Clay dispersivity and aggregate stability effects on seal formation and erosion in effluent-irrigated soils

Seal formation in soils exposed to rain is determined by aggregate disintegration and clay dispersion. Aggregate disintegration is related strongly to rate of aggregate wetting, whereas dispersion is dictated by soil solution electrolyte concentration and composition at the soil surface. It has been hypothesized that the relative importance of these two mechanisms in determining soil susceptibility to sealing depends on clay content. We used a laboratory rainfall simulator to study dependence of infiltration rate (IR), runoff, and interrill erosion on (i) a fast wetting rate (WR) (64 mm h) serving as the control, (ii) a fast WR combined with surface application of 5 Mg ha phosphogypsum (PG), designed to prevent clay dispersion, and (iii) a slow WR (2 mm h) designed to prevent aggregate slaking. Experiments were conducted with samples of five Israeli soils of varying clay content (8–65%) taken from fields that had been irrigated with effluents for more than 15 years. Final IR values for sandy clay (38.6% clay) were 3.5, 11.75, and 6.0 mm h for fast WR (i.e., control), fast WR+PG, and slow WR, respectively. Similar trends were noted in the other soils. Consequently, runoff and soil erosion losses were significantly lower in fast WR+PG and in slow WR compared with fast WR. In soils with clay content more than 40%, final IRs were higher and runoff and soil loss were lower in slow WRs compared with fast WR+PG. The opposite held true for soils with less than 40% clay. These results suggest that management practices selected for decreasing soil susceptibility to seal formation in effluent-irrigated soils should consider clay content. In soils with less than 40% clay, prevention of physicochemical clay dispersion by PG application is preferable to prevention of aggregate slaking by use of slow WR. In clay soils (>40% clay), prevention of aggregate slaking during the wetting process of the soil could be more beneficial than prevention of the physicochemical clay dispersion.