Variation of phosphorus leached from Pennsylvanian soils amended with manures, composts or inorganic fertilizer

The continued application of P as fertilizer or manure at rates exceeding crop requirements can increase soil P concentrations and the potential for P loss to surface waters. Limits for eutrophication of flowing waters not discharging directly into water bodies of 0.01 mg dissolved reactive P (DRP) and 0.1 mg total P (TP) l−1, can be readily exceeded in areas with P enriched soils. Little information is available on how long P discharge will remain above environmental thresholds, particularly for subsurface flow. Thus, this work looked at the effect of applying P (as mineral fertilizer, dairy manure and compost, and poultry manure and compost at 0–200 kg P ha−1 per year for 5 years), to a Hagerstown silt loam on P leached from lysimeters (30 cm deep) with rainfall generating high flow (330 cm per 5 months; high-flow lysimeters) and low flow conditions (165 cm per 5 months; low-flow lysimeters). For high-flow lysimeters, most P was leached from dairy compost (88 mg DRP and 148 mg TP) and poultry manure (41 mg DRP and 83 mg TP) amended soils compared with mineral superphosphate (26 mg DRP and 43 mg TP) and dairy manure (26 mg DRP and 59 mg TP). Similar trends but lower losses were observed for the low-flow lysimeters. Among the 50 lysimeters, Mehlich-3 soil P thresholds of 100 and 130 mg kg−1 occurred for DRP and TP, respectively. Above these thresholds, DRP and TP loss was enhanced. The decrease of P concentrations leached (inorganic and organic P forms, y) with time (t, days) could be described by a power function (y=αtβ). The rate of decrease in P leached (β) was quicker at high than low flow rates. Using this equation, it was estimated that for a soil of 100 mg kg−1 Mehlich-3 P under high-flow conditions would take about 3 years for DRP to fall below 0.01 mg L−1 and 6 years under low flow conditions. Clearly, once soil P concentrations become elevated via management, it can take several years for P enriched drainage waters to decline below “acceptable” levels.