PHOSPHORUS MOVEMENT DOWN A TOPOSEQUENCE FROM A LANDSCAPE WITH TEXTURECONTRAST SOILS

Pedological processes culminating in physical and chemical differences to the soil profile down a toposequence also changed the migrating po tential of phosphorus (P) within the landscape. Considerable erosion h ad occurred resulting in a large movement of soil from the upper to th e 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 tha t in the upper parts of the toposequence there was enhanced macropore or bypassing flow resulting in a greater and deeper leaching of P comp ared with the lower slopes. The P that moved was almost entirely in th e dissolved form and clearly originated from commercial phosphate fert iliser that had been added to the soil surface immediately prior to th e start of irrigation. We could find no evidence to support the common ly held view that P often moves through soil in association with very fine clay-sized particles. When considering land management options fo r reducing phosphate movement in catchment areas, our results suggest that the upper parts of the landscape need to be given special conside ration.