Ratio and rate effects of 32P-triple superphosphate and phosphate rock mixtures on corn growth

the availability of phosphorus (p) from " patos de minas" phosphate rock (pr) can be improved if it is applied mixed with a water-soluble p source. the objective of this study was to evaluate 32p as a tracer to quantify the effect of the ratio of mixtures of triple superphosphate (tsp) with pr and the rates of application on p availability from pr. two experiments were conducted in a greenhouse utilizing corn (zea mays l.) plants as test crop. in the first experiment, the p sources were applied at the rate of 90 mg p kg-1 soil either separately or as compacted mixtures in several tsp:pr ratios (100:0, 80:20, 60:40, 50:50, 40:60, 20:80 and 0:100 calculated on the basis of the total p content). in the second experiment, the tsp was applied alone or as 50:50 compacted mixtures with pr applied at four p rates (15, 30, 60 and 90 mg p kg-1) while the sole pr treatment was applied at the 90 mg kg-1 p rate . the mixture of pr with tsp improved the p recovery from pr in the corn plant and this effect increased proportionally to the tsp amounts in the mixture. when compared with the plant p recovery from tsp (10.52%), pr-p recovery (2.57%) was much lower even when mixed together in the ratio of 80% tsp: 20% pr. there was no difference in pr-p utilization by the corn plants with increasing p rates in the mixture (1:1 proportion). therefore, pr-p availability is affected by the proportions of the mixtures with water soluble p, but not by p rates.