first two factors is termed initial subsidence and is normally accomplished in about 3 years after 

 lowering the water table. Oven-dried samples of organic layers of a Kenner soil lost 85% of the 

 original volume. Artificial drainage, however, does not reduce water content to that extent, and 

 volume change under field conditions is less. Initial subsidence of organic soils is estimated to 

 result in a reduction of thickness of the organic materials above the water table by about 50%, and 

 it is accompanied by permanent open cracks that do not close when the soil is rewet. After initial 

 subsidence, shrinkage will continue at a fairly uniform rate due to biochemical oxidation of the 

 organic materials. This is termed continued subsidence and progresses until mineral material or 

 the water table is reached. The rate of continued subsidence depends upon temperatures, the 

 mineral content, and depth to water table. The rate increases with depth to the water table. 

 Mineral soils with the fluid layers (n-value of 1 or more) have a potential for initial subsidence due 

 to loss to water and consolidation after drainage. 



Classification of the soils is based upon selected chemical and physical properties as defined in 

 Soil Taxonomy (U.S. Department of Agriculture 1974). The soils in Louisiana's coastal marsh may 

 be classified as Aqualfs, Aquents, Saprists, or Aquepts. 



Aqualfs are mineral soils that have subsurface horizons that have formed structure and contain 

 alluvial silicate clay. They have poor natural drainage and ground water stands close to the 

 surface at some time during the year, but not during all seasons. Regional subsidence of coastal 

 lands relative to mean sea level causes progressive inundation of former uplands. This class of soils 

 is presumed to have developed in a previous weathering regime and subsequently submerged. This 

 class constitutes about 3% of the coastal marsh. 



Aquents are mineral soils that have little or no evidence of development of pedogenic horizons. 

 They are gray and permanently saturated with water. They have never dried; consequently bulk 

 densities are low (about 0.6 g/cm 3 ) and water contents are high (over 100%). Because of the high 

 water content, soil strength is low, commonly too low to support grazing animals. The soil material 

 is fluid. This class constitutes about 36% of the coastal marsh. 



Saprists are soils that are dominantly organic material. The organic layers contain more than 

 18% organic carbon if the mineral fraction is more than 50% clay, or more than 12% organic 

 carbon if the mineral fraction has no clay. In addition to the required organic carbon content, 

 saprists are organic soils that, except for thin mineral layers, extend from the surface to a depth 

 of 40 cm and the organic material has a bulk density of 0.1 g/cm 3 or more. The organic carbon 

 consists of almost completely decomposed plant remains and the rubbed-fiber content is less than 

 15%. They are saturated with water most of the year, but fluctuation has allowed aerobic decom- 

 position of some of the fibrous material. This class constitutes about 59% of the coastal marsh. 



Aquepts are mineral soils that have subsurface horizons that also have formed structure, but 

 lack illuvial horizons enriched with silicate clay. Water stands close to the surface at some time 

 of the year, but not during all seasons. These soils are on low ridges in the Chenier Plain. This 

 class constitutes about 2% of the coastal marsh. 



SUMMARY AND CONCLUSIONS 



Although the soils of Louisiana's coastal marshland have a common characteristic of wetness, 

 other generalizations are impossible unless the location and characteristics unique to each soil are 



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