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Iron and Aluminum Oxide Influence on Agronomic Effectiveness of Modified Phosphate Rocks

Research and Development Division, Int. Fertilizer Development Center, P.O. Box 2040, Muscle Shoals, AL 35662

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

ABSTRACT

A greenhouse experiment was conducted to study the influence of Fe and Al oxide in phosphate rocks (PR) on partially acidulated phosphate rock (PAPR) effectiveness and to evaluate compaction of PR with soluble phosphates as an alternate method of utilizing PR with high Fe and Al oxide. Five PR with Fe + Al oxide ranging from 19 to 320 g kg^–1 were acidulated with 50% of the H₂SO₄ (PAPR, 50% H₂SO₄) required to fully convert the PR to triple superphosphate (TSP) or were compacted with TSP so that 50% of the total P was in TSP form (PR + TSP). They were applied to Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapludult, pH 4.8) at rates 50, 100, 200, and 300 mg P kg^–1. Two successive crops of maize (Zea mays L.) and one of rice (Oryza sativa L.) were grown. Biomass yield of the first crop from PAPR-treated soils decreased from 86% of that from TSP to 16% and P uptake from 72 to 12% with increasing Fe and Al oxide from 1.9 to 32%. The decrease in effectiveness of PAPR was directly related to a decrease in water-soluble P in PAPR with increasing Fe and Al oxide content. The compacted PR = TSP were 86 to 104% as effective as TSP in increasing biomass yield and 78 to 94% as effective in increasing P uptake. There were no great differences between PAPR and the compacted PR + TSP in terms of residual effect on the second crop of maize, except for Matongo PR. The difference in residual effectiveness between PAPR and compacted PR + TSP was not as pronounced for rice as for maize.

Received for publication March 11, 1994.

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