AN EMPIRICAL-METHOD FOR DESCRIBING THE JOINT EFFECTS OF ENVIRONMENTALAND OTHER VARIABLES ON CROP YIELD

There is considerable interest in developing understanding of the spat ial variation of crop yield within fields in relation to the spatial v ariation of environmental factors. Such understanding might allow inpu ts to crops to be used more efficiently by spatially variable applicat ion. The joint action of factors on crop yield (whether controlled tre atments or environmental variables) may be additive, interactive or po tentially limiting in the sense of von Liebig's ''Law of the Minimum'' . The nature of the joint action of an input and environmental factors has profound implications for the possibility of improving the effici ency of use of the input by spatially variable application. Convention al methods for analysing data on such factors and yield (e.g. multiple regression) make specific assumptions compatible only with a narrow r ange of permissible models. A procedure is proposed and demonstrated i n this paper for the exploratory analysis of data from experiments and /or crop surveys in order to describe the joint action of important fa ctors on crop yield. The procedure could be applied to specific experi ments in order to indicate whether there is any scope for optimising r ates of an input with respect to one or more Spatially variable factor s, and for indicating whether the relationship between the local optim um rate of the input and values of these factors is simple (factors ac t additively or in accordance with the Law of the Minimum) or complex (factors interact).