Odén has shown that colour intensity comparisons between alkali extracts of peat soils of varying origin can be made since, despite the possible difference in the nature of their humic matter, the absorption spectra are almost identical within the visible range. These absorption spectra are also comparable with that of Merck's Acidum Huminicum, which is a standard preparation. Odén developed his colorimetric method as a means of measuring the degree of humification of the organic matter of peat soils, that is to say, the proportion of the total organic matter which could be classed as humic matter. In this way the disadvantages of Grandeau's method, in which there is no discrimination between the coloured and the colourless parts of the soil organic matter, are obviated.