Alkaline soils with solid CaCO3 can be reclaimed with grass cultures, organic compost, waste hair, organic garbage, etc ensuring the incorporation of much acidifying organic material into the soil, and enhancing dissolved Ca in the field water by releasing CO2 gas. Deep plowing and incorporating the calcareous subsoil into the top soil also helps.
Manytimes salts migration to the top soil takes place from the underground water sources rather than surface sources. Where the underground water table is high and the land is subjected to high solar radiation, ground water oozes to the land surface due to capillary action and gets evaporated leaving the dissolved salts in the top layer of the soil. Where the underground water contains high salts, it leads to acute salinity problem. This problem can be reduced by applying mulch to the land. Using poly-houses during summer for cultivating vegetables/crops is also advised to mitigate soil salinity and conserve water / soil moisture. Poly-houses filter the intense summer solar radiation in tropical countries to save the plants from water stress and leaf burns.
Where the ground water quality is not alkaline / saline and ground water table is high, salts build up in the soil can be averted by using the land throughout the year for growing plantation trees / permanent crops with the help of lift irrigation. When the ground water is used at required leaching factor, the salts in the soil would not build up.
Plowing the field soon after cutting the crop is also advised to prevent salt migration to the top soil and conserve the soil moisture during the intense summer months. This is done to break the capillary pores in the soil to prevent water reaching the surface of the soil.
Clay soils in high annual rain fall (more than 100 cm) areas do not generally suffer from high alkalinity as the rain water runoff is able to reduce/leach the soil salts to comfortable levels if proper rain water harvesting methods are followed. In some agricultural areas, the use of subsurface "tile lines" are used to facilitate drainage and leach salts.
It is also possible to reclaim alkaline soils by adding acidifying minerals like pyrite.
Alternatively, gypsum (calcium sulfate, CaSO4. 2H2O) can also be applied as a source of Ca++ ions to replace the sodium at the exchange complex. Gypsum also reacts with sodium carbonate to convert in to sodium sulphate which is a neutral salt and does not contribute to high pH. There must be enough natural drainage to the underground, or else an artificial subsurface drainage system must be present, to permit leaching of the excess sodium by percolation of rain and/or irrigation water through the soil profile.
Calcium Chloride is also used to reclaim alkali soils. CaCl2 converts Na2CO3 in to NaCl precipitating CaCO3. NaCl is drained off by leaching water. Spent acids (HCl, H2SO4, etc) can also be used to reduce the excess Na2CO3 in the soil.
Where urea is made available cheaply to farmers, it is also used to reduce the soil alkalinity / salinity primarily. The NH4 (Ammonium) present in urea which is a weak cation releases the strong cation Na from the soil structure in to water. Thus alkali soils absorb / consume more urea compared to other soils.
To reclaim the soils completely one needs prohibitively high doses of amendments. Most efforts are therefore directed to improving the top layer only (say the first 10 cm of the soils), as the top layer is most sensitive to deterioration of the soil structure. The treatments, however, need to be repeated in a few (say 5) years time.Trees / plants follow gravitropism. It is difficult to survive in alkali soils for the trees with deeper rooting system.
It will be important to refrain from irrigation (ground water or surface water) with poor quality water.
One way of reducing sodium carbonate is to cultivate glasswort or saltwort or barilla plants. These plants sequester the sodium carbonate they absorb from alkali soil into their tissues. The ash of these plants contains good quantity of sodium carbonate which can be commercially extracted and used in place of sodium carbonate derived from common salt which is highly energy intensive process. Thus alkali lands deterioration can be checked by cultivating barilla plants which can serve as food source, biomass fuel and raw material for soda ash and potash, etc.
Read more about this topic: Alkali Soils
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