Phosphorus movement down a toposequence from a landscape with texture contrast soils

Pedological processes culminating in physical and chemical differences to the soil profile down a toposequence also changed the migrating potential of phosphorus (P) within the landscape. Considerable erosion had occurred resulting in a large movement of soil from the upper to the lower slopes. This, in turn, had led to a decrease in profile depths on the upper slopes and an increase on the lower slopes. We found that in the upper parts of the toposequence there was enhanced macropore or bypassing flow resulting in a greater and deeper leaching of P compared with the lower slopes. The P that moved was almost entirely in the dissolved form and clearly originated from commercial phosphate fertiliser that had been added to the soil surface immediately prior to the start of irrigation. We could find no evidence to support the commonly held view that P often moves through soil in association with very fine clay-sized particles. When considering land management options for reducing phosphate movement in catchment areas, our results suggest that the upper parts of the landscape need to be given special consideration.