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Phosphorus Availability from Phosphate Rock as Enhanced by Water-Soluble Phosphorus

Research and Development Division, International Fertilizer Development Center (IFDC), P.O. Box 2040, Muscle Shoals, AL 35662

*Corresponding author (ifdc.research{at}cgnet.com).

ABSTRACT

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 ³²P 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) ³²P solution and central Florida PR (CFPR), (ii) ³²P-tagged TSP, and (iii) ³²P-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.

Received for publication May 12, 1995.

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