Diagnosis and yield prognosis model and method were developed on the basis of data from factorial field experiments on soybean (Glycine max(L) Merr.) conducted at AIT, Bangkok. Composition of N and P in the aerial parts of the plant at a fixed dry matter weight formed the basis of the model. The method included the determination of absolute and relative nutritional status of the individual nutrients expressed by diagnosis and the complete nutritional status of the young plant expressed by the final yield as yield prognosis.