Quantitative Approach for Recovering Legacy Phosphorus in Soils.

Legacy P from agricultural fields constitutes waste of a vital resource and can negatively affect water quality. We are looking at a quantitative approach – the use of a soil P storage capacity (SPSC) index – to predict how long a healthy crop can be grown from legacy P availability prior to resuming P fertilization. Field experiments at three sites in Florida: northwest panhandle (Ultisols), north central peninsula (Spodosols) and central peninsula (Entisols) for a rye-silage corn-sorghum cropping cycle are underway. At each site, two treatments were replicated 6 times (12 plots per site). The two treatments are i) N fertilizer applied according to University of Florida (UF) recommendations with all other nutrients, including P, supplied according to soil test, and ii) N fertilizer applied according to UF recommendations and all other nutrients except P supplied according to soil test (control). We collected soil samples by depth (0-15 cm and 15-30 cm) at each plot at all three sites for obtaining background information on SPSC. The first two cropping cycles (rye and corn) are now complete; tissue and soil samples are currently being analyzed in our laboratories. We will compare SPSC at each of the plots by depth before and after the cropping cycles to evaluate P mining by rye and corn at the three locations. We anticipate decrease in SPSC in the control plots, with SPSC remaining the same (or decreasing) in the P fertilized plots. Depending on the soil type, SPSC values in the surface and subsurface samples could differ with likely minimal differences in the subsurface for the more P retentive soil during subsequent harvests.