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Aquic Moisture Regimes in Soils with and without Low Chroma Colors¹

ABSTRACT

Four seasonally saturated Alfisols along a toposequence 56 km north of Houston, Tex., were studied to determine if each soil had an aquic moisture regime. Water table fluctuations were monitored weekly during 1978 and 1979, and oxidation-reduction (redox) potentials and groundwater Fe contents were monitored weekly during 1979 only. Water tables of soils on backslope and toeslope positions were perched and rose to the surface during periods of heavy rainfall. Water tables of lowland soils were probably permanent, and their rise lagged behind those of the upland soils by as much as 3 months. Redox potentials ranged from –200 to +900 mV across the toposequence during the year. Anaerobic conditions existed in each soil for variable periods. The periods when redox potentials (at a depth of 75 cm) were sufficiently low to reduce Fe(III) compounds generally increased from 6 to > 197 cumulative days from upper to lower topographic positions. Groundwater Fe contents were low, ranging between 0.1 and 2.7 ppm. Apparently little Fe reduction occurs in this landscape, because organic matter levels are low (< 1.0%). On the basis of the water table and redox potential measurements, each soil was considered to have significant periods of saturation and reduction within 75 cm of the surface. Low chroma ( 2) colors were not found in horizons (at a depth of 75 cm) which were reduced for 120 cumulative days, but color chromas were 3 or less along ped surfaces and ped interiors had mottles with chromas of 4 or more. Limitations of the low ( 2) chroma color criterion commonly used to identify aquic moisture regimes are discussed. Recommendations for use of color chromas of 3 or less for identification of aquic moisture regimes are proposed.

¹ Contribution from the Soil and Crop Sciences Department, Texas A&M University. Paper no. 17844 of the Texas Agric. Exp. Stn., College Station.

² Former Research Assistant and Professor of Soil and Crop Sciences Department, Texas A&M University, College Station, TX 77843, respectively. Current address of Senior Author is Soil Science Dep. North Carolina State University, Raleigh, NC 27650.

Received for publication August 31, 1981. Accepted for publication September 24, 1982.

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