Phosphorus fertilizer requirements as estimated by phosphate sorption

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Abstract
Soil samples [29] representing 12 soil series and varying levels of P fertility were obtained from cooperating agricultural experiment stations in southern U.S.A. Fertilizer P requirements were estimated from P sorption curves and correlated with P requirements established by field experiments. Estimated and measured P requirements were highly correlated (r 2 = 0.93**) with the quantitative relationship being nearly 1:1. With 1 exception, P sorption also accurately estimated needs of fertilized soils. In that 1 case, fertilizer needs were estimated at 45 kg P/ha, although there was no response to P in the field. The quantity of sorbed P in equilibrium with 0.02 .mu.g P/ml apparently was a good estimate of fertilizer P requirement. The value 0.02 was somewhat lower than was reported from other research. This lower value may be associated with somewhat lower yields, warm soil temperatures, and relatively long growing season in southern U.S.A. It probably was also caused by under-evaluating the effectiveness of residual fertilizer P or by a more stable P concentration in soil solution for soils receiving long-term applications of fertilizer P.