Soilless container medium components such as peatmoss and perlite have almost no capacity to retain PO4-P, and preplant amendments of triple superphosphate (TSP) are readily leached. Al amendments were tested to reduce P losses from these media. Al2(SO4)3 solutions at rates of 320 and 960 μg Al/cc were applied to a 70 peat: 30 perlite medium and dried at 70C. Adsorption isotherms were created at 25C for the Al2(SO4)3-amended media and an unamended control using solutions of Ca(H2PO4)2 at concentrations of P ranging from 0 to 500 μg·ml–1. Isotherms showed that P retention increased as Al concentration increased. In a greenhouse study, Dendranthema ×grandiflorum `Sunny Mandalay' was grown in these media with 100 g P/m3 from TSP incorporated into the mixes before planting. PO4-P, soluble Al, and pH were determined on unaltered medium solutions collected throughout the cropping cycle and foliar analyses were determined on tissue collected at mid- and end-crop. The highest rate of Al was excessive and resulted in low pH and soluble Al levels in the medium solution early and in the cropping cycle, which were detrimental to plant growth. When Al was applied at 320 μg/cc, soluble Al levels in medium solution were not significantly higher than in the unamended control, PO4-P leached from TSP was reduced, and sufficient PO4-P was released throughout the cropping cycle to result in optimal plant growth.
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