Phosphate Reactivity in Long‐Term Poultry Litter‐Amended Southern Delaware Sandy Soils

Eutrophication caused by dissolved P from poultry litter (PL)‐amended agricultural soils has been a serious environmental concern in the Delaware–Maryland–Virginia Peninsula (Delmarva), USA. To evaluate state and federal nutrient management strategies for reducing the environmental impact of soluble P from long‐term PL‐amended Delaware (DE) soils, we investigated (i) inorganic P speciation; (ii) P adsorption capacity; and (iii) the extent of P desorption. Although the electron microprobe (EMP) analyses showed a strong correlation between P and Al/Fe, crystalline Al/Fe‐P precipitates were not detected by x‐ray diffraction (XRD). Instead, the inorganic P fractionation analyses showed high levels of oxalate extractable P, Al, and Fe fractions (615–858, 1215–1478, and 337–752 mg kg −1 , respectively), which were susceptible to slow release during the long‐term (30‐d) P desorption experiments at a moderately acidic soil pH water The labile P in the short‐term (24‐h) desorption studies was significantly associated with oxalate and F extractable Fe and Al, respectively. This was evident in an 80% reduction maximum in total desorbable P from NH 4 oxalate/F pretreated soils. In the adsorption experiments, P was strongly retained in soils at near targeted pH of lime (≈6.0), but P adsorption gradually decreased with decreasing pH near the soil pH water (≈5.0). The overall findings suggest that P losses from the can be suppressed by an increase in the P retention capacity of soils via (i) an increase in the number of lime applications to maintain soil pH water at near targeted pH values, and/or (ii) alum/iron sulfate amendments to provide additional Al‐ and Fe‐based adsorbents.