Histosol reaction families - pH 4.5
I do not recall hearing a specific reason for using pH 4.5 in 0.01 M CaCl2 to separate dysic and euic families in Histosols. I do not now find any written rationale.
I observed one practical application in the Maritime Provinces of eastern Canada, where vegetables, carrots in particular, were grown on sphagnum bogs. With low pH, roots of carrots bifurcated. I don't think they were harmful, but they were not marketable. Above a pH 4.5 the problem did not exist. I latched on to that as practical reason. The pH may well separate sphagnum peats from wooded peats and marsh developed from rushes, sedges, and mosses other than sphagnum. I do not have evidence in hand, however.
The dysic-euic criteria does seem to work well to separate soils where acid loving plants such as blue berries, cranberries, sun dews and pitcher plants grow well. Most organic soils are clearly euic or dysic. There seems to be few soils that are border-line to the pH criterion. I think it may have something to do with whether soil water is moving into or out of the soil.
