Effects of long-term use of sodic water irrigation, amendments and crop residues on soil properties and crop yields in rice-wheat cropping system in a calcareous soil

One of the options to ameliorate the deleterious effects of sodic water irrigation is to apply gypsum to soil. We examined whether the application of organic manures or crop residue can reduce the need for gypsum in calcareous soils. A long-term field experiment with annual rice-wheat cropping rotation was conducted for 15 years (1991-2006) on a non-saline calcareous sandy loam soil (Typic Ustochrept) in northwestern, India. The irrigation water treatments included good quality canal water (CW) and sodic water (SW) with residual sodium carbonate (RSC) of 10 mmol_c L⁻¹ from 1991 to 1999 and of 12.5 mmol_c L⁻¹ from 2000 onwards. Gypsum was applied at 0, 12.5, 25, and 50% of the gypsum requirement (GR), to neutralize RSC of the SW. Three organic material treatments consisted of application of farmyard manure (FYM) at 20 Mg ha⁻¹, Sesbania green manure (GM) at 20 Mg ha⁻¹, and wheat straw (WS) at 6 Mg ha⁻¹. The organic materials were applied every year to the rice crop. Continuous irrigation with sodic water for 15 years without gypsum or organic materials resulted in a gradual increase in soil pH and exchangeable sodium percentage (ESP), deterioration of soil physical properties, and decrease in yields of both rice and wheat. The cumulative yield loss in SW irrigated plots without gypsum and organic materials remained <1.5 Mg ha⁻¹ for up to eight years in the case of rice and up to nine years in the case of wheat. Thereafter, marked increase in pH and ESP resulted in further depression in yields of rice by 1.6 Mg ha⁻¹ year⁻¹ and wheat by 1.2 Mg ha⁻¹ year⁻¹. Application of gypsum improved physical and chemical properties of the soil. The beneficial effects on crop yields were visible up to 12.5% GR in rice and up to 50% GR in wheat in most of the years. All the organic materials proved effective in mobilizing Ca²⁺ from inherent and precipitated CaCO₃ resulting in decline in soil pH and ESP, increase in infiltration rate, and a increase in the yields of rice and wheat crops. Although the application of organic materials resulted in comparable reductions in pH and ESP, the increase in yield with SW was higher for both crops with FYM. Pooled over the last six years (2000-2006), application of FYM resulted in 38 and 26% increase in rice and wheat yields, respectively, over SW treatment; corresponding increases in 50% GR treatment (recommended level) was 18 and 19%. During these years, application of GM and WS increased wheat yields by 20%; for rice, GM resulted in 22% increase compared to 17% in WS amended SW irrigated plots. Combined application of gypsum and organic materials did not increase the yields further particularly in the case of FYM and GM treated plots. This long-term study proves that organic materials alone can be used to solubilize Ca from inherent and precipitated CaCO₃ in calcareous soils for achieving sustainable yields in sodic water irrigated rice-wheat grown in annual rotation. The results can help reduce the dependency on gypsum in sodic water irrigated calcareous soils.