COMPARISON OF MEHLICH 3-EXTRACTABLE AND BRAY 1-EXTRACTABLE PHOSPHORUSLEVELS IN A STARR CLAY LOAM AMENDED WITH POULTRY LITTER

Two soil tests commonly used to characterize the availability of soil phosphorus (P) are Bray P1 (B1) and Mehlich 3 (M3) extractants. The ob jective of this investigation was to compare M3- and B1-extractable P levels in a Starr clay loam (fine-loamy mixed thermic Fluventic Dystro chrepts) amended with relatively low to high surface applications of p oultry litter (PL). The following eight treatments were applied to the soil for pasture renovation in 1991 and 1992: 1) a control, 2) P appl ication as monocalcium phosphate [Ca(H2PO4)(2). H2O], 3) P application as Ca(H2PO4)(2). H2O and nitrogen (N) application as ammonium nitrate (NH4NO3) and urea, and 4) five levels of poultry litter (PL) based on N content. The five P levels as PL were 56.6, 113.2, 169.8, 226.4, an d 283.0 kg ha(-1) in 1991, and 49.2, 98.5, 147.7, 196.9, and 246.1 kg ha(-1) in 1992, respectively. Soil samples were obtained from the 0- t o 5-cm layer in the spring of 1992 and 1993 for extractable P determin ation. Levels of P extracted from all treatments by the M3 and B1 extr actants were linearly correlated both years (r(2)=0.96 in 1991 and r(2 )=0.99 in 1992). However, M3 extracted more P from the PL treatments, whereas B1 extracted more P from the control, P application, and N and P application treatments. Curvilinear relationships were obtained bet ween P uptake and levels of either M3- or B1-extractable P for the PL treatments (r(2)=0.713 for M3 and 0.663 for B1 in 1991 and r(2)=0.925 for M3 and 0.933 for B1 in 1992). These close relationships in 1992 be tween extractable P and uptake of P for the PL- treated soils indicate that both the B1 and M3 extractants could be used to evaluate excess P in PL-amended soils.