FATE OF PHOSPHORUS IN FLORIDA SPODOSOLS CONTAMINATED WITH CATTLE MANURE

Phosphorus loading from dairies and beef ranches in the Lake Okeechobe e watershed and the subsequent movement of the P into the drainage wat ers is a major factor influencing the eutrophication of Lake Okeechobe e. The soils of this area are mainly Spodosols with the watertable lyi ng between surface and spodic horizons for extended periods each year. In this study, the quantity of total P (TP) within the soil profile ( A, E, Ph and Bw horizons) of dairies and beef ranches in the Lake Okee chobee basin was determined to evaluate the magnitude of P loading in these soils. The effect of cattle density was evident in TP concentrat ions throughout the soil profile. In the A horizon, mean TP concentrat ions were 1680, 165, and 34 kg P ha-l for high, low, and nonimpacted a reas, respectively. The same trend, although at lower concentrations, was evident in the E, Bh, and Bw horizons. The quantity of P considere d to be potentially mobile under leaching conditions (water-soluble P, Mehlich I extractable or NH4Cl extractable), also followed similar tr ends as the TP concentrations. Based on chemical fractionation data, n early 80% of TP in the A horizon of the highly impacted soils may be c onsidered leachable. We calculated that about 4000 kg P ha(-1) would b e available for leaching in the soil profile of the high intensity are as immediately adjacent to the dairy barns. This ''labile P'' appears to be solubilized slowly over a long period of time (likely several ye ars). There seems to be no natural mechanism whereby the P is stabiliz ed through the formation of minerals, and even if such processes do ta ke place, a vast amount of P still remains in a form in which it is re adily transported along with the drainage water. The A and E horizons had poor P retention capacities while the retention capacity of the Bh horizon varied with the soil type, Myakka greater than or equal to Im mokalee > Pomello. Due to the low P-retention capacity in the upper ho rizons of these soils, there is potential for significant subsurface l ateral P transport.