The Seagram Center for Soil and Water Sciences, The Hebrew University of Jerusalem, PO Box 12, Rehovot, 76100, Israel

EPP values for the three soils ranged as follows: 0.8–1.3, 5.5–9.2, 16.0–21.0 and 58.0–76.0 for PAR's of 0.0, 0.72, 3.74 and ∞, respectively. HC incrased slightly (20%) up to EPP values of about 20% for the loamy sand and heavy clay soil, while a decrease in HC corresponding to any increase of EPP was observed for the light clay soil. This soil was richer in illite and also exhibited higher affinity for K⁺. At the highest EPP values (58.0–76.0) HC of the three soils decreased to about 20% of the values measured for the Ca²⁺ saturated soils.

SEM observations were performed and Ca²⁺ saturated soils compared with the K⁺ enriched ones. Ca²⁺ treated loamy sand exhibited discrete clay aggregates located in the spaces between sand particles or attached to them. K⁺ enrichment resulted in the formation of a dense network of clay microaggregates filling up the pore space. The microaggregate structure of the two Ca²⁺ clay soils changed to a dense layer composed of much smaller particles following K⁺ enrichment. Pores were mostly smaller than 10 μm in the K⁺ soil compared to several tens of microns in the Ca²⁺ form.

SEM observations and the fact that clay content did not vary with depth suggest that dispersion of clay microaggregates and their rearrangement in situ were the major mechanisms involved in HC reduction, rather than long-range clay migration and the formation of a clay enriched layer with impeded drainage.