Kind and amount of ions released and sodium hazard in an alkali soil under saline/sodic irrigation waters

The kind and amount of ions released from soil and consequent changes in composition of applied waters were estimated in sandy loam alkali soil of pHs 9.2, sodium adsorption ratio (SAR) 31.9 and electrical conductivity (ECe) 3.2 dS m⁻¹. The soils were equilibrated with waters having three levels of total electrolyte concentration (TEC) (TEC: 10, 20 and 40 m.e. L⁻¹), four levels of SAR (10, 20, 30 and 40) and two levels of residual sodium carbonate (RSC: 0 and 5 m.e. L⁻¹). The analysis of effluent collected at equilibrium showed that Na followed by Ca+Mg were the major cations and CO₃ and HCO₃ were the major anions released, as a result of which total salt concentration increased by 4.0 to 7.0 m.e. L⁻¹ compared to applied waters. Owing to the proportionately higher release of Ca+Mg, effluent SAR was drastically reduced by 6 to 72%, particularly where initial total electrolyte concentration of water was low and SAR was high. Higher release of CO₃+HCO₃ than Ca+Mg increased RSC of the effluent by 0.2 to 1.7 m.e. L⁻¹. To find out the ions released from mineral weathering, previously equilibrated soil columns were again leached uniformly with pore volumes of good quality water having SAR 1.5 and TEC 6.1 m.e. L⁻¹ at biweekly intervals till the steady composition of coming out solution was achieved. Here also a continuous released Na varied from 3.9 to 6.8 m.e. L⁻¹ and that of Ca+Mg varied from 0.8 to 1.2 m.e. L⁻¹ in the displaced soil solution at steady state. From break-through curves, it was found that the relative release of Na in soil solution from soil, saturated with waters of different SAR and TEC, both of RSC and non-RSC, was higher and continued for longer duration than Ca+Mg. Though more Na was released than Ca+Mg but ratio of Na:Ca+Mg ions released into the soil solution varied only from 3.5 to 6.8 which would reduce the sodic hazards (SAR) of irrigation waters having SAR > 9.6. The release of Ca+Mg reduced sodic hazards of high SAR irrigation water, which will not be as high as suggested by their chemical composition.