My main problem is RSC in sub soil water and alkaline nature of soil, is there any method to treat brackish (high rsc) tubewell water?
One suggested solution is removal of carbonates from subsoil by acidification and then by simple leaching etc. sodium may be overcomed.
Residual Sodium Carbonate (RSC) index of irrigation / soil water is used to indicate the alkalinity hazard of soil and find the suitability of water for irrigation .When dissolved sodium in comparison with dissolved calcium & magnesium is higher in water, clay soil swells or undergoes dispersion which drastically reduces its infiltration capacity.
The plant roots are unable to spread deeper in to the soil due to lack of moisture. Clay soils irrigation with high RSC index water leads to fallow alkali soils formation.
RSC index = HCO3/61 + CO3/30 Ca/20 - Mg/12.
It is expressed in meq /L.
Under agro-climatic conditions of Pakistan evapo-transpiration is several times higher than rainfall (2025 mm Vs. 150 mm) which is responsible for net upward movement of salts. In Pakistan, for water RSC the safe limit of 2.5 mmolL-1 has been proposed by Directorate of Land reclamation while limit 5.0 mmol/L by WAPD.There are 3 ways to control lRSC and alkalinity;the first two being commonly used in every country but third one is an improved method.
[1]The poor quality water can be improved considering the sodicity hazard, by increasing calcium through addition of chemical amendment rich in calcium, i.e. with the use of proper amount of GYPSUM.
[II] By decreasing its carbonate and bicarbonate contents with the addition of ACIDS/ACID FORMERS , either to soil or water.
Both the methods have the advantages/ disadvantages:
(a) Use of gypsum is economical but has low solubility (0.24-0.30 g/100 mL water at 25 ċ) yet it has an advantage for sustained availability of calcium.
(b) Use of pure acids is about 5-7 times costly than gypsum and handling is also difficult yet can control RSC in a better way by making the following changes as given in3 rd methos
[III]As an alternate Sulfurous Acid Generator (SAG) is a recently introduced technology to treat saline-sodic/sodic waters. Sulfur (S) is burnt to produce SO2 in a chamber which gets dissolved in a fraction (1/15th – 1/20th) of tubewell water to form sulfurous acid (H2SO3) although solubility in water is limited. This H2SO3 neutralizes CO32- and HCO3- ions of water so RSC of such a water is reduced.
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