Turbidimetric methods, using Ba ions to precipitate SO4, are frequently used to determine soil sulfates extracted with phosphate solutions. These methods, as routinely performed, seriously underestimate SO4 in some soils of the tropics because phosphate is removed from the extractant by soil adsorption and because many extracts fail to yield satisfactory precipitate even if the extracting procedure is adequate. Decolorizing the extracts with carbon black, treating extracts with strong oxidizing agents, adding SO4 spikes, and seeding the extracts with BaCl2 seed‐crystals improve precision, but some extracts, especially those from soils derived from volcanic ash, do not yield reliable precipitates even though these procedures are employed. This paper presents a method that consistanlty yielded more SO4 than other turbidimetric procedures with which it was compared. The proposed method was further validated against an ion‐chromatographic method for SO4 determination. The two methods yielded virtually identical results. The proposed method consists of extracting SO4 with 0.04 M Ca(H2PO4)2 pH 4, at a soil‐to‐solution ratio of 1: 10. Repeated extraction is necessary for phosphate‐retentive soils. (A. single extraction was approximately 40% effective for removing indigenous SO4 from a Hydric Dystrandept subsoil, approximately 78% effective for an Eutrustox.) Organic materials are removed from the extracts by adsorption on charcoal; SO4 is concentrated in the extract by volume reduction; a SO4 spike is added; BaCl2‐seed crystal is added, after which volume is increased by adding BaCl2 solution. Optical density is read at 600 nm.
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Fox et al. (1987) studied this question.