A COMPARISON OF 3 MATHEMATICAL-MODELS OF RESPONSE TO APPLIED NITROGEN- A CASE-STUDY USING LETTUCE

Modern fertilization recommendations must optimize crop yield and qual ity and minimize chances of negative environmental effects due to over fertilization. Data from fertilizer studies can be fitted to several m athematical models to help determine optimum fertilizer rates, but res ulting recommendations can vary depending on the model chosen. In this research, lettuce (Lactuca sativa L.) was used as a case study vegeta ble crop to compare models for estimating fertilizer N requirements. G reenhouse studies were conducted with 'South Bay' and 'Sierra' cultiva rs of crisphead lettuce to measure yield response to applied N. Indivi dual plants were grown in pots and received six rates of N (0,0, 0.2, 0,4, 0.6, 0.8, and 1.0 g/plant) as ammonium nitrate in split applicati ons. Data for plant fresh mass and N uptake were recorded. The logisti c model described the data for both cultivars quite well, with correla tion coefficients of 0.98 and above. The logistic model was also appli ed to field data for average head mass of 'South Bay' lettuce followin g application of N at 0, 56, 112, 168, 224, and 280 kg.ha(-1). Logisti c, linear-plateau, and quadratic models were compared for the field da ta. Coefficients for the linear-plateau model were derived from the lo gistic model. All three models for lettuce production were compared gr aphically and analytically. The model coefficients were then used to m ake improved estimates of fertilizer recommendations for field product ion of lettuce.