ICPAES, in conjunction with vacuum-purged optics, provides an efficient alternative for the measurement of S in soil extracts.
Comparative testing (GA Barry, GE Rayment and DJ Lyons, unpublished data) has shown that use of ICPAES for the measurement of S in 0.01 M Ca(H2PO4)2 extracts gives results that are highly correlated with those obtained by Method 10B2. However, as ICPAES measures total S in the 0.01 M Ca(H2PO4)2 extracts, including some that may avoid detection by Johnson-Nishita distillation, slightly higher S values can be expected for most soils. For a range of soils from Queensland, a value of 5 mg S/kg by Method 10B2 may increase to around 7 mg S/kg. At very high levels of phosphate-extractable S, wider differences can occur, but these are well beyond the range likely to be associated with S deficiency in plants.
Extracting Solution – 0.01 M Ca(H2PO4)2 at pH 4.0
As for Method 10B1.
Sulfate Standard Solutions
As for Method 10B1 but make up Working Standards equivalent to 0–200 mg S/kg.
Prepare clear soil extracts as described for Method 10B1. Determine S concentrations in these extracts using ICPAES. Follow manufacturer’s instructions for instrument settings and for the preferred wavelength. The most likely wavelength is 182.037 nm, but other wavelengths are possible, such as 180.731, 182.037, and 182.625 nm. Avoid using the S emission doublet at 182.619/182.635 nm. At 182.037 nm, there is virtually no interference from Ca2+.
Report phosphate-extractable S (mg S/kg) on an air-dry basis.