FERTILIZER AND SOIL P UPTAKE BY MAIZE, AS AFFECTED BY SOIL P LEVEL, GRANULE SIZE, AND SOLUBILITY OF PHOSPHATE SOURCES

Two greenhouse pot experiments were conducted to evaluate labelled concentrated superphosphate (CSP) and labelled dicalcium phosphate (DCP) alone and in mixtures for maize grown on soils low, high and very high in available P. On the low P soil yield of dry forage, total uptake of P and uptake of fertilizer P increased linearly with 0, 20, 40 and 80 ppm of P applied as the various fine and granular sources varying widely in P water solubility. Hence, all of these criteria were highly correlated and suitable for evaluating effectiveness of the P on the low P soil. On the soil high and very high in P, only uptake of fertilizer P, which also increased linearly with rate of application, was suitable for evaluating the sources. Total uptake of fertilizer P decreased with increase in soil P level, but relative differences among granular sources were similar at the three soil P levels. Effectiveness of the P fertilizers decreased markedly with decreasing water solubility of the granular sources on all soils; the same trend was true for the fine sources at high soil P levels. Actual uptake of fertilizer P or percentage recovery of labelled P applied was a better basis for source evaluation at high soil P levels than were calculated A values. There was little advantage in this study of labelling with 32P for P source evaluation at the low soil P level over use of multiple rates of the unlabelled fertilizers. However, labelling was essential at the higher soil P levels. © 1969, Cambridge University Press. All rights reserved.