Moisture regimes and associated soil mottling patterns were investigated over a 2‐y period in a Paxton‐Rainbow‐Ridgebury‐Scarboro hydrosequence in central Massachusetts. Changes in physical conditions in these coarse‐loamy fragipan soils were monitored with well points, tensiometers, soil temperature probes, and redox potential electrodes. Well‐drained soils appeared uniformly brown, reflective of a generally strongly oxidized environment. They also showed the greatest seasonal fluctuations in soil temperature ranging from 0 to 23°C. Matrix colors of highest chromas (6 or higher) occurred in the moderately well‐drained (MWD) soils where reducing and oxidizing environments coexist within the profile at some time during the growing season. Prominent ferrans and albans in the 2Cx horizons of MWD soils were related to sustained groundwater levels, although higher water tables often occurred for periods too short or too cold for development of strongly reducing conditions. The characteristics grey streaking patterns of fragipans were best expressed in the 2Cx horizons of MWD soils and can be attributed to reducing environments during periods with soil temperature above biologic zero when conditions are wet but unsaturated. Significant translocation and leaching of iron occurred in the somewhat poorly drained soils, as evidenced by olive‐grey matrix colors. Channel neoferrans and some neoalbans indicated that a reducing environment may persist within the B2 horizon long after it has become unsaturated. Lowest chroma colors in this hydrosequence was found in the very poorly drained soils where perennial conditions of saturation or near saturation exists up to the soil surface resulting in the strongest reducing environment and least fluctuations in seasonal temperatures.
Conventional redox potential measurements in the field frequently become erratic during the summer season due to low moisture contents in the surface soil. A KCl-salt bridge which circumvents this problem was designed for in situ, long-term, nondestructive redox determinations in the subsoil. Measurements are rapid, and redox values are not affected by the use of the salt bridge, irrespective of depth of measurement.
Identification of fragipans is often difficult due to the lack of quantitative differentiating criteria. The unconfined compression (UC) test was used to characterize strength properties of three indurated horizons. Tests were performed in the laboratory on undisturbed core samples equilibrated to one of four moisture conditions: wet, 0.1 MPa, 0.5 MPa, or air‐dried. Fragipans developed in basal till exhibited maximum UC strength values of about 100 kPa at moisture contents < 0.10 kg kg−1 and minimum UC strength of < 25 kPa at wetnesses of 0.14 kg kg−1 or more. Between these two critical moisture contents, strength varied inversely with wetness. A pan formed in post‐glacial colluvial deposits showed similar maximum UC strength values but still retained about 70% of its UC strength at the wet moisture condition. The UC testing data enabled separation of the fragipans developed in the glacial till from indurated horizons formed in the colluvial material. Pocket and Proctor penetrometers were investigated to determine their suitability for routine recognition of fragipan horizons in the field, especially during wet soil conditions. Their application did not improve upon current field techniques.
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