Patterns of, and a model for, dry matter yield response in grass/clover pastures to annual applications of triple superphosphate fertiliser

Abstract

Pasture herbage dry matter (DM) responses to annual applications of triple superphosphate (TSP) at 0.5, 0.75, 1.0, and 2.0 times the calculated maintenance requirement were measured in 12 field trials for 6 years. The trials were on well-established, permanent pastures which had typical fertiliser histories and represented major pastoral soils of North 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 responses in total DM over 6 years ranged from 4 to 24%. Responses increased steadily with time at four sites, but at other sites changes with time were erratic. Mitscherlich curves were fitted to the 6-year total DM production data for each site. However, the standard errors in the parameters of these fitted curves were very large, making it impossible to predict with any useful degree of accuracy the rate of TSP required for 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 compartmental model of P in the soil combined with a Mitscherlich relationship between pasture production and available soil P. The observed response patterns were adequately accounted for by this model with the exception of yields from the nil P control treatment. These declined more slowly than model predictions, suggesting that without P fertiliser, plants made relatively more use of less-available P, e.g., from deeper in the soil profile or from mineralisation. © 1994 Taylor & Francis Group, LLC.