaching. Moreover, gypsum should not be applied every year.
Sometimes sulphuric acid is also applied to the land. The top 20 cm soil layer is usually treated with 1 to 5 percent solution of sulphuric acid to neutralise alkalies in the soil. It improves the growth of plants. Acid-forming fertilisers are also effective.
4) Adopting rice cultivation:
Salinity of the land is reduced by adopting rice cultivation. In rice cultivation, the large depth of water over the land leaches the salts and keeps them at a safe depth below the surface.
Rice cultivation also causes a reduction in the alkalinity of the soil. The roots of the rice plants produce carbon dioxide which lowers the pH value, increases the percolation and brings the exchangeable sodium ions of the soil into solution.
However, during rice cultivation, nitrogen in the soil, which is essential for the growth of plants. is reduced due to excess water and absence of adequate air. To compensate the nitrogen in the soil, a leguminous crop, such as gram, is grown during the next Rabi season. Guara and San also improve the nitrogen deficiency.
5) Crop rotation:
Salinity of a soil can be further reduced after leaching by adopting crop rotation in which rice or maize is introduced. The following two crop rotations are commonly adopted.
(i) Rice in rotation
(ii) Maize in rotation
6) Green manuring:
A crop called Jantar is generany used as a green manuring crop. It has a rapid growth in saline and poorly drained soil. Other green manuring crops are San, Senji, Guara and Berseem, etc.
Green manuring also improves the structure of the soil. It reicases the organic acids which lower pH value and add nitrogen to the soil.
7) Addition of agricultural waste products:
The salinity of the soil can be reduced by adding agricultural waste products such as groundnut hull, saw dust, molasses with lime sludge, distillery waste, sunflower hull, tamarind seed powder. etc.
Molasses alone are not effective for the reclamation of alkali soils, However, when combined with lime sludge, they are quite effective. Distillery wastes are acidic and quite effective in reducing alkalinity and in replacing sodium with calcium.
8) Use of argemona:
Argemona is a plant which grows on the waste land. The plant is highly acidic and can be effectively used to reduce the alkalinity of the soil. Other similar plants are also sometimes used.
9) Use of processed coal:
The alkanity of a soil can be reduced by the addition of a small quantity of processed coal to the soil.
Electro-dialysis can be used to reduce the alkalinity of the soil. When an electric current is passed through the soil, it renders it porous and permeable.
Hence the infiltration rate is increased and soluble salts are washed out. However, the method is quite expensive. It can be adopted where cheap power is available.