EXTENDED LOGISTIC MODEL OF FORAGE GRASS RESPONSE TO APPLIED NITROGEN,PHOSPHORUS, AND POTASSIUM

Models provide a quantitative means to describe response of forage gra ss to applied nutrients. An extended NPK model of forage grass respons e to applied nitrogen (N), phosphorus (P), and potassium (K) is presen ted, which provides estimates of dry matter yield and plant nutrient u ptake, as well as plant nutrient concentrations. Yield and plant nutri ent uptake are each estimated by logistic equations. Plant nutrient co ncentration is defined as the ratio of plant nutrient uptake to yield, and thus involves the ratio of two logistic equations. For fixed leve ls of two of the three nutrients, the equations reduce to the simple c ase for a single nutrient, developed in previous articles. A rigorous procedure was developed for estimating the model parameters, which was then applied to field data from a complete 4 x 4 x 4 factorial of N x P x K for Coastal bermudagrass on a Cecil sandy loam. It described da ta from the field study rather well, with scatter diagrams which gave correlation coefficients of 0.98, 0.98, and 0.97 between estimated and observed plant uptake of N, P, and K, respectively Estimated maximum potential yield of 29.8 Mg/ha was close to the value 30.1 Mg/ha measur ed at Tifton, Georgia. The model can be used to evaluate interactions among applied N, P, and K, and to estimate nutrient balances and suffi ciency levels for the three major elements. Implementation on a pocket calculator is very simple.