Theories and methods on plant nutrition and growth

A theory comprising two basic concepts relating nutrition and growth is presented. The first concept is a nutrient flux model and is based upon studies of plants at constant internal nutrient concentrations, where a formal mathematical derivation shows that the relative uptake rate (R U ) and the relative growth rate (R G ) are equal. Deviations from equality are results of experimental insufficiencies and errors. The second concept is based on the observation that R G is linearly related to the internal nutrient concentration. The slope represents nutrient productivity (P n ), an important parameter expressing growth rate per unit of nutrient. Light and the plant genome, for example, influence the value of the proportionality factor, P n , but not the formal relationship between the internal nutrient concentration and R G Not only the theory itself but many results and conclusions are very different from those obtained with traditional methods. In experiments where R U was controlled during the exponential period of growth, the relationships between treatment (the relative addition rate, R A ), nutrient uptake (R U ) and growth (R G ) were reproduced with extremely low variability. In agreement with theory, internal nutrient concentration and R G remained stable over time (steady‐state). An extension of the theory is based upon the empirical assumption that after exponential growth, self‐shading and ageing reduce P n in proportion to biomass. This assumption has been successfully applied in predicting growth of forest stands, but the nature of the growth reduction is little understood. The generalized model has few parameters and can easily be rewritten to suit different experimental aims, for example to establish reference values and to model changes in soil fertility. Further extension and understanding of the model and different interpretations of the parameters are discussed.