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 Zea land, 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) in dex in the Waitarere and Motuiti dune phase sands, of age <130 years a nd 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 z ero MED values were measured in the 1600-6000 year old Foxton dune pha se sands and 10 000-25 000 year old Koputaroa dune phase sandy loams u nder either pasture or native bush. There was no consistent relationsh ip 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, desp ite a low organic carbon content. The carbon content profiles of most of the soils did not appear to be related to the respective MED profil es of repellency severity. The MED values of the surface layer from fi ve 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 soil s ranged from 5.61 to 6.89, with no apparent relationship between pH a nd MED. A study of soil water content indicated that repellency reduce d rainfall infiltration into the Waitarere and Motuiti sands and the H imatangi sand, found on elevated sand plains. The most severely repell ent sands had the greater variability in soil water content after rain fall.