Phosphorus Storage within Soil Profiles of P-Impacted Aquods.

 With the excessive use of fertilizers and manures, soil phosphorus (P) contents in agricultural lands have increased over the year, leading to eutrophication of surface water. Spodosols (Aquods ) are characterized by sandy textures, fluctuating water table and, below an A-E horizon sequence, the presence of a spodic horizon (Bh) which accumulates C, Al and/or Fe, thus acting as P sink. Environmental risk of P loss from Aquods can be calculated from the concepts of P saturation ratios (PSR) and soil P storage capacity (SPSC). The SPSC is a calculation of the amount of P that can be added to a certain volume or mass of soil before the soil becomes an environmental concern. The objectives of this study were to a) determine the capacity of different Aquods horizons to retain P and b) establish the relationship between SPSC and soluble P forms.  Thirty-two Aquods profiles from four sites located in South Florida were sampled by horizon.  Water soluble P (WSP) was determined for each profile using a 1:10 soil:water ratio. Soils were also analyzed for P, Al and Fe using a Mehlich 1 solution at a 1:4 soil:water ratio. Oxalate-extractable P, Fe and Al were determined using a 0.1 M oxalic acid and 0.175 M ammonium oxalate solution. Results showed that generally the Bh horizons had higher SPSC values than surface A and E horizons, probably due to high Al concentration. WSP values were low for positive SPSC values and increased linearly for negative SPSC values for both surface and subsurface horizons. Therefore, when SPSC is negative (PSR >0.15), soil is a P source for both surface and Bh horizons.