Efficiency of different methods in extracting selenium from agricultural soils of Finland

Selenium (Se) has been added to fertilizers in Finland since 1984, but the fate of the annual selenate addition not removed by the crops is unclear. Residual Se is assumed to be reduced from soluble selenate into insoluble species, which accumulate in the soil, but this has not been verified by soil analyses. In order to develop an appropriate extraction method for tracing the fate of fertilizer Se, Se reserves of two mineral soils were characterized with two sequential extraction procedures. In addition, we compared the efficiency of single extractions with hot water, phosphate-buffer solutions, acid ammonium oxalate, and aqua regia. As for the sequential extractions, approximately 1% of soil Se was soluble in a salt solution, 15–20% was adsorbed on oxide surfaces, and 50% was associated with organic matter, 10% being elemental and 20% in recalcitrant forms. About 50% of Se extractable with aqua regia was recovered with a single extraction with oxalate. The extraction efficiency of phosphate buffers strongly depended on the pH and phosphate concentration of the buffer, which enabled differentiation of Se by sorption strength. A 0.1 M buffer at pH 4.4 extracted only 5–10% of the aqua regia extractable Se, whereas with neutral, 1 M buffer 30% was recovered. Hot water proved to be an inefficient extractant that removed only 5–10% of soil Se. Due to the low level of readily soluble Se in Finnish soils, extraction methods directed at releasing adsorbed Se are needed. Therefore, phosphate buffers and oxalate seemed to be the most promising extractants for routine monitoring purposes of our soils.