Water repellency in a New Zealand development sequence of yellow brown sands

A series of sands on the west coast of the lower North Island, New Zealand, were studied to investigate the effects of time, topography and vegetation cover upon the development of soil water repellency. Severe repellency was measured with the molarity of ethanol droplet (MED) index in the Waitarere and Motuiti dune phase sands, of age <130 years and c. 500 years respectively. In each dune phase, the dune sands were more repellent than the lower lying soils of the sand plains. Low or zero MED values were measured in the 1600-6000 year old Foxton dune phase sands and 10 000-25 000 year old Koputaroa dune phase sandy loams under either pasture or native bush. There was no consistent relationship between bush or pasture cover and repellency severity in the Foxton and Koputaroa soils, however, the species composition of the pasture and bush differed. The Waitarere sand was the most repellent soil, despite a low organic carbon content. The carbon content profiles of most of the soils did not appear to be related to the respective MED profiles of repellency severity. The MED values of the surface layer from five dune sands were generally related inversely to the fulvic acid (FA) content and proportionally to the humic acid to fulvic acid ratio (HA/FA), which were measured in a previous study. The pH of the five soils ranged from 5.61 to 6.89, with no apparent relationship between pH and MED. A study of soil water content indicated that repellency reduced rainfall infiltration into the Waitarere and Motuiti sands and the Himatangi sand, found on elevated sand plains. The most severely repellent sands had the greater variability in soil water content after rainfall.