In the context of agricultural problem soils, Histosols are soils in which the organic matter content dominates the problems related to agricultural land use. They are characterized by a layer of organic matter (>30%) of more than 40 cm either extending down from the surface or taken cumulatively within the upper 80 cm of the soil (histic horizon).
Histosols have specific characteristics (low bulk density, colloidal nature and specific thermal properties) which differ substantially from those of mineral soils. Variations in these characteristics depend on the flora which has formed the Histosol. The floristic composition and evolving decomposition products (“humus substances” as humid and fulvic acids) determines the organic matter which forms the Histosol. Histosols are formed because the production of organic debris exceeds its decay; the accumulated dead plant material is only slightly decomposed due to varying combinations of waterlogging (and resulting anaerobic conditions), low temperatures and severe acidity.
The occurrence of Histosols depends on a permanent plant cover and a shallow water table. Natural drainage is very poor and the occurrence of water lenses within the peat is fairly common. Histosols occur mainly in areas of high atmospheric humidity, low evapotranspiration and consequently wet soils or form in wet depressions and flat sites where run-off water accumulates and prevents the rapid decomposition of the plant litter.
Histosols occupy some 270 million hectares worldwide, mainly in boreal and temperate regions. Some 225 million hectares are situated in the boreal and temperate regions of North America, Europe and Asia. An estimated 40 million hectares occur in the tropics and subtropics, of which almost 25 million are in the coastal lowlands of Southeast Asia.
Histosols are used for various forms of extensive forestry and/or grazing or lie idle. If carefully reclaimed and managed, Histosols can be very productive under capital-intensive forms of arable cropping and horticulture.
Histosols have a high pore volume (low bulk density) which causes considerable problems for the reclamation. The removal of water through drainage is necessarily associated with shrinkage and compaction of the loose organic matter and subsequent considerable subsidence of the land surface. The low bearing capacity hampers also the construction of roads, buildings, and water works.
After removal of the plant cover the low thermal conductivity of the organic material allows very high temperatures to build up in the upper few centimetres of Histosols exposed to direct solar radiation. This causes irreversible transformation of colloids and makes the organic material crumble to a dry “powder” which is very susceptible to wind erosion. The lower parts of the solum, in contrary, heat up slowly which can be unfavourable for plant development in colder environments.
The fertility of Histosols is normally low when the natural vegetation is abruptly replaced with agricultural crops. The cycling of plant nutrients is interrupted and leads to chemical exhaustion, particularly where annual crops are grown on oligotrophic material. Especially micro-nutrients (as boron, copper and zinc) may be deficient. Controlled burning is often (particularly in subsistence agriculture) used as a means to liberate nutrients. However, burning has to be considered as an inappropriate practice for sustainable agriculture because in the long run soil structure is seriously damaged and most of the nutrients are lost to the atmosphere or leached out of the rooting zone.
The specific characteristics (low bulk density, colloidal nature and specific thermal properties) of Histosols require careful soil management for their sustainable agricultural use in order to deal with the degrading effects of rapid decomposition, mineralization and compaction of organic material after drainage; and irreversible shrinkage resulting in adverse water retention characteristics and increased sensitivity to wind erosion.