PHOSPHATE AND SULFATE RETENTION BY NONVOLCANIC SOILS WITH ANDIC PROPERTIES

Abstract

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 (PO 4 ) and sulfate (SO 4 ) sorption capacity of soils with andic properties from Galicia, NW Spain, was investigated. Phosphate sorption experiments were conducted by adding 1.6, 16.1, and 48 mM KH 2 PO 4 to soils, and those of sulfate by adding 0.4 and 1.6 mM K 2 SO 4 (ratio of soil:solution, 1:10). The pH of the extracts was measured, as were the levels of PO 4 (only in the PO 4 experiments), SO 4 , Si, and dissolved organic carbon. The soils retained much higher amounts of PO 4 than of SO 4 . Surface horizons displayed a higher capacity than subsurface horizons for retaining PO 4 but not for retaining SO 4 . The PO 4 retention capacity was related to the sum of 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 PO 4 primarily produced displacement of dissolved organic carbon, Si, and SO 4 into solution and increases in solution pH, whereas sorption of SO 4 only generated increases in solution pH. The low ability of SO 4 anions to displace sorbed organic C mainly explains 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 PO 4 and SO 4 for reactive surfaces and the interaction of organic C in sorption processes.