Entisols-Fluvents and Fluvaquents: Problems Recognizing Aquic and Hydric Conditions in Young, Flood Plain Soils
- David L. Lindbo
Most Entisols are found principally in flood plains (Fluvents) or low-lying areas (Aquents). Entisols have minimal horizon development (A and C horizons) due to a combination of factors including recently deposited or resistant parent material, landscape position (areas subject to flooding), wetness, climate, and/or time. Identifying aquic conditions and hydric soils in Entisols poses problems due to their lack of redoximorphic features. A review of data from 128 Fluvents and 42 Fluvaquents indicated that colors inherited from parent materials, the effect of high pH, and low amounts of organic C probably prevent development and/or recognition of redoximorphic features more typical of hydric soils. Fluvaquents and Fluvents from the Connecticut River Valley and western Massachusetts have similar colors, often with chromas <2, throughout the profile regardless of whether they have been reduced and saturated or not. Hydric Fluvaquents with similar morphologies in bottomland hardwoods of the lower Mississippi River were identified as hydric and were anaerobic for 10 to 100% of the year. Thus variable conditions may result in the development of similar morphologies and redoximorphic features. Lack of reducing conditions in alluvial soils of the Puget Lowlands of Washington was explained by low soil temperatures and low C contents (<8 g kg−1). A confounding factor in the Puget Lowland study was a change in natural drainage. This results in a soil morphology that no longer reflects the current conditions. Carbon contents (<8 g kg−1) and a thermic temperature regime contributed to reducing conditions and formation of redoximorphic features on some barrier island soils of Texas in the Gulf of Mexico. A study of constructed wetlands in Illinois suggests that an organic matter (OM) content of approximately 30 g kg−1 (approximately 16 g kg−1 organic C) is essential for rapid development of redoximorphic features. This review suggests the problems associated with the identification of hydric conditions in some Entisols may only be overcome by a better understanding of the parent material, climate and chemical properties of the soil, and by monitoring the site.
Entisols are soils that have not developed much beyond the formation of an ochric epipedon (Buol et al., 1989; Soil Survey Staff, 1994). The relative lack of horizon development can be attributed to one or more of the factors of soil formation. Entisols may be geologically young in the sense that there has been insufficient time for appreciable soil formation to occur. For example, an Entisols is formed in an area that floods frequently when sediment accumulates in layers >50 cm thick results. On the other hand, Entisols also occur in areas where the climate is extreme (i.e., cold and wet, cold and dry, or hot and dry) where little biological activity or weathering occurs. Parent material may retard soil development in locations where the material is highly resistant to weathering as in the case of coarse textured quartzitic material. Entisols also are common in situations where a soil is developing on fresh bedrock or thinly deposited sediments. Topographic position also plays a role since soils in low lying areas are often waterlogged (saturated), and soil development is slowed by waterlogged conditions. Finally, the actions of man may result in a soil whose horizons are mixed or removed, resulting in a soil being classified as an Entisol.
Fluvaquents and Fluvents are two of the more important Entisols. These soils occur in areas containing alluvium (flood plains, fans, and others) and in wet mineral areas (marshes, lagoons; Grossman, 1983). In most regions of the USA they rarely exceed 5% of any area (Table 8-1). Their location in fertile, highly cultivated flood plains, however, make them agriculturally important. In addition, these soils often are located near major population centers and are exposed to significant developmental pressure. Therefore, an understanding of the hydric status of these soils along with the role of periodic flooding and sediment deposition is necessary. The objectives of this chapter are to: (i) evaluate Fluvents and Fluvaquents from a survey of the NRCS data base for factors relating to their hydric soil status; and (ii) identify problems with Entisols in determining hydric soils using six case studies. Although this chapter discusses hydric conditions, the same discussion can be applied to aquic conditions since a hydric soil meets the requirements for aquic moisture regime.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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