Gypsum (CaSO4·2H2O) is the only pedogenic calcium sulfate mineral that has been found in soils with ustic, xeric, and aridic moisture regimes. It has been found in soils in 14 of the 17 conterminous western states by the National Soil Survey Laboratory and likely will be found in the other three. In these arid to subhumid soils, parent material differences in large part control the occurrence of gypsum. But gypsum in soils can be from other sources too. For example, drainage of coastal wetlands oxidizes sulfides to acid sulfates, and in their reclamation, if not before, the acid sulfates are neutralized by carbonate to form gypsum. Gypsum is also formed in minesoils by neutralization of the acid sulfates released by oxidation of sulfides. Pedogenic gypsum, in contrast to allogenic gypsum, accumulates in subsurface horizons relative to surface and underlying horizons mostly as euhedral to subhedral spindle-shaped crystals in pores and veins. As the s-matrix in a developing gypsic horizon becomes plugged, the pore volume decreases and the restricted hydraulic conductivity keeps the soil moist longer allowing the growing gypsum crystals to interlock and indurate the horizon. Subsidence of soils through solution and removal of gypsum can crack building foundations, break irrigation canals, and make roads uneven. Concrete in slab structures, irrigation canals, and building foundations deteriorates and cracks as extremely high pressures develop during formation of highly hydrated ettringite [Ca6·Al2(SO4)3(OH)12·26H2O] or during conversion of thenardite (Na2SO4) to mirabilite (Na2SO4·10H2O) if the temperature in the concrete and the soil drops low enough.

Thirty-nine soils were studied on Holocene and late Pleistocene geomorphic surfaces. Granodiorite, sandstone, and alluvium derived from these rocks are the parent materials. Climate is Mediterranean. Chamise (Adenostoma faciculatum) is on the drier sites and redwood (Sequoia sempervirens) on the moister sites. Our objectives are twofold, (1) Find if today's two-season wet- dry, subhumid climate explains the general noncalcic nature of the soils, or if not (2) accept that their noncalcic nature results from more moist past climates and define some indicator soil properties. The depth to carbonate in the soils formed in calcareous materials on Holocene surfaces corresponds roughly to the average annual depth of water movement, not to the predicted wettest years. We accept then, that the relict paleosols formed under one or more pluvial cycles because they are free of carbonate below their B horizons. Defined levels of pedon clay accumulation, dithionite-citrate extractable Fe (Fe_d) accumulation in the B horizons, cation exchange capacity at pH 7 (CEC_7) to clay ratios, and the minimum base saturation at pH 7 (BS_7) in the pedons are useful properties for separating these relict paleosols from the Holocene age soils. A further evidence of the relict nature of the soils on the Pleistocene surfaces is the weathering reversal noted in these previously weathered materials.

Paleosols formed on prior surfaces of the geologic past and include buried, exhumed, and relict kinds. Exhumed and relict ones are subaerial and have been classified according to systems of classification for other soils at this interface. One of these systems, Soil Taxonomy, includes relict and exhumed paleosols with the subaerial (ground) soils, but would need extensive modification to include buried paleosols. Our approach to paleosol classification, like that of the recently published Mack-James-Monger system, uses some Soil Taxonomy orders. However, we use proxy criteria to incorporate plaeosols that have become lithified and add the prefix krypt (hidden or covered) to all order names. Enduring properties are used as proxy criteria. Because base saturation by the NH4OAc method and cementation of paleosols tends to increase with additions of salt, gypsum, and carbonate to buried paleosols, we chose as proxy criteria the weatherable mineral content of sands and silts, cation exchange capacity (NH4OAc method) to clay ratio, total analysis of the < 2 mm soil and clay mineralogy. For the same reasons, petrogypsic and petrocalcic horizons and duripans are used instead of gypsum, carbonate, or silica content. Five of our proposed orders follow Soil Taxonomy criteria with some modification to accommodate related paleosols that have become lithified. New paleosol orders are proposed only for soils so modified by profile welding (overprinting), diagenesis, and metamorphism that their properties do not closely relate to Soil Taxonomy criteria. Additional paleosol subclasses are provided for describing the complete profile and its horizons, its oxidation state, the kind of overburden, and the geomorphic extent of the paleosol.

