Sulfur has long been known to be one of the essential plant food constituents. It has always been believed, however, that there was sufficient present in all soils for the optimum growth of crops. This assumption has been very largely based on Wolff's analyses of the ashes of various crops which showed the presence of very small amounts of sulfur.
The recent work of many investigators has demonstrated, however, that the amount of sulfur in plant materials as determined in the ash is, in most cases, entirely too low; that there is a considerable loss of sulfur in the process of igniting; and that the amount found in the ash may therefore be a very small part of that originally present in the plant tissues
Brown, P. E. and Kellogg, E. H.
"Sulfofication in soils,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 2
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol2/iss18/1