Comparison of the phosphorus status of soils managed organically and conventionally

AbstractSoils from adjoining farms, one managed organically and the other managed conventionally, were used in a greenhouse study to compare soil P status and the efficiency of concentrated superphosphate (CSP) and North Carolina rock phosphate (RP). Soil and plant parameters were measured as indicators of levels of soil P forms and availability of soil P to soybeans (Glycine max L.). Management did not affect dry matter yield of soybean plants. Conventional management resulted in higher P concentration in the plant and higher P uptake when CSP was the P source. However, when RP was the source, management effect was not significant. RP was only 15% as effective as CSP in increasing yield. When no P was applied, organic management resulted in greater total soil P, organic P, and Ca phosphate (CaP); conventional management resulted in greater Al and Fe phosphate (AlFeP) and occluded AlFeP (OcP). Addition of CSP increased AlFeP and OcP. Addition of RP increased CaP. CSP was more effective than RP in increasing extractable soil P. Multiple regression analysis showed that extractable P was related to AlFeP when CSP was the P source and to CaP when RP was the source. Yield was related to extractable P when CSP was the P source but when RP was the source, no significant regression models were found for yield.