Discuss reclamation of alkali soil.
Answers
Stages of Reclamation of Saline and Alkali Soils:
1. Leaching and Amending Stage:
Saline soils are leached by ponding an appreciable depth of water with adequate drainage.
The objectives and measures to be considered in the course of leaching are:
1. Removal of excess salts to the depth of deepest root system of the crops in rotation.
2. Reduction of groundwater salinity to below the critical level.
3. Lowering the groundwater table sufficiently to remove the danger of it resalinising the soil.
4. Avoiding alkalisation of saline-alkali soils by application of gypsum late in leaching and when the rate of infiltration slows down. Gypsum should not be applied before leaching to avoid its removal with other soluble salts.
2. Cultivation and Improvement Stage:
Protection of leached and amended soil against desalinization or realkalisation can be achieved by:
1. Securing drainage to keep groundwater table below the critical depth.
2. Continuing periodic leaching to maintain the desired salt balance in the soil.
3. Starting to grow salt tolerant crops or those needing high irrigation water, such as rice.
4. Land levelling, furrow irrigations, pre-planting/seeding flooding, etc.
Physical, chemical and biological soil properties can be improved by:
a. Deep ploughing to loosen compacted layers for improving soil structure.
b. Application of organic manures and balanced fertiliser use to improve soil fertility.
Reclamation of alkali soils depends mainly on the addition of suitable fertilisers after loosening the compacted layers and breaking hard pans, in addition to periodic flooding and drying with effective drainage. The cost and time of reclamation increase as texture becomes heavier and clay fraction becomes dominated by montmorillonite type.
It is not necessary to use chemicals as a source of soluble calcium in calcareous soils. Calcium carbonate may be converted into more soluble from through application of acids or high rates of organic manures including green manures.
Reclamation of Alkali, Saline and Saline-Alkali Soils:
The excessive accumulation of alkali salts in the soils is injurious for plants growth It is necessary, therefore, to reduce the percentage of salts to optimum or normal level so that plants may grow luxuriantly in such soils.
There are several methods of reclamation which can be grouped as follows:
(A) Chemical method in which some chemicals are added to the soil in order to brine the alkalinity to desired level.
(B) Mechanical practices such as improving drainage and leaching, mechanical shattering of clay pans, and scrapping.
(C) Cultural method (growing salt tolerant plants).
Since fundamental causes in various groups of salty soils are different, their reclaiming techniques are different. Hence, these are discussed separately.
1. Reclamation of Alkali Soils:
Alkali soils are best reclaimed by the following methods:
(A) Chemical method:
(1) By cationic exchange (replacement of alkali from soil colloids by calcium ions). Application of calcium sulphate (gypsum) in the soil reduces alkalinity to a great extent and makes the soil fertile.
The reaction proceeds in the following way:
2Na-clay + CaSO4 → Na2SO4 + Calcium-clay
Na2CO3 + CaSO4 + CaCO3, + Na2SO4
Good drainage leaches away Na2SO4.
(2) Alkali salt percentage can also be reduced in the soil by the use of acid forming chemical amendments such as sulphur, ferrous sulphate and limestone. Sulphur, when applied to the soil, oxidises and forms sulphuric acid which converts carbonates of sodium and potassium to Na2SO4 and K2SO4 respectively that may be removed from top soil by drainage water. The amount of gypsum and sulphur required to reclaim the alkali soils will be different depending upon the degree of alkalinity, drainage and buffering capacity of soils.