Gypsum: An Economical Amendment for Amelioration of Saline- Sodic Waters and Soils, and for Improving Crop Yields

The Indus Plains of Pakistan are situated in arid to semi-arid climate where monsoon rains are erratic and mostly fall in the months of July, August and March, which are quite insufficient to grow even a single crop without artificial irrigation. To make the agriculture a success under the ambient agro-environment, a net work of gravity flow surface irrigation canals is handling 111.1 MAF water, about 48 MAF pumped ground water from > 0.4 million tube wells and sewage irrigation around urban dwellings. At present, 6.3 mha soils are salt-affected and 70-80% of the pumped ground water is hazardous for irrigation. Competition among the agricultural and non-agricultural uses has decreased the sweet water availability for the former sector, which is expected to continue in future. As a consequence, brackish ground water (high EC, SAR, RSC) is being pumped more and more to practice irrigated agriculture that might be a sustainability risk in the long run. Water quality parameters include EC for total soluble salts, and SAR (high sodium with low Ca 2+ +Mg 2+ ) and RSC (high CO3 2- +HCO3 - or low Ca 2+ +Mg 2+ ) reflect the sodicity hazards. The ground water, drainage water and sewage become hazardous because of high EC (>1.0 dS m -1 ), SAR (>10.0) and/or RSC (>2.5 mmolc L -1 ). For lowering high EC of water, only dilution with low electrolyte water is the option. In this case, use of any amendment (gypsum, acids, acid formers) will increase it further without any beneficial effect. To lower high water SAR, gypsum is the most economical amendment, dilution will decrease it by the square root times of the dilution factor, while use of any acid (sulphurous acid or sulphuric acid) or acid former has to do nothing with high water SAR rather will induce cost-intensiveness without any gain rather may deteriorate the soil health (physically and chemically) if acids or acid formers are used for longer periods. For high RSC, dilution with low CO3 2- +HCO3 - water will decrease it proportionately to the dilution factor, Ca-salts will increase Ca 2+ +Mg 2+ to affect a decrease in water RSC, while acids or acid formers will neutralize CO3 2- +HCO3 - to decrease water RSC. Among RSC treatment amendments, the use of gypsum is economical and safe, while acids could accomplish the same but at a much higher cost. For reclaiming saline soils (ECe ≥ 4.0 dS m -1 , SAR ≥ 13.0), no amendment is required rather simple leaching with all the types of water (canal, ground water, agricultural drainage) is useful during early phase of reclamation following a gradual shift toward sweet water application. For saline-sodic (ECe ≥ 4.0 dS m -1 , SAR ≥ 13.0) and sodic soils (ECe ≥ 4.0 dS m -1 , SAR ≥ 13.0), Ca-carriers (gypsum, calcium chloride, calcium nitrate, phosphogypsum, later three being industrial by-products) are economical, acids (H2SO4, HCl, HNO3) or acid formers (sulphur, calcium poly-sulphide, pyrite, ferrous sulphate) can reclaim such soils relatively at a faster rate but at 5-10 times higher cost.