Ar. Overman et Sr. Wilkinson, EXTENDED LOGISTIC MODEL OF FORAGE GRASS RESPONSE TO APPLIED NITROGEN,PHOSPHORUS, AND POTASSIUM, Transactions of the ASAE, 38(1), 1995, pp. 103-108
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.