VARIATION OF PHOSPHORUS LEACHED FROM SOILS AMENDED WITH MANURES, COMPOSTS OR INORGANIC FERTILIZER

Authors: MCDOWELL, R - AGRESEARCH LIMITED; Sharpley, Andrew

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2003
Publication Date: 3/20/2004
Citation: McDowell, R.W., Sharpley, A.N. 2004. Variation of phosphorus leached from soils amended with manures, composts or inorganic fertilizer. Science of the Total Environment. 278:113-125.

Interpretive Summary: Areas with intensive livestock farming often have soils enriched with phosphorus due to the addition or disposal of manure. Phosphorus levels in these soils can be twice or three times those required for optimum plant growth and can increase the potential for phosphorus loss to surface water and therefore eutrophication. Little information is available on how long phosphorus discharge will remain above environmental thresholds, particularly in water draining through soils. Thus, we determined the effect of applying phosphorus as fertilizer or manure on the amounts of phosphorus draining through soils in wet and average precipitation years. Phosphorus loss was much greater in wet years but followed similar predictable trends. Using these trends, it was predicted that for a soil with a phosphorus concentration of 100 ppm, it would take about 6 years for drainage phosphorus to fall below environmental thresholds (0.1 mg/L) with average precipitation and about 3 years in wet years. Clearly, once soil phosphorus concentrations become elevated via management, it can take several years for phosphorus enriched drainage waters to decline below "acceptable" levels.

Technical Abstract: The continued application of P as fertilizer or manure at rates exceeding crop requirements can increase soil P and 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, 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, we determined the effect of applying P (as mineral fertilizer, dairy manure, dairy compost, poultry manure, poultry compost, and swine slurry at 0 - 200 kg P/ha/yr for 5 yrs), to a Hagerstown silt loam on P leached from 30-cm lysimeters with rainfall simulating wet (150 cm 5/months; high-flow lysimeters) and average precipitation years (75 cm 5/months; low-flow lysimeters). For high-flow lysimeters, most P was leached from dairy compost (187 mg DRP and 351 mg TP) and poultry manure (87 mg DRP and 176 mg TP) amended soils compared with mineral superphosphate (56 mg DRP and 92 mg TP) and dairy manure (55 mg DRP and 126 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 occurred for DRP and TP, respectively. Above these thresholds, DRP and TP loss was enhanced. The decrease of P leached (inorganic and organic P forms) could be described by a power function (y = at^b). The rate of decrease in P leached (b) was quicker at high than low flow rates. Using this equation, it was calculated that for a soil of 100 mg/kg Melhich-3 P under high-flow conditions would take about 6 years for DRP and 3 years for TP to fall below respective environmental limits of 0.01 and 0.1 mg/L. Clearly, once soil P concentrations become elevated via management, it can take several years for P enriched drainage waters to decline below "acceptable" levels.