A plant test for sufficiency of phosphorus based on the phosphorus-nitrogen interaction in sunflower nutrition.

Sunflower plants were alternated between complete nutrient sol. and sol. devoid of either P or N. Successive harvests of top growth were analysed for P and N fractions to follow the time course of depletion and restoration. Under adequate P the TCA-insoluble protoplasmic substance contained 25 mol P/1000 mol N. Unrestrained incorporation of P needed the presence of at least 25 mol inorganic P/1000 mol protoplasmic N; P was adequate with at least 50 mol of total P/1000 mol N converted to proteins and nucleic acids. This ratio agreed with the ratio total P:total N at which max. growth was attained in published work of both field and pot experiments, presumably because most of the N was protein bound. Because of the possibility of higher contents of TCA-soluble N, it is better that total P be related to the TCA-insoluble N. (Abstract retrieved from CAB Abstracts by CABI’s permission)