Shainberg, I., Institute of Soils and Water, ARO, The Volcani Center, P.O. Box 6, Bet Dagan, Israel
Levy, G.J., Institute of Soils and Water, ARO, The Volcani Center, P.O. Box 6, Bet Dagan, Israel
Rengasamy, P., State Rivers and Water Supply Commission, Irrigation Research Institute, Tatura, VIC, 3616, Australia
Frenkel, H., Institute of Soils and Water, ARO, The Volcani Center, P.O. Box 6, Bet Dagan, Israel

In soils exposed to rain, aggregate disintegration is the first process which leads to seal formation. The objective of this study was to evaluate the relative importance of aggregate stability in seal formation. The effects of raindrops’ impact energy, exchangeable sodium percentage (ESP), electrolyte concentration in the applied water, and addition of an anionic plyacrylamide (PAM) on aggregate stability and seal formation were studied on three smectitic cultivated soils from Israel using laboratory drip-type rain simulators. Aggregate slaking took place much faster than seal formation; only 9 mm of rain were needed to disintegrate the aggregates compared with >40 mm of rain needed for seal formation. Soil ESP enhanced aggregates’ breakdown at the upper ESP range, whereas the effect of ESP on seal formation was at the lower ESP range. Electrolyte concentration in the applied water had no affect on aggregate disintegration but affected the rate and final infiltration rate of the seals formed. Adding PAM to the soil improved aggregate stability and increased the permeability of the seal. Aggregate breakdown was suggested as the first step in seal formation, to be followed by surface compaction and clay dispersion. © 1992 Williams & Wilkins.