This is the second of a series of blog posts where I will describe some of the interesting features of each of the 12 soil orders in Soil Taxonomy – the soil classification system developed for the US by the United States Department of Agriculture. Each order will be “profiled” (forgive the bad soils pun) in the order in which they are “keyed out” in Keys to Taxonomy.
Histosols are soils with organic matter as the primary parent material. They occur when conditions allow organic matter to accumulate at a faster rate than it can be decomposed. This is usually under wet conditions such as a wetland (think the Florida Everglades) or in areas where it’s just too cold for the microbial community to decompose plant material fast enough (think the Arctic Circle).
Histosols have organic surface layers at least 40 cm deep that are at least 12-18% organic carbon (not living roots) depending on clay content. Some frozen soils that were once classified as Histosols have been now reclassified as Histels once the Gelisol soil order was introduced.
Histosols occur where wet conditions exist because saturated conditions don’t allow air (and oxygen) to enter the soil profile, thus eventually creating anaerobic conditions. Under these conditions decomposition or organic matter is slowed, and what would have been converted into carbon dioxide by microbial respiration in dry conditions instead remains in the soil as organic matter. This organic matter builds up over time and eventually forms a Histsol.
There are different types of organic soil materials including fibric, hemic, and sapric soil materials. Fibric soil materials are organic soil materials that contain three-fourths or more plant fiber material after rubbing [to soil scientists, rubbing means rubbing the material between two fingers 10 times]. Sapric soil materials are organic soil materials that contain less than one-sixth plant fiber materials by volume after rubbing. Sapric materials will feel “greasy”. Hemic soil materials are in-between fibric and sapric.
For Histosols, the formative element for the order is “ist”. As described by Buol, et al (1997) Histosols have five suborders and they are classified in the following order in Soil Taxonomy. Wassists are Histosols that are floating on top of free water. Folists are Histosols that formed not due to wet conditions, but from high rates of accumulation of organic matter (relative to decomposition). The rest of the suborders are classified based on the state of decomposition. Fibrists are Histosols with fibric materials, Hemists are Histosols with hemic materials, and Saprists are soils with sapric materials.
As said above, Histosols form anywhere that has a rate of organic matter accumulation greater than decomposition.
At the scale of this map, it’s difficult to pick out many expansive areas of Histosols. However, they do occur throughout the world, but usually in small areas. This map of the US shows a better depiction of their occurrence.
Histosols are used for crop production and forestry, as well as wildlife and recreation. The organic material can also be harvested for horticultural potting soil and for heating and electricity. They can be production crop soils, however extensive drainage is required.
Unfortunately, drainage leads to subsidence. Subsidence is the loss of soil depth. Subsidence occurs when water is drained from the profile. The organic materials “float” in saturated conditions and become more compact when drained. Once drained, the soil begins to oxidize and microbes consume the organic matter and slowly turn it into carbon dioxide with time. Subsidence, as a rule of thumb, occurs at a rate of 1 inch of soil per year. This creates problems for drainage ditch maintenance and long term uses of agricultural soils.
Histosols, when drained, are also vulnerable to fires. Here is a recent news story about a fire that occurred in Alligator River National Wildlife Refuge here in North Carolina <http://outerbanksvoice.com/2011/06/10/million-of-gallons-of-water-pumped-on-peat-fire/>.
Here’s an excerpt from that article:
Flammable organic peat ranges from a few inches to 8 feet deep in the ground, said the multi-agency team fighting the fire.
Because of the lack of rain, peat, which was once used as a fuel, continues to burn even when the surface fire is put out, according to the North Carolina Incident Management Team.
“The ground fire will continue until the fire consumes all the peat down to mineral soil, the fire burns down to a level of high moisture content, or the soil moisture level rises to the fire as a result of an extended heavy rain or pumping operations,” the team said in a statement Friday.
My current research involves Histosols in a Carolina Bay wetland. The wettest part of the wetland (the center) is where the Histosols occur. They’re really interesting soils (I might be biased because I’m a wetland soils guy).