the relationship between N and P is well known, as is the relationship between P and copper (Cu), Fe, manganese (Mn), and zine (Zn), ratios of 3 to 1 between N and P, and 200 to 1 between P and Zn are considered critical. the ratio of N to P is used as a DRIS norm for interpreilting a plant analysis.
soluble P (in 2 % acetic acid) is present as the orthophosphate (PO) anion in main stems and leaf petioles of the actively growing portions of the plant. Its concentration ranges from 100 to 5000 ppm of the dry weight and can be used to evaluate the P status of the plant. critical concentrations occur at approximately 2500 ppm. a list of critical soluble P values for several crops are given in table 2.6