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Book: Quantifying Soil Hydromorphology
Published by: Soil Science Society of America

 

This chapter in QUANTIFYING SOIL HYDROMORPHOLOGY

  1.  p. 61-76
    SSSA Special Publication 54.
    Quantifying Soil Hydromorphology

    M.C. Rabenhorst, J.C. Bell and P.A. McDaniel (ed.)

    ISBN: 978-0-89118-949-7

     

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doi:10.2136/sssaspecpub54.c4

Soil Morphology and Frequency of Diagnostic Wet Soil Conditions

  1. A. A. Szögi and
  2. W. H. Hudnall
  1. USDA-ARS Florence, South Carolina
    Louisiana State University Baton Rouge, Louisiana

Abstract

Soil hydrology is the factor that drives the development of hydromorphic features, saturation, and reduction in wet soils. These three soil characteristics are all required to recognize aquic conditions and are important in the development of hydric soils, but they may vary throughout the year. Since soil hydrology variables are nearly always affected by random components due to annual, seasonal, and daily fluctuations, reliable prediction of wet soil conditions is difficult. However, time-dependent hydrological variables can be transformed into cumulative probability distributions in order to improve predictions of wet soil conditions. Our purpose was to examine differences in cumulative probability distributions of three hydrological variables [water table (WT) levels, soil redox potential, and tension] within a soil and to compare these distributions with locations of hydromorphic features within that soil. Water table levels, soil redox potential (Eh), and soil saturation data, taken periodically at different soil depths, were transformed into cumulative probability distributions. We examined the following two soils in a toposequence on the natural levee of the Mississippi River: Cancienne silt loam (fine-silty, mixed, hyperthermic Aeric Endoaquepts) and Schriever clay (very-fine, smectitic, hyperthermic, Chromic Epiaquerts). The use of cumulative frequency analysis helped to better understand the relationship between soil hydromorphology and frequency of WT levels above a horizon or group of horizons in both soils of this study. The approach also was useful for comparing the location of hydromorphic features vs. the probability of occurrence of WT levels above diagnostic soil depths according to Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys (Soil Surv. Staff, 1975). Water table level probability patterns corresponded well with probabilities of occurrence of reduction at 50-cm depth for both soils. But at 100-cm depth, the probability of soil reduced conditions were lower than the probability of occurrence of WT levels above 100-cm depth. As expected, since for soil reduction an organic C source in addition to soil saturation is required, more intense reduction and lower WT frequencies than at greater soil depths took place nearer the soil surface. Although soil moisture tension data was necessary to verify soil saturation, the comparison of saturation to either probabilities of occurrence of WT levels or location of hydromorphic features was not straightforward. Saturation patterns based on tensiometer data were not consistent with WT level frequencies due to differences between piezometer and tensiometer data at the same soil depth. Aquic conditions in both soils were verified by the presence of redoximorphic features, frequencies of occurrence of saturation, reduction, and WT levels at diagnostic soil depths according to the Keys to Soil Taxonomy (Soil Surv. Staff, 1992). When hydric soil conditions were tested, we found that the Schriever soil is a hydric soil because hydric soil criteria and field indicators of hydric soils corresponded well with probability of occurrence of WT levels. Morphological field indicators in the Cancienne soil indicated possible hydric conditions. However, the Cancienne soil did not meet hydric soil criteria, and this soil is not a hydric soil. The Schriever soil provides an example of how field indicators agree with the hydrology of a hydric soil and Cancienne provides an example of how field indicators do not agree with the hydrology of a hydric/nonhydric borderline soil.

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