Pasture responses to phosphorus and nitrogen fertilisers on East Coast hill country: Total production from easy slopes

• Published in: New Zealand journal of agricultural research 2007-09, Vol.50 (3), p.307-320
• Main Authors: Gillingham, A. G., Morton, J. D., Gray, M. H.
• Format: Article
• Language: English
• Subjects: hill country; nitrogen fertiliser; pasture production; phosphate fertilizer
• ISSN: 0028-8233; 1175-8775
• DOI: 10.1080/00288230709510299

A series of small, mowed plots, excluded from grazing, were established on flat to easy sloping (up to 15°) pastures on farms from Wairoa (northern Hawke's Bay) to Moeraki (North Otago) with contrasting degrees of spring, summer and autumn dryness, to evaluate the pattern of pasture responses to a range of nitrogen (N) fertiliser rates, and associated soil phosphorus (P) levels. Mean annual rainfall ranged from 474 (Marlborough) to 1348 mm (Wairoa) with all farms having an associated range in spring/summer/autumn rainfall. In mid-late winter 2000, fertiliser P and N were applied at 0, 30, 60 or 90 kg ha -1 in a completely randomised design with incomplete replication. In winter 2001 and 2002 the rates of N fertiliser (urea) treatments were repeated. The application of P fertiliser (triple super) varied between treatments, sites and years in an attempt to generate and stabilise a range of soil P tests at each site. The rates of P applied in 2001 and 2002 were based on the soil Olsen P test in the previous spring. The pasture response to N fertiliser differed between the early spring (August-October) and late spring-autumn seasons. The pasture response in early spring to N fertiliser ranged from 1.58 kg DM kgr 1 N in Marlborough to 17.9 kg DM kg -1 N fertiliser at Wairoa. Conversely, at almost all sites, there was a negative pasture response in late spring-autumn to increasing rate of N fertiliser. The net difference between the positive pasture responses to fertiliser N in early spring and the negative effects in late spring-autumn showed a positive net response over the total season. The overall average for all sites was 12.4 kg DM kg -1 N response at the 30 kg N ha -1 rate, and 7.6 kg DM kg -1 N fertiliser at the 90 kg N ha -1 rate. The relative pasture responses at most sites to increasing soil P status was similar in both the early spring, and in the late spring-autumn seasons. Asymptotic regression relationships (Mitscherlich type), derived for data from each of the Wairoa, Puketapu, Waipawa, Wallingford and Moeraki sites suggested that near peak responses were obtained at Olsen soil P test of 20.