Residual Effects of Various Phosphates as Measured by Yields, P32 Uptake, and Extractable Phosphorus

In 1930 an experiment was started on a Decatur silt loam and on a Greenville fine sandy loam to compare the effectiveness of different sources of phosphorus for crops grown in a rotation of corn and cotton with vetch. At the end of 16 years, phosphate applications were discontinued in order to study residual effects on yields. In 1950 and 1951, tagged superphosphate was applied to all treatments and available phosphorus, based on P³² uptake, was calculated and expressed as A values. Yield data from both locations show that all sources had considerable residual effects. For any particular source, the residual effect was in proportion to the amounts that had been added. Basic slag gave the greatest residual effect on the Decatur soil as measured by vetch and cotton yields. It also gave the greatest residual effects on the Greenville soil as measured by cotton yields. Tricalcium phosphate and raw phosphate showed a much greater residual effect, as indicated by vetch yields, in the Greenville soil than in the Decatur soil. A values show that all sources had considerable residual effects as measured by radiophosphorus uptake. The A values for basic slag at both locations were larger than for an equivalent amount of any other source. For the Greenville soil, A values show that superphosphate had a greater residual availability than rock phosphate applied at twice the rate of P₂O₅. However, A values on the Decatur soil indicate that rock phosphate had about the same residual availability as superphosphate applied at an equivalent rate of P₂O₅. For any particular source, A values were in proportion to the amounts which had been added. Yield data for the first 4 years of the residual period show only a very general relationship with A values when all sources are considered together. However, correlation coefficients show a fairly close relationship between yields and A values in most cases when the analysis is limited to rates of one source. Extractable phosphorus data show that phosphate applications resulted in phosphorus accumulation and that the amount accumulated was proportional to the rate of application.