Phosphorus Translocation as Impacted by Subsurface Hydrology.

In most soils, phosphorus can rapidly form highly insoluble compounds requiring frequent additions of P fertilizers to meet crop requirements. However, soluble phosphorous can be mobile and has the potential to contribute to non-point source pollution of ground and surface water.  The objective of this study was to examine the effect of subsurface hydrology on concentration of soluble phosphorous. Two terraces under pasture management were sampled using National Cooperative Soil Survey methods. Transects were established across each landscape along a 3% gradient to the edge of a creek. One pasture was underlain with karst topography and outcropping springs. The second site had neither springs nor karst topography. Fertilizer (19-19-19) was applied (224.2 kg/ha) in March. Transects were sampled prior to and following fertilizer application. Soil moisture was measured using a gravimetric method.  Phosphorous was determined using the Mehlich I procedure. As expected following fertilizer application, both sites had the highest concentrations of P occurred in the upper 65 cm.  At depths of 170 cm or greater, concentrations decreased substantially.  In the karst landscape, P content ranged from 2 to 5 mg/kg in horizons where subsurface flow was active. The presence of a relatively consistent flow of moisture, indicated some phosphorus remained soluble. In pedons downslope from obvious sinkholes, phosphorus at similar lower depths increased to a range of 10 to 15 mg/kg indicating phosphorus was possible moving into the sinkholes.  In the non- karst landscape, horizons at lower depth had low levels of phosphorus (~1 to 4 mg/kg).  Sites adjacent to the creek had significantly higher P concentrations (10 to 15 mg/kg).  In managing soils for soluble P, both the underlying geologic strata and hydrology need to be considered in determining frequency, concentration, and timing of P fertilizer application.