Evaluation of Residual Soil Phosphorus in Permanent Fertility Plots

Areas of four long‐time fertility experiments that were planted to oats in 1952 were used to study the effect of different past treatments upon the level of available soil phosphorus. Phosphorus A values, as derived by Fried and Dean, were compared with the Bray soil phosphorus test, and with crop yields as a means of evaluating the soil phosphorus level. The A values appeared to be very satisfactory for this purpose, and correlated significantly with soil test phosphorus values and with oat yields. At three of the four locations, a higher degree of correlation existed between A values and yields than between soil test phosphorus and yields. Phosphorus A values, determined from plant samples harvested at maturity were higher than those determined from an earlier sampling, but did not correlate any better with yields or soil test phosphorus. The 1952 results from the long‐time experimental treatments indicated that ordinary superphosphate, concentrated superphosphate, and calcium metaphosphate applied at equal rates of P 2 O 5 over a period of years appear to have an equal effect upon the residual level of available soil phosphorus and upon the yield of oats. Fused tricalcium phosphate compared favorably with these materials on an acid soil but was less effective as a phosphate source on a calcareous soil. Rock phosphate applied at equal or higher rates of P 2 O 5 was a less effective source at all locations and by all three standards of comparison.