SODICITY IN IRRIGATED SOILS IN SASKATCHEWAN - CHEMISTRY AND STRUCTURAL STABILITY

Irrigation with sodic waters may damage soil structure, but neither th e processes involved nor the critical levels of exchangeable Na have b een well defined for prairie soils. We examined two irrigated soils fr om southern Saskatchewan on which sodicity damage had occurred to dete rmine the processes and the chemical conditions (exchangeable Na and e lectrolyte 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 scannin g electron microscopy (SEM) showed that sand- and silt-size grains wer e stripped of binding colloidal particles and that large pore spaces h ad formed, creating very loose aggregates. In one of the soils, physic al instability was observed at an exchangeable-Na percentage (ESP) of only about 10%, indicating that some soils in Saskatchewan are relativ ely sensitive to sodicity. With a 1:5 (wt vol(-1)) soil/water extract, the electrical conductivity (EC) needed to prevent clay dispersion wh en 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 adsorpt ion ratio of 20 (mmol, L(-1))(0.5) Sodic conditions greatly altered so il chemical behavior, with the most sodic soil having an extremely hig h level of water-extractable P. In a laboratory experiment, addition o f Ca (as CaCl2 or gypsum) to replace Na reduced water-extractable P fr om 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 pote ntial as exchangeable Na increased. Increased solubility of P along wi th the potential for runoff and erosion from Na-affected soils could r esult in increased inputs of P to surface waters.