Simulated rainfall, using low conductivity water, was applied at an intensity of 63 mm h#x2212;1to a field soil of the Doveton series (a kaolinitic, sesquioxidic clay soil at Cedara in Natal, recently cultivated out of pasture) after pre-treatment of the scarified soil surface with a mulch of maize stover, 200 kg ha#x2212;1of sodium hexametaphosphate (applied in solution), or 10 000 kg ha#x2212;1of industrial phosphogypsum. The decline in infiltration rate with cumulative rainfall was accelerated by the hexametaphosphate (final infiltration rate after 126 mm rain was 27 mm h#x2212;1compared with 42 mm h#x2212;1for the untreated control) and retarded by the gypsum (final IR=47 mm h#x2212;1) and mulch (53 mm h#x2212;1) treatments. After 60 mm of a second rain, final IR was 26 (control), 24 (hexametaphosphate), 35 (gypsum) and 26 mm h#x2212;1(mulch). Laboratory-simulated rain produced a similar result with gypsum to that found in the field. Ultrasonic treatment of suspensions of the same soil showed (i) that with decreasing aggregate size, an increase in clay dispersion occurs for a given energy input, and (ii) that the presence of low concentrations of citrate, orthophosphate and hexametaphosphate in solution greatly increases the yield of dispersed clay per unit of energy applied to the suspension. A hypothesis is advanced which states that the presence of adsorbed humic substances neutralizes positively charged surfaces and greatly reduces the beneficial effect of electrostatic attraction on aggregate stability. This runs counter to the usual assumption of humus providing enhanced soil aggregate stability. Provided enough mechanical energy is supplied, many apparently stable soils may show a high capacity for clay dispersion and consequent physical deterioration.
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