Phosphorus forms in soil solution and leachate of contrasting soil profiles and their implications for P mobility

Abstract

Purpose: Policy changes to reduce the transport of phosphorus (P) from agricultural land have triggered research in many parts of the world to improve understanding of processes controlling P mobility so that P management practices can be designed to limit losses to waterways. Previous studies on soils from cropping land in the Mediterranean-type climate of south-west Western Australia suggested that dissolved unreactive P (DURP) comprises a large proportion of extractable soil P, but the implications of these findings for P mobility in soil solution were not examined. Materials and methods: Intact columns of three contrasting soil profiles (sand, loam, loamy sand) from cropping land of the south coast region of Western Australia were leached to examine P forms in leachate (bottom of 20 cm columns) and extracted soil solution (5, 10 and 15 cm depths) after application of three P rates (0, 20 and 40 kg P/ha). Leachate and soil solutions were analyzed for total dissolved P (TDP0.45 μm and 0.2 μm) and dissolved reactive P (DRP0.45 μm and 0.2 μm). Dissolved unreactive P (DURP0.45 μm and 0.2 μm) was calculated by difference. Results and discussion: Initial high P concentrations in leachate were followed by a sharp decrease with successive leaching events at high P rates (sand > loamy sand > loam), suggesting that macropore flow dominated in early leaching followed by matrix flow which favours P sorption by soil. The major fraction of P in leachate and soil solution even after the application of KH2PO4-P was DURP. The leachate (0.45 μm pores) and soil solution (0.2 μm pores) both had similar predominance of DURP fraction, although the proportion of DURP0.2 μm in soil solution was less than in leachate. The difference suggests that a significant proportion of the DURP fraction was fine colloidal material in 0.2–0.45 um size class, but its composition remains unknown. Conclusions: The predominance of DURP in leachate and soil solution was the most striking finding of relevance to P mobilization on intact columns of sand, loamy sand and loam soils. Phosphorus sorbed on the surfaces of the colloidal particles and occluded P within the colloids may account for some of the calculated DURP fraction. Determination of the composition of DURP in soil solution collected might provide useful insights for P mobility since this more effectively excluded particulate P than the <0.45 μm filtrate used for leachate analysis.