Assessment of phosphorus leaching losses from a free draining grassland soil

Intact soil monoliths (70 cm deep, 50 cm diameter), collected from a free draining Lismore silt loam soil (Udic Haplustept) under grassland, were used to evaluate phosphorus (P) leaching for two years. The objective of the study was to investigate the effect of the application of mineral P fertiliser (at 45 or 90 kg P ha^–1 y^–1) and/or farm dairy effluent (FDE) (30 to 60 kg P ha^–1 y^–1) on P losses by leaching. Annual mean total P (TP) concentrations and losses were higher from the treatments that received both FDE and P fertiliser (203–429 g L^–1; 1.4–2.5 kg ha^–1) compared with P fertiliser alone (77–151 g L^–1; 0.6–1.3 kg ha^–1). The form of applied P influenced the pattern of P forms leached. For example, significantly higher P losses in different P forms were observed for the combined mineral P fertiliser and FDE treatment (P45/FDE200) than fertiliser alone (P90/N200/U). This is due to the inclusion of liquid FDE in the former treatment although the total P inputs were similar for both treatments. This illustrates the potential of these soils to adsorb soluble inorganic P applied from mineral P fertiliser, while FDE contained unreactive P forms that were mobile in the soil profile. There was a distinct pattern of P forms leached in the following order: particulate unreactive P (PUP: 40–70%)>dissolved unreactive P (DUP: 14–53%)>particulate reactive P (PRP: 5–12%)>dissolved reactive P (DRP: 1–11%). Results also suggest that changing the irrigation method from flood to spray may be the most effective means to reduce P loss in these stony, free-draining soils.