Histosols are soils that are composed mainly of organic materials. They contain at least 20-30% organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 g cm3. Most are acidic and many are deficient in major plant nutrients.
Most Histosols form in settings such as wetlands where restricted drainage inhibits the decomposition of plant and animal remains, allowing these organic materials to accumulate over time. As a result, Histosols are ecologically important because of the large quantities of carbon they contain. These soils occupy ~1.2% of the ice-free land area globally and ~1.6% of the US. Histols form whenever organic matter forms faster than it is destroyed. Histols store large quanities of organic carbon and if accumulation continues for a long time period, coal forms.
Histosols are often referred to as peats and mucks and have physical properties that restrict their use for engineering purposes. These include low weight-bearing capacity and subsidence when drained. They are mined for fuel and horticultural products.
Folists - Histotols that are not saturated with water for long periods of time during the year. Many of these soils are in very humid climates. Most of the USA Folists are in Hawaii and Alaska and some in the mountains of the western US, northern Lake States, and the northeastern USA.
Fibrists - Histosols that are primarily made up of only slightly decomposed organic materials, often called peat. Often the botanic origin of materials can be readily determined. Many Fibrists have ground water near the soil surface nearly all the time. Sometimes found in closed depressions and in broad flat areas such as coastal plains. Most are under natural vegetation.
Hemists - Histosols that are primarily made up of moderately decomposed organic materials. Botanic origin of materials cannot be readily determined. They occur in closed depressions and in broad flat areas, such as coastal plains and outwash plains. Most are under natural vegetation and are used as woodland, rangeland or wildlife habitat.
Saprists - Histosols that are primarily made up of highly decomposed organic materials. Tend to occur in areas where the ground water table tends to fluctuate within the soils or in areas where the soils were aerobic during drier periods in the past. When drained, fibric and hemic materials can decompose to form sapric materials. If deep Fibrists and Hemists are drained, they can convert to Saprists after some decades.