PATTERNS OF, AND A MODEL FOR, DRY-MATTER YIELD RESPONSE IN GRASS CLOVER PASTURES TO ANNUAL APPLICATIONS OF TRIPLE SUPERPHOSPHATE FERTILIZER

Pasture herbage dry matter (DM) responses to annual applications of tr iple superphosphate (TSP) at 0.5, 0.75, 1.0, and 2.0 times the calcula ted maintenance requirement were measured in 12 field trials for 6 yea rs. The trials were on well-established, permanent pastures which had typical fertiliser histories and represented major pastoral soils of N orth and South Islands of New Zealand. The DM response to TSP averaged over all rates and sites was 5% in Year 1, increasing to 23% in Year 4 with little change thereafter. At individual sites, average response s in total DM over 6 years ranged from 4 to 24%. Responses increased s teadily with time at four sites, but at other sites changes with time were erratic. Mitscherlich curves were fitted to the 6-year total DM p roduction data for each site. However, the standard errors in the para meters of these fitted curves were very large, making it impossible to predict with any useful degree of accuracy the rate of TSP required f or any specified yield. Response curves for the 12 sites combined were much more clearly defined. Annual combined response curves after Year 1 were accurately described by the Mitscherlich equation. The overall pattern of response in relation to rates of phosphate (P) fertiliser and years of annual application was interpreted on the basis of a comp artmental model of P in the soil combined with a Mitscherlich relation ship between pasture production and available soil P. The observed res ponse patterns were adequately accounted for by this model with the ex ception of yields from the nil P control treatment. These declined mor e slowly than model predictions, suggesting that without P fertiliser, plants made relatively more use of less-available P, e.g., from deepe r in the soil profile or from mineralisation.