Soil phosphorus storage capacity in manure-impacted Alaquods: Implications for water table management

Phosphorus (P) applied as fertilizers and manures can accumulate in soils if not recovered by crops. Excess P accumulated can contribute to eutrophication of surface water. Spodosols (Alaquods) have spodic (Bh) horizons enriched in organically complexed Al and Fe having high P retention capacity. Environmental risk of P loss from soil horizons can be evaluated from phosphorus saturation ratio (PSR; molar ratio of P to [Al + Fe]) and the soil phosphorus storage capacity (SPSC). A threshold PSR (“change point”) above which release of P from soil to solution abruptly increases can be determined for a given population of soils. SPSC is a calculation of how much P can be added to a soil before reaching this critical PSR threshold. The primary objective of this study was to compare P release characteristics of surface and spodic horizons to allow an evaluation of soil P storage capacity of the Spodosol profile. Six Spodosol beef or dairy manure-impacted sites in Florida differentially P-impacted were sampled by horizon. Change point PSR for spodic horizons was determined using oxalate (PSRox), Mehlich-1 (PSRM1) and Mehlich-3 (PSRM3) extracts. Mean SPSC of Bh horizons (95 mg kg−1) was significantly higher than that of A horizons (−360 mg kg−1) due to much lower metal content and greater P loading in the surface horizon. Status of SPSC of any horizon may be used for predicting consequences of water table management regimes with respect to P transport; for example, maintaining water table below the Bh could favor P retention by Bh-horizons with high positive SPSC.