Classification

Foresters generally use the term forest humus form rather broadly when referring to any organic portion of the soil profile, but they have known for a long time that there are differences in humus beneath different forest ecosystems, and maybe even beneath the same forest types growing on different types of soils (Fisher and Binkley, 2000).

Several systems of forest humus forms classification have been historically used in British Columbia. Currently, the most commonly used system is one proposed by Green et al. (1993) as a modification of the system of Klinka et al. (1981). This classification system is based on the field-observable (morphological) features of humus forms. There is an ongoing need to test (1) whether humus forms that appear different are in fact different in their physical, chemical, and biotic properties and (2) the portability of the classification system outside the area in which it was developed.

In the taxonomic hierarchy of the system by Green et al. (1993), two levels are recognized:

• Orders (mull, moder, and mor orders are differentiated by the type of F horizon and the relative prominence of Ah horizon) and
• Groups (16 groups reflect differences in the nature and rate of decomposition processes).

Summary of Humus Form Taxa