Transport of phosphorus through soil in an effluent-irrigated tree plantation

Abstract

A young plantation of Pinus radiata in south-eastern Australia was irrigated with secondary-treated sewerage effluent for 48 months, when evapotranspiration exceeded rainfall. Concentrations of phosphorus (P) in saturation pastes of soil were measured after 30, 37, 42, and 48 months to monitor vertical transport of P, and results were compared with depth of transport predicted from P sorption isotherms. Standard laboratory isotherms greatly underestimated the capacity of soil to retain P. Thirty months after plantation establishment, 165 kg/ha of P had been applied in effluent, and P in soil solution was predicted from sorption isotherms to be at a concentration of 5 · 7 mg/L to a depth of 0 · 29 m. In comparison, the concentration of P in saturation paste at this time decreased exponentially from 3 · 5 mg/L at 0 · 025 m to 0 · 43 mg/L at 0 · 30 m. Similarly, 48 months after plantation establishment, 318 kg/ha of P had been applied, and P was predicted to be in equilibrium to a depth of 0 · 51 m. At this time, concentration of P in saturation paste was 4 · 7 mg/L at 0 · 25 m but decreased to a concentration of 0 · 11 mg/L at 0 · 45 m. The concentration of P in saturation paste in the bore-water control was low (<0 · 36 mg/L) thoughout the profile. The discrepancy between predicted and observed results was found from subsequent experiments to be due largely to the short (17 h) equilibration time and to the wide solution : soil ratio (10 : 1) used during construction of standard sorption isotherms. In the 0-0 · 1 m layer of soil, P-retention capacity determined in saturation paste and after 21 days incubation was about 9-fold greater than when the standard method was used. Leaching of dissolved P is unlikely to threaten the sustainable life-time of the effluent-irrigated plantation studied.