Amelioration of water repellency: application of slow-release fertilisers to stimulate microbial breakdown of waxes

Our research effort into improving agricultural production on water repellent sands concentrated on the use of slow-release fertilisers to stimulate indigenous wax-degrading microorganisms to reduce the repellency. Laboratory and glasshouse experiments conducted with two slow-release sources of nitrogen and phosphorus (MaxBac (N:P:K:S 22:5.7:0:0.6) and MagAMP (N:P:K:Mg 7:20:5:9)) added to water repellent sand, in the absence of plant growth, resulted in a significant drop in hydrophobicity values apparently due to stimulation of wax-degrading microorganisms already present in the soil. Accordingly, two field experiments were set up in the south east of South Australia in which three different rates of the slow-release fertilisers were applied together with a low rate of kaolinitic, Mundulla clay. Subterranean clover was sown, but weeds were not controlled due to the unknown effect of herbicides on the soil microbial population. There was a significant decrease in water repellency at one site in the spring of the second year for the highest rates of MaxBac compared to the unfertilised control at a depth of 0-5 cm. At the end of the summer, however, the water repellency had risen to the same value as the untreated controls at both sites. The following winter and spring, there was a decrease in water repellency at both sites, though there was no clear trend between treatments. The presence of plant growth appeared to be a key factor in the lack of a sustained effect of the fertilisers. The reduction in hydrophobicity, either due to degradation of waxes or the movement of dissolved organic matter, was reversed when temperatures were elevated in summer. Dissolved organic matter was found to decrease the severity of water repellency and may be an important factor in developing an amelioration strategy.