A Unifying Concept for the Dependence of Whole-crop N : P Ratio on Biomass: Theory and Experiment

Numerous estimates have been made of the concentrations of N and P required for good growth of crop species but they have not been defined by any unifying model. The aim of the present study was to develop such a model for the dependence of the N : P ratio on crop mass, to test its validity and to use it to identify elements of similarity between different crop species and wild plants. A model was derived between plant N : P ratio (R(w)) and its dry biomass per unit area (W) during growth with near optimum nutrition by considering that plants consist of growth-related tissue and storage-related tissue with N : P ratios R(g) and R(s), respectively. Testing and calibration against experimental data on different crop species led to a simple equation between R(w) and W which was tested against independent experimental data. The validity of the model and equation was supported by 365 measurements of R(w) in 38 field experiments on crops. R(g) and R(s) remained approximately constant throughout growth, with average values of 11.8 and 5.8 by mass. The model also approximately predicted the relationships between leaf N and P concentrations in 124 advisory estimates on immature tissues and in 385 wild species from published global surveys. The N : P ratio of the biomass of very different crops, during growth with near optimum levels of nutrients, is defined entirely in terms of crop biomass, an average N : P ratio of the storage/structure-related tissue of the crop and an average N : P ratio of the growth-related tissue. The latter is similar to that found in leaves of many wild plant species, and even micro-organisms and terrestrial and freshwater autotrophs.