AN EXTENDED NITROGEN, PHOSPHORUS, AND POTASSIUM MODEL FOR TALL FESCUE

Dry matter yields and plant nutrient uptake of forage grasses are infl uenced by levels of applied nitrogen (N), phosphorus (P), and potassiu m (K). Response to one element generally depends on levels of the othe r two. In this article, a mathematical model is presented which includ es the major elements, N, P, and K as inputs. It consists of triple lo gistic equations with a total of thirteen parameters. The model is eva luated for Kentucky 31 (KY 31) and Kenwell tall fescue (Festuca arundi nacea Schreb.) grown at Watkinsville, GA on Cecil sandy loam (clayey, kaolinitic, thermic, Typic flapludult). Procedures are described for p arameter estimation. The model provides high correlation between yield s and plant uptake of N, P, and K with applied N, P, and K for both cu ltivars of tall fescue, as demonstrated in response graphs and scatter diagrams. Intercept and N response coefficients from this study agree closely with those from previous work. Data from this study support t he hyperbolic relationship between dry matter yield and plant N uptake predicted by the model. The model is mathematically well-behaved and is relatively easy to use in practice.