Partially Acidulated Rock Phosphate as a Source of Phosphorus to Plants: III. Uptake by Corn from Soils of Different Calcium Status

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Abstract
Rock phosphate was treated with H3PO4 to give 5, 10, 50, and 100% acidulations relative to triple superphosphate. The resulting materials were applied to three Ohio soils brought to different calcium levels. The P uptake and the percentage recovery of added P by corn plants (Zea mays L.) were determined. The soils 1/3‐3/4 Ca saturated generally resulted in better yields, P uptake, and P fertilizer recovery than other Ca saturations. Acidulations beyond 10% in soils less than 1/3 Ca saturated and beyond 50% in soils 1/3‐3/4 Ca saturated were generally not significantly beneficial to P uptake or percentage recovery of added P. Comparison of fertilizers with similar amounts of soluble phosphate but different amounts of insoluble (unreacted) rock phosphate indicated that on the average 1.3 to 1.5 times as much of the added “available” phosphorus was recovered where more insoluble P was present. This was attributed to the reaction of the soil‐formed H3PO4 with unreacted rock phosphate so that less phosphate was fixed by solubilized Al and Fe compounds.

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