Soil sample depth in pasture soils for environmental soil phosphorus testing

Phosphorus (P) in runoff from agricultural land is a major contributor to eutrophication of surface waters. This study investigated the relationship between dissolved P in surface runoff and soil-test P measured at different sample depths (0-2 and 0-10 cm). Soil at these depths was collected from 136 sites in southeast New South Wales, Australia, under pastoral agriculture, covering a wide range of soil types and land-use intensity, from native pasture to intensive dairying. Bicarbonate-extractable soil P concentrations at these two depths were curvilinearly related over a very wide range of P (r² = 0.91). Small-plot rainfall simulations were conducted at 14 of these locations, for which the relationship between the sampling depths was even closer (r² = 0.95). After dividing the soils into two groups based on parent material (basalt and nonbasalt), linear relationships were found between extractable soil P and runoff dissolved reactive P, with more of the variance being accounted for with the sedimentary soils (r² = 0.89, 0-10 cm; 0.91, 0-2 cm) compared with basalt soils (r² = 0.63; 0-10 cm; 0.57, 0-2 cm). The results suggest that agronomic soil P testing in pastoral soils (typically 0-10 cm depth) is sufficient for estimating the potential for losses of P in runoff and that there is no need to collect shallow soil samples especially for this purpose.