Predictability of existing indices and an alternative coefficient for estimating sodicity build-up using Adj.R_Na, and permissible limits for crops grown on soils irrigated with waters having residual alkalinity

Various indices in vogue (SAR¹, adj.SAR², adj.R_Na³) for determining the sodicity hazards of irrigation waters having residual alkalinity were tested with long-term field data available for different cropping sequences viz., fallow-wheat, paddy-wheat, millet/maize-wheat and cotton-wheat from various parts of north-west India. The EC, SAR, adj.SAR, adj.R_Na and RSC of these waters ranged between 0.4–10 dS m⁻¹, 0.6–43 (mmol L⁻¹)^½, 0.6–102 (mmol L⁻¹)^½, 0.6–57 (mmol L⁻¹)^½ and 0–15 me L⁻¹, respectively. Only those data sets were selected for comparisons where the soils had been irrigated with alkali waters for more than 5 years. Empirical relations between exchangeable sodium percent (ESP, 0–0.3 m soil) and above indices like SARiw (R² = 0.44–0.76), adj.SAR (R² = 0.45–0.83) and adj.R_Na (R² = 0.36–0.76) could only be employed for particular cropping sequences and there was lack of general validity for pooled data of different rotations (R² = 0.29–0.31). Even when the leaching fraction (LF) and mineral-weathering coefficient were put into equations, the predictions of sodicity did not improve (R² = 0.20–0.28). However, the predictability using adj.R_Na improved remarkably (R² = 0.69**) with an alternative coefficient based on annual quantities of alkali waters applied (D_IW),the rainfall (D_RW) at the site and the evapotranspiration (ET) demands of the crops grown in sequence viz., ESP = (D_IW/D_RW) (√(1+ D_RW/ET) (adj.R_Na). Maas & Hoffman type stepwise linear response functions fitted for wheat, paddy, millet/maize and cotton irrigated with alkali waters showed the threshold sodicity levels (ESP_t, 0–0.3 m soil) to be 16.2, 20.1, 6.1 and 14.9 while the slopes were 1.86, 1.62, 1.34 and 1.26 per cent/ESP, respectively. Predictions of sodicity, based upon the proposed equation, and the relative yields from sodicity response functions were utilised to propose permissible limits in terms of adj.R_Na for the sustainable use of alkali waters for different rainfall regions and the cropping sequences.