Sodicity in irrigated soils in Saskatchewan: Chemistry and structural stability

Abstract

Irrigation with sodic waters may damage soil structure, but neither the processes involved nor the critical levels of exchangeable Na have been well defined for prairie soils. We examined two irrigated soils from southern Saskatchewan on which sodicity damage had occurred to determine the processes and the chemical conditions (exchangeable Na and electrolyte concentration) that cause structural damage. Dispersion of clays in the upper 20 cm of the profile seemed to be the primary cause of structural deterioration. Examination of irrigated soil by scanning electron microscopy (SEM) showed that sand- and silt-size grains were stripped of binding colloidal particles and that large pore spaces had formed, creating very loose aggregates. In one of the soils, physical instability was observed at an exchangeable-Na percentage (ESP) of only about 10%, indicating that some soils in Saskatchewan are relatively sensitive to sodicity. With a 1:5 (wt vol−1) soil/water extract, the electrical conductivity (EC) needed to prevent clay dispersion when soil suspensions were mechanically agitated was about 0.2 dS m−1 in the absence of Na, increasing to 1.5–2 dS m−1 at a sodium adsorption ratio of 20 (mmolc L−1)0.5. Sodic conditions greatly altered soil chemical behavior, with the most sodic soil having an extremely high level of water-extractable P. In a laboratory experiment, addition of Ca (as CaCl2 or gypsum) to replace Na reduced water-extractable P from 78 mg kg−1 to less than 20 mg kg−1. The effect of sodicity on P solubility was likely due to a decrease in surface electrostatic potential as exchangeable Na increased. Increased solubility of P along with the potential for runoff and erosion from Na-affected soils could result in increased inputs of P to surface waters. Key words: Sodicity hazard, clay disperson, phosphate solubility