Present classification systems for buried paleosols are inadequately developed. Several classification systems for paleosols, based on the US Soil Taxonomy, have been developed recently, but their use is still very limited. We propose appilcation of the World Reference Base for Soil Resources, which uses only substantial profile characteristics, as a background for the development of a new buried paleosol classification. The system was adapted for the use for buried paleosols. Dynamic criteria, which could be altered by diagenesis, were disregarded, and the definitions of most horizons were modified. A new prefix "infra-" was applied for modified diagnostic horizons and properties, and for refernce groups of buried paleosols. As a result, 11 diagnostic horizons, 2 properties and 2 materials were left with their definitions and names; 9 diagnostic horizons and 2 properties were modified, and named with the prefix "infra"; 14 diagnostic horizons, 6 properties and 4 materials were excluded or joined with others; and two new diagnostic horizons were proposed. This relationship reflects the balance between stable and dynamic properties in the diagnostic criteria used by the WRB. The number of soil units proposed for buried paleosols is fewer, than for surface soils in the WRB (25 instead of 30). The use of WRB modifiers, reflecting dynamic soil properties, was tentatively recommended for the second-level buried paleosols classification with prefixes "pedo-" for pedogenically derived properties, and "dia-" for the properties caused by post-burial processes. The application of the proposed system to a set of paleosols, previously described in the literature, showed its utility for buried paleosol classification.

The uppermost surficial stratigraphic unit south of the Arkansas River in the Kansas High Plains is a previously unidentified middle Holocene or younger loess. The unit fits both a loess-thickness and a particle- size model for loess distribution with increasing distance from source. A soil immediately below this unit is radiocarbon dated 6000 to 6700 years BP. The radiocarbon ages indicate that the loess unit is younger than the commonly reported ranges for Bignell Loess in the Great Plains and demonstrate a need for re-examining Holocene loess stratigraphy of the Great Plains. In some locations closest to the Arkansas valley source, this middle Holocene unit is overlain by, or interfingers with, dune sands. The presence of these aeolian sands indicates that, following a period of relative landscape stability and soil development, a shift toward a middle- Holocene climate in which aeolian processes dominated occurred about 6000 years ago on the High Plains of west-central Kansas.

Climate is, in many instances, the dominant variable controlling the storage of carbon in soils. It has proven difficult, however, to determine how soil properties influenced by climate, such as soil temperature and soil moisture, actually operate to determine the rates of accumulation and decomposition of soil organic matter. Our approach has been to apply a relatively new tool, the comparison of C-14 in soil organic matter from pre- and post-bomb soils, to quantify carbon turnover rates along climosequences. This report details the progress made toward this end by work under this contract.

Nettleton, W.D., Brasher, B.R., Baumer, O.W. and Darmody, R.G., 1994. Silt flow in soils. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 361–371.

Soil erodibility is influenced by several soil properties including the extent to which the clay fraction will disperse in water. Because early methods for estimating soil erosion were empirical methods and did not utilize water-dispersible clay as a parameter, few data have been collected. The recent development of the Water Erosion Prediction Project (WEPP) model, a process-based model for predicting water erosion that uses water-dispersible clay in the algorithm for computing interrill erodibility, resulted in an increased demand for these data. In order to accommodate this and similar models, a method for estimating the water-dispersible clay content of soils based on existing information is needed. Data collected by the National Soil Survey Laboratory in support of the WEPP were used to identify soil properties that were significantly correlated with water-dispersible clay and to develop equations to estimate the water-dispersible clay content of soils based on those properties. The property most strongly correlated with water-dispersible day is total clay. Other properties significantly correlated with water-dispersible clay are the water content at 1.5 MPa, dithionite-citrate-extractable Fe and Al, the coefficient of linear extensibility, Wischmeier's M, the very-fine-sand content, the ratio of cation-exchange capacity (CEC) to total day, Bouyoucos' clay ratio, and the CEC. A simple linear regression of water-dispersible clay vs. total clay revealed that, for the soils included in this study, approximately one-third of the total clay was water dispersible. However, the model only bad an R2 of 0.604. When the ratio of the CEC corrected for organic carbon (CCEC) to total clay was included in the model, the R2 improved to 0.723. However, sorting the data by the ratio of CCEC to total clay instead of including it in the model improved the overall fit of the model and increased the R2 to 0.879.

