Research suggests that relatively stable, fine fraction soil organic matter (FF-SOM) has almost constant concentrations of nitrogen, phosphorus and sulphur (NPS) per unit of carbon (C), although there are considerable differences in the datasets used. Such differences could, in agricultural situations in particular (where inorganic nutrients are often added), result from substantial concentrations of inorganic-N, inorganic-P and inorganic-S that remain in the soil. We assessed the efficacy of removal of inorganic nutrients with different washing procedures (0.1 or 0.5 m hydrochloric acid solution). Even after repeated additions of inorganic-N, inorganic-S and inorganic-P to soil as part of a long-term incubation experiment, washing soil with 0.1 m HCl was effective at removing residual inorganic-N and inorganic-S, but not effective at removing inorganic-P. We demonstrate that estimates of C:N, C:S and C:P ratios in FF-SOM can be substantially underestimated when these washing procedures are not carried out as part of the soil analyses. This leads to overestimation of the amounts of N, P and S that are required for co-sequestration of C during the formation of FF-SOM. Because P is essential for the formation of FF-SOM, more research is needed to obtain an accurate measure of the P required per unit of FF-SOM. Although washing with 0.1 m HCl also removed some FF-C, we suggest a method to compensate for this. We tested the procedure on soil with clay contents ranging from 8 to 60%; however, the stronger acid might be more suitable for soil with larger clay contents. Highlights: What is the best method to measure the size and stoichiometry of the stable soil organic matter pool? Enlarging this soil organic matter pool could help restore fertility and mitigate climate change. Washing soil with dilute acid can remove excess inorganic nutrients, but it also removes organic carbon. We recommend the size of the pool on unwashed soil and the stoichiometry on washed soil is measured.