Region 3: The Coastal Plain

The only Histosols in the South Central occur in coastal Louisiana, especially in the Mississippi Delta. These organic soils form from the slow decomposition of organic detritus in wetland environments (Figure 8.19). Swampy, waterlogged soils are often drained to accommodate human settlement, which causes them to condense and subside. In addition, a series of floodwater-diverting levees block the coast of Louisiana from receiving new alluvial sediments that would counteract subsidence. These factors and others have contributed to the loss of Louisiana’s wetlands, in turn reducing the protection such wetlands provide against hurricanes.

Figure 8.19: An example of a Histosol soil. These soils are rich in organic matter and are often referred to as peats, bogs, or mucks.

Ultisols are common in the forested part of the Coastal Plain region, spanning northwestern Louisiana and eastern Texas. These clay-rich soils are interspersed with Alfisols, which are common in southeastern Missouri, eastern Arkansas, central Louisiana, and southeastern Texas. The Alfisols in Texas show a climate gradation from warm, moist soils in the coastal areas to drier soils in the inland parts of the Coastal Plain.

Most Entisols in the South Central are found along the Mississippi River, but they are also scattered along the Red River in central Louisiana and in Texas near the coastline. These soils, generated by the deposition of floodplain alluvium (Figure 8.20), are often highly productive for agricultural use.

Most of the South Central’s Vertisols are found in the Coastal Plain. During dry periods, these clayey soils shrink and form wide cracks at the surface (Figure 8.21); these cracks seal shut again when moisture enters the soil. Because Vertisols shrink and swell so readily, it is extremely difficult—and even dangerous—to build structures or roads on top of them. The action of shrinking and swelling within the soil also prevents the formation of distinct horizons. Drier-condition Vertisols are very common in a belt that follows clay-rich Cretaceous rocks corresponding to the Balcones Escarpment; they develop from parent materials rich in magnesium and calcium, such as limestone and basalt. Wetter-condition Vertisols are found along the lower Mississippi River, the Red River in Louisiana, and some of the larger rivers in southeastern Texas, such as the Brazos River.

Figure 8.20: Entisols at the mouth of Long King Creek, Polk County, Texas.

Figure 8.21: Cracks in a dried-out Vertisol clay found in Arkansas.

Aridisols do occur in the Coastal Plain region, but not near the coast. They are limited to the hot interior of southern Texas, in areas that are generally higher in elevation.

Mollisols are relatively uncommon and are found mostly in southeastern Texas, away from the coast. These soils form under drier conditions, like the Mollisols in the Texas portion of the Central Lowland.

Inceptisols are scattered along streams in eastern Louisiana and eastern Texas, and along the edge of northeastern Arkansas in a belt of slightly higher-elevation terrain.