The soil organic matter plays an essential part in the function of ecosystems. It predetermines the direction and intensity of the soil processes, as well as the range of the organisms living on the surface or inside the soil (Kononova, 1966). The morphology of humus horizons determines the specific microclimatic conditions that form for organisms and nutrient migrations. The morphological structure, in turn, is the product of multisided interactions between the climate, soil and vegetation.

In Europe, just like in North America, numerous classifications have been developed for organic profiles, starting with the classic papers by Muller (late 19th century). The Canadian (Green et al. 1993) and French (Brethes et al. 1998) classifications are often used in multidisciplinary international studies, but the organic profiles of some European forest ecosystems are not described there (Jabiol et al., 2004). New national classification systems have lately appeared in Austria, Germany and the Netherlands.

The basic principles of most classification systems are similar. Yet, the methods currently used to describe and distinguish between organic profiles need to be elaborated more thoroughly, as they do not fully reflect the diversity of morphotypes or morphological components of the organic matter, their combinations in organic profiles and spatial distribution. The literature suffers from terminological confusion. Different authors describe the hierarchical levels of the morphological organization of organic profiles differently. Many authors mix the terminology of the profile, horizon and aggregate (microstructure type) levels together.

Forest soils of Karelia were taken as the example to describe the forms of organic material at the following hierarchical levels: humus morphotype, humus stratotype and organic profile. The humus micromorphotypes described were grouped into categories: plant remains, new biogenic microforms, finely dispersed humus and colloidal humus. Each stratotype had its own characteristic set of micromorphotypes. The division of the organic-bearing soil horizons into types enabled classification of organic profile types.

Determination of organic profiles using various classifications has shown that any one of them is applicable for diagnosing the organic profiles of the mor-, moder- and mull types. Difficulties arise when trying to identify the transitional forms – mor-moder and moder-mull. Humus identification at a lower taxonomic level is particularly challenging. It therefore appears very important to attempt combining the concepts of different taxonomic systems, description methods and classifications.

Forest soil classifications make very disproportionate use of the parameters of the organic profile structure: the litter structure type is hardly used at all, whereas peat influx is considered at the type and subtype levels. We tend to support O. Chertov's (1981) suggestion that the organic profile type be used in soil classification at a certain taxonomic level.

The study was supported by the Russian Science Support Foundation and the Russian Foundation for Basic Research (grant ¹ 05-04-97529-g_north).