The fabric of humus cover (HC) may be very variegated, depending on local pedo-ecological conditions (origin and composition of deposits, topography, moisture regime, soil climate, plant cover composition). This multiplicity can be expressed by the HC types (or humus forms). On natural areas, where the HC is formed as a result of long-term equilibrated mutual influences of soil cover and plant cover, the fabric and properties of the HC may be used as an ecological indicator in the evaluation of the functioning character and intensity of the soil–plant system (or also the whole ecosystem). In Estonia, besides natural areas, the classifications of HCs are also elaborated for arable areas. Regardless of the profound changes in the fabric of the HC occurring with land-use change (from natural land to arable and vice versa), certain inherited properties are sustained in newly formed HCs. In this paper the fabric and properties (thickness, carbon and nitrogen stocks) of the main HC types (19 in total) are analysed and ecological aspects of HC formation are explained. Data on tree layer taxation, agro-chemical parameters of the soil cover, and ground vegetation productivity are presented with the HC types used as the ecological background. The relationships of soil cover with soil and forest site type are analysed against the background of soil matrix tables and ordination net of forest sites, respectively. Comparative analysis of the Estonian HC classification with the European Reference Base for humus forms (ERB) showed a relatively good correspondence of the Estonian classification of HC types with ERB’s humus forms.
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