The Soil Orders in Mississippi

Pedogenesis - How do soils develop?
Soil development, according to the American soil scientist Hans Jenny, is a function of numerous interacting factors, notably time, parent material, vegetation and other organisms, climate, and topography (landscape position). This idea was first suggested by earlier soil scientists, including Mississippi's state geologist E.W. Hilgard in the mid-1800s. Soil development or pedogenesis ultimately involves the combinations resulting from additions, subtractions, and in situ (in-place) transformations.

Over time, the combination of factors and processes results in a soil unique to an area. When similar conditions occur in various places around the world, the soils will develop somewhat alike, but due to local variations, these rarely will be exactly the same. Interestingly, in the study of the ecosystems that develop in the various global biomes, we generally find that similar ecosystems will support similar kinds of soils.

Soils in Mississippi, as Hilgard remarked in 1860, are quite diverse. In fact, eight of the twelve know soil orders (U.S. Taxonomy) occur with Mississippi's borders. The high diversity in Mississippi is due to latitudinal differences in climate from north to south, and more importantly, to parent materials and topography. Some of the older soils date to the Cretaceous and even older in the northeast to the very young Holocene soils of the Delta region. The majority of soils in the state owe their origins to Cretaceous marine, Holocene alluvial or post-Pleistocene aeolian deposits. Soil orders that occur in Mississippi are described below, as well as summaries of the four non-Mississippi orders.

Histosols (< histos = tissue) are soils that are composed mainly of organic materials, mostly plant remains. They contain at least 20-30% organic matter by weight and are more than 40 cm thick. Histosols typically form in settings where restricted drainage inhibits the decomposition of plant and sometimes, animal remains, allowing these organic materials to accumulate over time.

Wetland histosols are categorized based upon the degree of decomposition. Fibrists contain 1/2 or more easily identified plant remains. Hemists contain less than 50% identifiable plant remains. Saprists are often called "mucks" because the plant remains have been decomposed to fine organic particles. Folists are non-wetland histosols that result from decomposition of deep accumulations of leaf litter, wood and other materials.

Due to their origins, 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. In Mississippi, histosols (here mostly saprists) are largely restricted to freshwater, brackish and tidal marshes along the Mississippi Sound in the Gulf of Mexico.

Hisotosols are commonly called peats and mucks and have physical properties that restrict their use for engineering purposes. In some areas (Canada, Ireland, Florida), peats are mined for use as fuel and/or horticultural products.

Spodosols (< spodos = ash) are acidic soils characterized by a subsurface accumulation of humus and Al & Fe oxides. These soils typically have a light-colored E horizon overlying a reddish-brown spodic horizon. Although spodosols often occur under coniferous forest in cool, moist climates, they also occur in the sandy Gulf and Atlantic coastal plains, most notably in Florida. Spodosols have recently been described in south Mississippi. Globally, they occupy ~2.6% of the ice-free land area. In the US, they occupy ~3.5% of the land area. Many Spodosols support forest. Because they are naturally infertile, Spodosols require additions of lime in order to be productive agriculturally.

Vertisols (< vertic = to turn) are smectite clay-rich soils that shrink and swell with changes in moisture content. They occur on every continent except Antartica and under climates that have a seasonal dry period. During the dry period, the soil volume shrinks and deep, wide cracks form. The soil volume expands considerably as it wets up, creating serious engineering problems. Because of the shrink/swell activity of these soils, and almost constant mixing, vertisols generally do not have distinct, well-developed horizons. Globally, Vertisols occupy ~2.4% of the ice-free land area. In the US, they occupy ~2.0% of the land area and occur primarily in Texas. Vertisols are common in slackwater and low lying areas of the Mississippi alluvial plain, and include the common Sharkey clay series.

Ultisols (< ultimate) are old, strongly leached, acid forest soils with relatively low native fertility. They are found in humid temperate and tropical areas of the world, typically on older, stable landscapes. Ultisols have a subsurface horizon in which hard clays accumulate. These argillic horizons often show strong yellowish or reddish coloration due to the presence of Fe oxides.

The so-called "red dirts" and "red clay'" soils of the southeastern United States are examples of ultisols. Ultisols are ubiquitous to the Gulf coastal plain and comprise much of the soil types occurring in central Mississippi and up into the upper coastal plain landscapes. Ultisols occupy ~8.5% of the global ice-free land area. They are the dominant soils of much of the southeastern US and occupy ~9.2% of the total US land area. Because of the favorable climate regime, Ultisols support productive forests in the southeastern US. They are poorly suited for continuous agriculture without the use of fertilizer and lime, but can be very productive with these inputs.

Mollisols (< mollic = soft) are soils typical of grassland ecosystems. They are characterized by a friable, thick, dark surface horizon - the mollic epipedon. This fertile surface horizon results from the long-term addition of organic materials derived from plant roots. Mollisols primarily occur in the middle latitudes and are extensive in prairie regions such as the Great Plains of the US. Globally, they occupy ~6.9% of the ice-free land area. In the US, they are the most extensive soil order, accounting for ~21.5% of the land area. In Mississippi, we see mollisols in the prairie physiographic regions and in a few wetland locations in the alluvial plain region. Well-drained mollisols are among some of the most important and productive agricultural soils in the world.

Alfisols (< Fe + Al) are moderately leached forest soils that have relatively high native fertility. These soils have well-developed horizonation and contain a subsurface horizon in which clays have accumulated. Alfisols are mostly found in temperate humid and subhumid regions of the world. Alfisols occupy ~9.7% of the global ice-free land area. In the US, they account for ~13.9% of the land area. Alfisols occur throughout Mississippi, but are well-represented in the loess deposits that border the alluvial plain. Alfisols also occur on ridges and ridge bottoms within the alluvial plain itself, and are the typical soil order found in the interior flatwoods region. The combination of generally favorable climate and high native fertility allows Alfisols to be very productive soils for both agricultural and silvicultural use. Alfisols that occur in low lying landscapes tend to perch water in the winter and spring, and may only be suitable as forest land.

Inceptisols (< inception) are very young soils that exhibit minimal horizon development. They are more developed than Entisols, but still lack the features that are characteristic of other soil orders. Inceptisols are widely distributed and occur under a wide range of ecological settings. They are often found on fairly steep slopes, young geomorphic surfaces, and on resistant parent materials. Land use varies considerably with Inceptisols. A sizable percentage of Inceptisols are found in mountainous areas and are used for forestry, recreation, and watershed. Sand dunes along the Mississippi Gulf coast and on its barrier islands, are comprised of mainly of inceptisols. Sand and mud bars in stream systems and recently deposited alluvium, are classified as inceptisols. Globally, Inceptisols occupy ~9.8% of the ice-free land area. In the US, they occupy ~9.7% of the land area.

Entisols (< recent) are soils of recent origin. The central concept is soils developing in or over unconsolidated parent material with usually no genetic horizons except an A horizon. Entisols are not old enough to have definite B horizons. Generally, any soils that do not fit into one of the other 11 orders are grouped into the Entisols. Thus, they are characterized by great diversity, both in environmental setting and land use. Many Entisols are found in steep, rocky settings. However, Entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide. These are well-represented in the soils along Mississippi's stream drainages, and in the alluvial plain. Globally, Entisols are the most extensive soil order, occupying ~16.2% of the Earth's ice-free land area. In the US, they occupy ~12.3% of the land area.