Phosphate and sulfate retention by nonvolcanic soils with andic properties

Soils with andic properties have a high anion retention capacity, which is mainly related to the large amounts of active Al and Fe compounds that they contain. The phosphate (PO4) and sulfate (SO4) sorption capacity of soils with andic properties from Galicia, NW Spain, was investigated. Phosphate s orption experiments were conducted by adding 1.6, 16.1, and 48 mM KH2PO4 to soils, and those of sulfate by adding 0.4 and 1.6 mM K2SO4 (ratio of soil: solution, 1:10). The pH of the extracts was measured, as were the levels of PO4 (only in the PO4 experiments), SO4, Si, and dissolved organic carbon. The soils retained much higher amounts of PO4 than of SO4. Surface horizons displayed a higher capacity than subsurface horizons for retaining PO4 but not for retaining SO4. The PO4 retention capacity was related to the sum o f contents of oxalate extractable-Al (Al-o) and -Fe (Fe-o), which tended to increase with soil organic carbon content and was mainly attributed to the presence of Al-humus complexes. Sorption of PO4 primarily produced displac ement of dissolved organic carbon, Si, and SO4 into solution and increases in solution pH, whereas sorption of SO4 only generated increases in solutio n pH. The low ability of SO4 anions to displace sorbed organic C mainly exp lains why surface horizons retained similar or even smaller amounts of SO4 than subsurface horizons, despite the higher Al-o and Fe-o contents of the former. The results emphasize the different affinities of PO4 and SO4 for r eactive surfaces and the interaction of organic C in sorption processes.