Effect of rates of applied phosphorus and soil storage temperature on P availability of a low-P loess-derived Alfisol

Overwintering soil temperature may influence crop response to phosphorus (P) and indices of P availability in the humid temperate transitional climate of Tennessee. The effects of P fertilization and soil incubation temperature upon sorghum-sudangrass (Sorghum bicolor x S. sudanese) grown on a Typic Hapludalf was investigated in two greenhouse experiments at Knoxville, Tennessee. Soils were incubated for 9 months at a constant temperature of 6°C or a diurnal fluctuation of 24 and 36°C. Reagent grade Ca(H₂PO₄)₂*H₂O was used as the fertilizer source and applied at rates of: 0, 10, 20, and 30 mg P kg^-1 for the first test and 0, 20, 40, 60, and 80 mg P kg^-1 for the second test. Critical P concentration in the shoots for optimum yield was 1.25 μg g^-1, corresponding to soil solution and labile P concentrations of 2.4 μmol L^-1 and 90 μg L^-1, respectively. Optimum yield occurred for applications of >40 mg P kg^-1 and was unaffected by soil incubation temperature. Phosphorus fertilization rates affected the amounts of P extracted by five chemical extractants (Bray I, Bray II, Mehlich I, Mehlich III, and Mississippi), but soil incubation temperature had no affect. These extractants however, were poorly correlated to plant P uptake and no one extractant was superior to the others as an indicator of P availability. Indices of P availability to plants in general were not affected by soil incubation temperature.