According to the 1999 USDA survey "Soil Taxonomy" and the 2007 addendum "Keys to Soil Taxonomy," hydric soil is any soil that exists in a saturated, flooded or submerged state for so long during the yearly growing season that it gives rise to the propagation of anaerobic bacteria. Based on this and other determinants, those soil orders and suborders that can become hydric soils include aqualfs, uderts and saprists.
Aqualfs are a suborder of alfisols (soils in semiarid to humid areas below hardwoods) found in Florida as well as from Louisiana to Illinois, following the Mississippi River. Within this suborder are 12 greater subgroups. All alfisols are moderately weathered or leached soils that are still high in nutrients. Aqualfs are characterized by their aquic conditions, meaning they drain very poorly and result in standing water if it should rain more than 3 inches in one hour. To qualify as an aqualf, the soil must have redoximorphic features, meaning it appears mottled when viewed in cross-section, in all layers between 10 to 15 inches below the surface. Chemically speaking, the process of oxidation and reduction known as redox must have reduced the non-metal nutrient content of the soil by 50 percent or more and created a chroma value of 2 or less according to the Munsell color system.
Uderts are a suborder of vertisol. All vertisols have a clay content of 60 percent or higher, giving them significant shrink-swell capacity and making them prone to poor drainage. Uderts occur in humid environments such as Alabama, eastern Texas and the lower Mississippi River Valley. They make up the catch-all category of vertisols. Any soil that contains enough iron to react to alpha,alpha-dipyridyl when the soil is not being irrigated, develops cracks that stay more than 1/8-inch wide at a depth of 10 inches during the dry season, or ever drops below freezing for more than 30 days is disregarded, leaving only those that stay warm year round, contain less clay than the other suborders, and are not acidic enough to kill off anaerobic bacteria.
Saprists are a suborder of histosol, which is made up of soils found in marshy environments and contain a minimum of 2/3 densic, lithic, or paralithic matter. These terms are used to indicate the extremity of organic matter's decomposition. Saprists specifically contain densic organic matter, which is organic matter at the highest degree of composition and therefore has the highest density. In addition, they do not contain layers of mineral aggregate. This means they are typically placed well below sea level and the water table isn't very far down. Add in the fact that the densic matter reduces this soil's ability to drain and hydric varieties of this are commonly found.