EFFECTS OF pH AND PHOSPHORUS RATES ON SOIL-SOLUTION PHOSPHORUS AND PHOSPHORUS AVAILABILITY

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Abstract
Results from short-term, absorption experiments with roots in Ca-free culture solutions have led to the concept that phosphorus is absorbed largely or exclusively as the monovalent ion, H2PO4-. In an effort to see if this concept has a useful application in soils, we treated an acid Ultisol surface soil with three rates of Ca(OH)2 and six rates of Ca(H2PO4)2 to give a wide range in pH and P contents. Rye (Secale cereale L.) was grown for 4 wk, and top growth was analyzed for P content. Soil solution was collected for each treatment and analyzed for inorganic ion composition. Soil-solution pH ranged from 4.6 to 7.5, and soil-solution P ranged from 1.9 to 191 .mu.M in unlimed soil and from 1.4 to 32 .mu.M in the highest limed soil. The critical P concentration for maximum growth was about 10 .mu.M. Lower P uptake (at equal P rates) at higher soil pH was primarily the result of lower solution P concentration. Across soil pH levels, P uptake was better correlated with total solution P concentration than with H2PO4- concentration.
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