An overview of land application of pig effluent-P using soil P chemistry and mass balance calculations

Abstract

Attention is directed at land application of piggery effluent (containing urine, faeces, water, and wasted feed) as a potential source of water resource contamination with phosphorus (P). This paper summarises P-related properties of soil from the 0–0.05 m depth at 11 piggery effluent application sites, in order to explore the impact that effluent application has had on the potential for runoff transport of P.

The sites investigated were situated on Alfisol, Mollisol, Vertisol, and Spodosol soils in areas that received effluent for 1.5–30 years (estimated effluent-P applications of 100–310 000 kg P/ha in total). Total (P_T), bicarbonate extractable (P_B), and soluble P forms were determined for the soil (0–0.05 m) at paired effluent and no-effluent sites, as well as texture, oxalate-extractable Fe and Al, organic carbon, and pH.

All forms of soil P at 0–0.05 m depth increased with effluent application (P_B at effluent sites was 1.7–15 times that at no-effluent sites) at 10 of the 11 sites. Increases in P_B were strongly related to net P applications (regression analysis of log values for 7 sites with complete data sets: 82.6% of variance accounted for, P < 0.01). Effluent irrigation tended to increase the proportion of soil P_T in dilute CaCl₂-extractable forms (P_TC: effluent average 2.0%, no-effluent average 0.6%). The proportion of P_TC in nonmolybdate reactive forms (centrifuged supernatant) decreased (no-effluent average 46.4%, effluent average 13.7%). Anaerobic lagoon effluent did not reliably acidify soil, since no consistent relationship was observed for pH with effluent application. Soil organic carbon was increased in most of the effluent areas relative to the no-effluent areas. The 4 effluent areas where organic carbon was reduced had undergone intensive cultivation and cropping.

Current effluent management at many of the piggeries failed to maximise the potential for waste P recapture. Ten of the case study effluent application areas have received effluent-P in excess of crop uptake. While this may not represent a significant risk of leaching where sorption retains P, it has increased the risk of transport of P by runoff. Where such sites are close to surface water, runoff P loads should be managed.

runoff, phosphorus, swine, hog.