The Use of a Modified Mitscherlich Model to Interpret Fertilizer Experiments

SummaryFertilizer experiments of a systematic design were carried out with up to 125 different combinations and levels of N, P or K on lettuce, French beans and summer cabbage in successive years on adjacent sites on a soil depleted of nutrients. The yields fitted a Mitscherlich model modified to account for adverse effects on yield associated with fertilizer dressings.The model allowed the response to each nutrient to be separated into four components : yield with no added fertilizer, the effectiveness of added fertilizer, the adverse effect of applications of fertilizer and the potential maximum yield. With nitrogen fertilizer, year-to-year variations in each of the components were considerable. Although lettuce was more sensitive than either of the other crops to the adverse effects of nitrogen, the beneficial effects were similar on all three crops. Inter-year variability was much less with phosphorus and potassium fertilizers, and depressions in yield with heavy applications were small. Responses to phosphorus were greatest with lettuce and least with summer cabbage. Potassium considerably increased yields of French beans but had little effect on summer cabbage.