Forms of phosphorus in soil profiles from dairies of South Florida

Soil P fractions of current and abandoned dairy systems in South Florida's Lake Okeechobee watershed were evaluated to develop an understanding of the stability of P in this basin. Land use within the dairies was classified as intensive (high cattle impacted) and pasture, forage, or native (low cattle impacted) areas. All soils (Spodosols) were characterized for labile P (1 M NH 4 Cl), Fe-Al-associated P (0.1 M NaOH), Ca-Mg-associated P (0.5 M HCl), residual P, and organic P in the NaOH fraction. The A horizons of the intensive areas had 9% of total P as labile P, though most of the Ca-Mg-associated P ( 70% of total P) in these particular systems could also be removed by sequential NH 4 Cl extractions. Therefore, almost 80% of the P in the surface horizons of the high cattle impacted areas had the potential to move eventually with drainage water into the lake. The P in the E horizons of the intensive areas consisted primarily of labile P and Ca-Mg-associated P forms as well, though the total P in these horizons was <5% of that for the A horizons. About 80% of the total P in the spodic (Bh) horizons was extractable by NaOH, of which only 4 to 18% was associated with organic matter. The total P contents for all horizons of the soil profiles from the low cattle impacted areas were considerably less than those from the high cattle impacted areas. There were apparently no major shifts of P to more stable P forms, with time, for soils of high cattle impacted areas. Therefore, the possibility of P release from high cattle impacted soils, even after dairies were abandoned for more than 12 yr, cannot be ruled out.