Phosphorus fractions in soil amended with organic and inorganic phosphorus sources

Abstract

Information on the P fractions in soils treated with different organic amendments is needed to better manage land application of organic amendments to agricultural soils. This study investigated the forms and distribution of P after 1, 4 and 16 wk in a Lakeland silty clay loam soil using a sequential fractionation procedure. Phosphorus was added at rates of 0, 123, 307 and 614 mg P kg -1 in the form of biosolids, hog manure, cattle manure and fertilizer P. The largest difference among the amendments was in the water-extractable P fraction, which was significantly lower in soil amended with biosolids. Regression analysis showed that the slope of P increment in the H 2 O fraction as a function of P application rate was four times lower for biosolids (0.06) than for hog manure (0.24) and seven times lower than for fertilizer (0.42) amended soils. In the biosolids-amended soil, there was a net increase of H 2 O-P, NaOH-P i and HCl-P fractions at the expense of organic P (NaOH-P o ) and residual P fractions after 16 wk of incubation, signifying net P mineralization. In hog manure amended soil, H 2 O-P decreased by 128 mg kg -1 with a corresponding increase in the NaHCO 3 -P, possibly due to P sorption during the 16 wk of incubation. The results for cattle manure indicated net immobilization of P as the H 2 O-P and NaHCO 3 -P declined while the inorganic P (NaOHP i ), HCl-P and residual P increased with incubation time. The transformation of P in fertilizer-amended soil was similar to that of hog manure amended soil. The amount of labile P (defined as H 2 O-P plus NaHCO 3 -P) in amended soils followed the order of fertilizer P > hog manure > cattle manure > biosolids. Key words: Phosphorus fractions, sequential P extraction, incubation, organic amendments