Abstract Regolith distributions and characteristics were related to slope processes along two transects in mica gneiss and mica schist terrane on the Blue Ridge Front of North Carolina. Regolith types are saprolite, soil residuum, and colluvium. Saprolite is thickest on low slope positions, where throughflow moisture promotes weathering and overlying colluvium protects the saprolite from mass wasting. Saprolite is thinnest on shoulders and backslopes, where mass wasting and rapid runoff inhibit its formation. Rock outcrop is most common on backslopes. Soil residuum, present on most landscape positions, comprises the entire regolith above saprolite only on the broad, nearly level ridge top. The pedogenic transformation of saprolite to soil residuum resulted in higher clay contents and redder hues through in situ weathering and illuvial accumulations. On sloping terrain, saprolite or soil residuum is overlain by colluvium. Lower slopes contain thick deposits of landslide colluvium. Colluvium from slope wash and soil creep comprise the upper 10 to 20 cm of regolith on all sloping surfaces. Colluvial deposits, excluding A and E horizons, are uniform in color, the result of homogenization during transport. Mass movement has also served to physically break down chemically weathered mica grains, releasing secondary minerals into finer size fractions. Knowledge of slope processes assists interpretation of regolith weathering history, which is important for understanding soil parent materials.
Abstract Upland residual soils formed from mica gneiss and schist in the Piedmont and Blue Ridge provinces of North Carolina represent developmental sequences where particle‐size class, profile development, and mineralogy are related to landscape position and slope. In this study the distribution of total and free iron in 10 Hapludult and Dystrochrept pedons is examined and interpreted with respect to degree of profile development. Primary mineral alteration and B horizon free iron/total iron ratios vary nearly systematically with particle‐size class and solum thickness. The latter two parameters were found to be good indicators of degree of profile development and hence of relative soil age. The free iron/total iron ratio was found to be a particularly sensitive indicator of relative soil development. The ratio reflects the degree of primary mineral alteration, the amount of clay formed, as well as the pedogenic process of illuviation. The B horizon ratios are much better indicators of relative soil development than those from either C or A horizons. Ratios of free iron/total iron in A horizons are influenced by downslope movement of surficial material, the low extractability of iron oxides of large particle size, and clay content differences. Free iron/total iron ratios of C horizons are dependent on sampling depth, the amount of clay formed, and the degree of illuviation that has occurred below the solum. All soils had oxidic ratios (%Fe 2 O 3 + %gibbsite/%clay) > 0.2; however, all but one was excluded from the oxidic mineralogy class due to having >40% mica. A simple free iron/total iron ratio in B horizons, which is systematically related to the degree of weathering of primary minerals and the degree of morphological soil development, better serves as soil family criteria than the present % Fe 2 O 3 + % gibbsite/% clay.
Abstract Thermodynamic models predict that Mn is a more mobile component of soil systems than is Fe and therefore subject to more extensive redistribution within soils and landscapes. This study was initiated to examine the accumulation of secondary Mn (Mn d ) relative to that of secondary Fe (Fe d ) as influenced by field‐scale water movement. Secondary Mn and Fe were measured using a citrate‐bicarbonate‐dithionite (CBD) extract of genetic horizons of 60 soils sampled in an Ultisol‐dominated landscape of the North Carolina Piedmont. Soil water pressure was measured using tensiometers installed at each soil sampling site to infer water flow patterns. For all well‐drained soil borizons, Fe d contents correlate well with clay content ( r = 0.878***), suggesting that relatively little redistribution of Fe released through weathering has occurred within the landscape. In contrast, Mn d contents are extremely variable among genetically similar horizons and are not well correlated with clay content ( r = 0.327***). These differences serve as the basis for using secondary Mn/Fe (Mn d /Fe d ) ratios as relative indicators of Mn accumulation. Total soil water potential contours for Ap and Bt horizons closely parallel elevation contours, indicating the strong influence of slope gradient and configuration on water movement. Secondary Mn/Fe ratios are highest in Ap horizons of soils occupying lower slope positions and concave areas of the landscape. Total soil water potential data indicate convergent throughflow occurs in these wetter areas of the landscape. In contrast, lower Mn d /Fe d ratios are found in soils occupying more convex landscape positions where soil water potential data show that divergent throughflow occurs. Results indicate that the difference in mobility between Mn and Fe in these acid soil systems allows secondary Mn/Fe ratios to be used as pedochemical indicators of field‐scale water movement.
