Effects on soil and paddy–wheat crops irrigated with waters containing residual alkalinity
Degradation of soils by irrigation with ground waters containing residual alkalinity poses a major threat to agriculture in semi-arid regions, particularly in South Asia. However, there is a lack of indices to define the soil degradation and crop performance under a monsoon climate. Therefore, an experiment was conducted during 2000–2004 to determine the responses of paddy rice and wheat crops in rotation to irrigation with alkaline waters (AW) having similar salinity (electrolyte concentration 30 me L−1) but varying ionic constituents (sodium adsorption ratio irrigation water, SARiw 10 and 25; adjusted sodium adsorption ratio, adj.RNa 13.6 and 29.2; residual sodium carbonate, RSC 5 and 10 me L−1 and Cl:SO4 4:1 and 1:4, respectively). The concentration factors, ECe/ECiw (ratio of electrical conductivity of soil's saturation paste extract to that of the irrigation water) were between 1.1 and 1.8 for soils deprived of rainfall, whereas it was almost 1 for soils not sheltered from rain. Similarly, saturation paste extract, SARe, was between 1.6 and 2.0 times SARiw and 2.0–2.3 times SARiw with and without rainfall, and the exchangeable sodium per cent (ESP) 1.0–1.8 times SARiw. Yields of paddy relative to yields of crops irrigated with good-quality water, averaged 56–74% during the period 2000–2004 compared with 81–88% for wheat, indicating the greater sensitivity of rice to irrigation with AW. Elevated levels of sulphate rather than chloride in the irrigation water lessened the impacts of the residual alkalinity. Production functions showed that the sodicity (ESP) did not solely explain the variation in crop yields because the salinity stress simultaneously inhibited growth. None of the sodicity indices (RSC, SAR and adj.RNa) adequately defined the relative impacts of AW, although residual alkalinity (RSC) was a better indicator than either of the other two. The monsoon rains played an important role in alleviating the effects of residual alkalinity. Data presented here should support the development of more reliable criteria for the assessment of sodicity/salinity hazards from AW in semi-arid regions.