The term humus form is not the same as the term humus. The humus form describes the various arrangement of organic and mineral horizons at the top of soil profiles. [1] It can be composed entirely of organic horizons, meaning an absence of any mineral horizons, or of a superposition of organic and mineral horizons, forming the 'humus profile', also called 'humipedon'. [2] Experts worldwide have developed different types of classifications over time, and common forest humus forms are mainly categorized into mull, mor, and moder. The most ancient contribution to the knowledge of humus forms was that of Peter Erasmus Müller, a Danish forester. In his seminal contribution Studier over Skovjord: som bidrag til skovdyrkningens theori, [3] [4] later translated in German [5] and French (while surprisingly not in English), [6] Müller described muld (later germanized as mull) and mor, two modes of assemblage of organic and mineral matter, which he associated to two opposite classes of high and low productivity and soil fertility of Danish beechwoods. His investigations embraced a thorough analysis of plant communities, and chemical as well as microscopic investigations in various soil horizons. Moder was later added as a third forest humus form by F. Hartmann, midway between mull and mor and previously described as 'insect mull' by Müller. [7]
HUMUSICA, a worldwide classification of humus forms, was achieved by an array of soil scientists working under the direction of Augusto Zanella, after numerous field sessions, workshops and exchanges between participants from various countries. [8] It described and classified humus forms from a wide array of terrestrial, semi-aquatic, cultivated and anthropic environments and was published in three special issues of the academic journal Applied Soil Ecology. [9] [10] [11] A freely available iOS called 'TerrHum' was developed in English, Italian and French, with an APK Android version, as a tool to identify humus forms directly on the field. [12]
One of the key principles of humus form classifications is that humus profiles (humipedons) may evolve at a different rate from soil profiles (pedons). Given the prominent part taken by soil organisms (from bacteria to mammals, passing by plants and invertebrates) in the spatial arrangement of organic matter and its links with mineral matter, humus forms may change under the influence of vegetation dynamics and other processes acting at the scale of the decade while centuries are necessary to see soil changing from a type to another. However, soil scientists like Walter Kubiëna considered that there was a parallelism between humus forms and soil types.