POLYMER EFFECTS ON RUNOFF AND SOIL-EROSION FROM SODIC SOILS

High levels of soil sodicity, resulting from intensive irrigation with saline-sodic waters, lead to an increased soil susceptibility to seal formation and to severe problems of runoff and soil erosion. The obje ctive of this study was to investigate the efficacy of the addition of small amounts of an anionic polyacrylamide (PAM) to the irrigation wa ter in controlling seal formation, runoff and soil erosion. Two predom inantly montmorillonitic sails were studied, a grumusol (Typic Haploxe rert) and a loess (Calcic Haploxeralf), having naturally occurring exc hangeable sodium percentage (ESP) >12. The soils were exposed to 60 mm of simulated irrigation with commonly used tap water (TW, electrical conductivity=0.8 dS m(-1); sodium adsorption ratio (SAR)=2), or saline water (SW, electrical conductivity=5.0 dS m(-1);SAR>12). PAM effectiv eness in controlling runoff and erosion from the sodic soils was compa red with runoff and erosion levels obtained from untreated soils havin g low ESPs (<4). For both soils and for both water qualities and polym er concentrations in the irrigation water, PAM was efficient in contro lling runoff at low ESP levels and inefficient at high ESP levels. At moderate ESP levels, PAM's efficacy in controlling runoff was inconsis tent and varied with water quality and polymer concentration. Converse ly, in general, soil loss originating from rill erosion, was significa ntly and effectively reduced in moderate and high ESP soils by additio n of PAM to the irrigation water, irrespective of water quality and po lymer concentration. PAM was more effective in reducing rill erasion t han in reducing runoff in the moderate and high ESP samples, because t he energy involved in generating runoff is much higher than that invol ved in rill erosion. PAM treated surface aggregates were not stable ag ainst the distructive forces leading to seal formation and runoff prod uction; but they were stable enough to resist the hydraulic shear exer ted by the runoff flow.