Quantitative estimation of the enhancement effect of water‐soluble P on P availability from phosphate rock (PR) has not been reported. The objective of this study was to use radioactive 32 P as a tracer to distinguish P availability from soil, PR, and TSP so that P uptake by crops from PR in the presence of TSP could be estimated. Three sets of 4‐kg soil samples of an acid Hartsells silt loam (fine‐loamy, siliceous, thermic Typic Hapludult, pH 4.8) were mixed with the following treatments: (i) 32 P solution and central Florida PR (CFPR), (ii) 32 P‐tagged TSP, and (iii) 32 P‐tagged TSP and CFPR at a P ratio of 50:50. The rates of P applied were 0, 12.5, 25, 50, 100, and 200 mg P kg −1 . For treatment (iii), an additional rate of 400 mg P kg −1 was also prepared. Maize ( Zea mays L.) and cowpea ( Vigna unguiculata [L.] Walp. ssp. unguiculata ) were planted and harvested at 42 d after planting for maize and 45 d for cowpea. The effectiveness of P sources in terms of increasing dry‐matter yield and P uptake followed the order of TSP >‐ (CFPR + TSP) > CFPR for maize and TSP = (CFPR + TSP) > CFPR for cowpea. Phosphorus uptake from CFPR in the presence of TSP was higher than P uptake from CFPR applied alone, indicating an enhancement effect of TSP on the effectiveness of CFPR. The increase in P uptake from CFPR due to TSP influence, across all the PR rates applied, was 3.48 mg P pot −1 for maize and 1.38 mg P pot −1 for cowpea. With respect to P uptake from CFPR applied alone, the corresponding relative increase in P uptake from CFPR due to TSP influence was 165% for maize and 72% for cowpea.
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Chien et al. (1996) studied this question.