Abstract Depending on the mineralogical nature of the parent material and underlying rock, Fe, Mn, Si, and bases are concentrated downslope as a result of lateral‐ and/or base‐water flow action. Dissolution at the rock‐saprolite interface is an important supplier of those elements, which are transported by the base‐water flow affecting mainly the soils in the footslope. Chemical weathering does not always proceed in the same direction. The alternate process of desilication‐resilication occurs horizontally downslope along the toposequence. This approximately corresponds to Walther's Law of Succession of Facies. The lateral resilication process is important in tropical regions because it counterbalances the allitization process and explains the occurrence of relatively less weathered clay minerals on lower slope positions in the soilscape.
Abstract Crop response to soil management practices is determined in part by soil properties. Soil classification systems provide a readily available measure of soil properties. If information about crop response to soil management practices can be transferred between locations based on soil classification information, this will aid in the adoption of optimum management practices. As a test of the usefulness of soil classification information in transfer of response information, a technical soil classification system was used to evaluate corn ( Zea mays L.) and soybean ( Glycine max L. Merr.) response to deep tillage across a range of soil, climate, and management conditions. Three deep tillage systems—subsoiling under the row, subsoiling under the row plus bedding, and chisel plowing—were compared with a shallow tillage system, disking only, in 16 experiments with corn and 13 with soybeans on North Carolina farms. Soils in the fields were classified according to a technical soil classification system adapted from the Fertility Capability Classification (FCC) system. Response to deep tillage varied significantly among soil groups in the technical classification system. Yield increases due to deep tillage varied by soil group from 36 to 3% with corn, and from 25 to −2% with soybeans. A general recommendation based on results across all soils would have led to nonoptimum choices of tillage practices on half of the soil groups. Response was related to texture and thickness of Ap and E horizons, and texture of Bt horizons. The largest yield increases occurred on soils with sandy Ap and E horizons over loamy Bt horizons. No significant yield responses occurred on soils with loamy Ap horizons overlying clayey Bt horizons. The technical soil classification system was found to have considerable potential as a basis for transfer of tillage response information.
Abstract Continuous clay skins are found on ped faces, in pores, and around root channels in the B22t horizon of Cecil soils in North Carolina. Thicker, discontinuous clay skins are also observed along vertical pores in the Cl horizon. Compared to the whole peds from the B22t horizon the clay skins contain more clay, fine clay (<0.2µ), total iron, free iron, Al 2 O 3 , K 2 O, P 2 O 5 , and total nitrogen; less silica and gibbsite than the whole soil peds. The fine clay fractions (<0.2µ) of the A2 horizon and the clay skins of the B22t horizon are similar, both being finely divided and poorly crystalline compared to the fine clay from the whole peds of the B22t horizon and the clay skins in the Cl horizon.
Abstract Soils of the Appling series (Typic Hapludults; clayey, kaolinitic, thermic) are extensive in the Piedmont Province of the southeastern United States. Typically, these soils exhibit colors grading from brown (7.5YR) in the upper B horizons to red (2.5YR) in the saprolite. Fine clay separates (<0.2µm) from the brown (B21t horizon) and red (C2 borizon) zones of a representative pedon of the Appling soil were analyzed to determine if the color transition reflected a change in iron mineralogy. Concentration of secondary iron forms by alkali dissolution of gibbsite and silicate clay minerals enabled the x‐ray identification of aluminous geothite and hematite in the C2 clay but only aluminous goethite in the B21t material. These results were substantiated by Mössbauer spectroscopic analyses of the natural clays, which gave spectra characteristic of finely divided hematite and/or goethite. Subnormal effective internal magnetic fields (relative to standard goethite and hematite) were also measured and were in keeping with the effect of Al substitution. Presence of both goethite and hematite in the saprolite but only goethite in the B21t material suggests a pedogenic transformation of hematite to goethite.
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