Long term effects of withholding phosphate application on North Island hill country: Ballantrae

Abstract

In a farmlet grazing trial at DSIR's Ballantrae Hill Country Research Station near Woodville the effect of withholding phosphate application was investigated. Soils are yellow-brown earths and related steepland soils with low phosphate retention, and average annual rainfall is 1200 mm. Four 10 ha farmlets, 2 each with histories of high (HF) or low (LF) superphosphate application, had annual applications continued at 125 (LF-LF) or 375 (HFHF) kg/ha; or discontinued on one farmlet in each case (LF-NF and HF-NF), starting in 1981. Soil Olsen P level decreased on the HF-NF area, but not on LF-NF and pasture production decreased by 4.6 and 1.7% p.a. respectively. Botanical composition was affected only on the HF-NF farmlet. Ewes were lighter as a result of not applying fertiliser. Ewe and lamb wool production, and lamb liveweight, were reduced by an average 1.2, 3.7 and 2.6% p.a., respectively, over the 7 years. A phosphate application trial suggested that reactive phosphate rock was as effective as superphosphate in stimulating legume responses in the 1st year after restarting phosphate application, and had greater residual activity. Keywords fertiliser, withholding, cessation, phosphate, superphosphate, hill country, pastures, soil fertility, wool, sheep