Should the soil contain organic admixtures, the filtrate is to be evaporated to dryness, and the residue carefully heated until all carbon is burned off. The sodium chloride content is calculated by multiplying the weight of chlorine by the factor 1648(NaCl/CL=58.5/35.5=1.648. If the moisture of the soil and the NaCl content of the dry soil  are known, it is easy to calculate the quantity of NaCl dissolved in the soil. (Braun-Blanquet 1932:200-1)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.