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Comparison of Seven SO4-S Extraction Methods for Analysis by Turbidimetry or ICP Spectrometry

Cihacek, L. J., Yellajosula, G., Jacobson, K. A., Swenson, L.
Communications in soil science and plant analysis 2015 v.46 no.20 pp. 2649-2659
ambient temperature, calcium, fertilizers, field experimentation, potassium chloride, soil, spectroscopy, sulfur, Great Plains region
Soil sulfur (S) analyses for fertilizer recommendations in the northern Great Plains often do not reflect crop S requirements. Seven SO ₄-S extraction methods with S determination by either turbidometry or inductively coupled plasma emission spectroscopy including Ca(H ₂PO ₄) ₂ and KH ₂PO ₄ (both containing 500 ug/l P), 0.25 M KCl (40 ºC) and 0.25 M KCl (room temperature), H ₂O, DTPA, and Mehlich 3 extractants. Three horizon depths of three soils from a previous field study were used for these comparisons. Average standard deviations for turbidometric determinations were 4.3 times greater than ICP determinations. With turbidometry, S values were H ₂O > KH ₂PO ₄ > Ca(H ₂PO ₄) ₂ > KCl (40 ºC) = KCl, while with ICP, the values were Mehlich 3 > KCl (40 ºC) = KCl > DTPA (diethylenetriaminepentaacetic acid) > KH ₂PO ₄ > H ₂O > Ca(H ₂PO ₄) ₂. Extraction with KCl at room temperature with ICP determination appears to show promise, but further method evaluation is necessary before it can be recommended as a SO ₄-S test method.