Pasture responses to phosphorus and nitrogen fertilisers on east coast hill country: 2. Clover and grass production from easy slopes

From 2000 to 2002 a trial series was established on seven farms from Wairoa (northem Hawke's Bay) to Moeraki (North Otago), with contrasting amounts of spring-summer rainfall, to evaluate the pattern of pasture responses to a range of rates of nitrogen (N) fertiliser and a range of soil phosphorus (P) levels. Small plots excluded from grazing were established on flat to gently sloping land containing pastures that had been established for many years. This paper describes the clover and grass component production responses to N fertiliser and to a range in soil P status at each site. In early spring, in all years, the grass components of pasture at all sites except Marlborough, showed a significant response in dry matter (DM) production to N fertiliser applied in mid winter. These responses continued into late spring-autumn at most sites/years. The consistency from year to year of the relative grass response in spring to N fertiliser allowed significant regression relationships to be developed for the 3-year dataset for the North Island sites. The same result did not occur in Marlborough, presumably because low rainfall limited pasture growth, or at Moeraki (not measured). The effects of N fertiliser on clover production throughout the total season tended to be the opposite of those for grass production. There was generally no response to N fertiliser in early spring but a marked and consistent decline in clover production in late spring-autumn with increasing rate of N fertiliser application. Compared with clover production from plots that received 90 kg N ha(-1), the average clover production in late spring-autumn from plots that received zero N application, was up to the order of three times greater at the Wairoa, Puketapu and Wallingford sites, and about 50% greater at the Waipawa and Wairarapa sites. There was a consistent response by clover in both early spring and in the late spring-autumn seasons, to an increase in soil Olsen P test. On the four northern-most sites an increase in Olsen P test from less than 10 to about 20 raised clover production by about 100% in early spring and by a greater proportion in late spring-autumn. This was in direct contrast to the absence of a response by the grass component during the same period. The management implications of the contrasting effects of N and P fertilisers on pasture clover and grass composition are discussed.