The Inland Bays watershed in southern Delaware is dominated by a large and growing poultry industry that uses land application as the primary means for disposal of manure. Runoff and drainage waters from an extensive ditch drainage system in the area eventually enter the Inland Bays. Phosphorus in these waters has been suggested as a contributing factor to eutrophication of the Inland Bays.
Soil test P (STP, Mehlich 1, 0.05N HCI + 0.025N H2SO4) levels in cultivated and field border areas and P sorption maxima (PSM) were determined for selected profiles of agricultural fields and non-cultivated field borders. Relationships between PSM and soil properties and a rapidly determined p sorption index (PSI) were also investigated. STP in cultivated topsoils from 48 sites ranged from 43 to 632 mg/kg and from 42 to 568 mg/kg at 0–5 and 5–20 cm, respectively, relative to a high value for STP for agronomic crops of 35 mg/kg. STP levels were consistently greater in cultivated profiles than in field borders. Downward movement of P in some cultivated profiles occurred to a depth of 40 cm. PSM ranged from 95 to 2564 mg/kg in cultivated soils and from 200 to 2000 mg/kg in field borders. PSM was highly correlated (r = 90) with clay content and was consistently higher in subsoil horizons. When clay contents were similar, P sorption was usually greater in field borders than in cultivated fields. PSI was highly correlated with PSM (r† = 0.94) except when PSI exceeded 1400 mg/kg.
This study suggests that past and current P management practices in the Inland Bays have increased soil P levels in this watershed to levels that may require much more intensive management of manure and fertilizer P in the future. Although subsoil horizons may prevent direct P leaching into groundwater, the high P values noted at the 0 to 5-cm depth and in some subsoils point to the need to assess the potential for soluble P losses in runoff and drainage waters.