Quantitative definitions of taxa and inclusion of the soil series as a category in soil taxonomy cause about three-fourths of the series-named pedons sampled, analyzed, and correlated in the USA to be taxadjuncts to the named series. Imposing the limits of soil taxonomy that circumscribe soils from without frequently divides natural soil bodies, i.e., collections of recurring contiguous pedons that are more similar to each other than they are to adjacent soils. We used the pedon data for six soil series to explore causes and alternatives to placement of pedons as taxadjuncts. Eight to 11 pedons per series were identified in the field, and sampled. Of the total of 56 pedons, 59% were taxadjuncts. We tried the two common approaches to handling these: (i) ignore misses that are just outside the limits of taxonomic criteria, or (ii) attribute near misses to errors in precision, which assumes that the error is not normally distributed. Neither approach is valid nor eliminates all of the taxadjuncts. We considered (i) dropping the series as a taxonomic category, (ii) adopting the cartographic series proposed by Knox, or (iii) classifying the central concept of the series but allowing characteristics to range across the limits between two families, or between two classes of any higher category. We propose option iii because, if accepted, (i) natural soil bodies would not be subdivided by artificial boundaries, (ii) soil taxonomy would be retained to facilitate technology transfer, (iii) the USDA-SCS prohibition against publishing data for taxadjuncts in soil survey reports would be nulified, and (iv) the exchange of information about the use and management of series would be facilitated.

AbstractSan Luis Valley is a semibolson in south central Colorado that has varying geomorphic and soil properties. Three sets of geomorphic surfaces were identified corresponding to young, intermediate, and old surfaces. A 14C date of 11 170 YBP on a peat deposit on the valley floor was used to separate Holocene geomorphic surfaces (Set 1) from late‐Pleistocene ones (Set 2). The oldest‐Pleistocene surfaces (Set 3) studied are believed to be Illinoian in age. They are above the late‐Pleistocene age valley floor and are more dissected than the other Pleistocene surfaces. Entisols have formed on the Holocene surfaces (Set 1). These Entisols have some accumulation of organic C and movement of carbonate, but none have calcic horizons. Their sand grains lack clay cutans or other evidence of soil formation. Most of the soils on late‐Pleistocene geomorphic surfaces (Set 2) have argillic and calcic horizons, and some have mollic epipedons. Grain argillans on sands are the most common form of illuvial clay and there are calcans in the calcic horizons. Some of the soils have natric horizons. The soils on mid‐Pleistocene geomorphic surfaces (Set 3) have a greater clay accumulation than any of the other soils and have calcic horizons. Clay accumulation is largely masked by the carbonate accumulation. The distribution of salt for the most part is in balance with today's arid climate in the valley. The occurrence of carbonate in horizons with illuvial clay, especially in the soils on the oldest surfaces, suggests an arid climate following one or more Pleistocene pluvials. Some of the salt and carbonate may have been added as dust from playas on the valley floor.

A computer data set of soil morphological data for most of the official U.S. soil series classified as Spodosols in December 1983 was developed to determine the characteristic morphology of Spodosol suborders. Included were 246 Orthods, 90 Aquods, and 35 Humods. Ferrods do not occur in the USA. The data for each suborder were divided into “cold” and “warm” subsets based on their temperature regimes and each of these subsets was divided into subsets with and without Ap horizons. Spodic horizons of Orthods were dominated by silt loams and fine sandy loams, while those of Aquods and Humods were sandy, reflecting parent material differences. Structure was weak or absent in all suborders but was most frequently present in Orthods. The Bh horizons were quite uniformly dark reddish brown (5YR hues). The Bs horizons were mostly brown to dark brown and dark yellowish brown (10YR and 7.5YR hues). Generally, Bh horizons had lower chromas than underlying Bs horizons. E and Bh horizons in “warm” Spodosols were thicker than those in “cold” Spodosols and E horizons in “warm” Humods were much thicker than those in “warm” Aquods. In “cold” plowed Spodosols the E and Bh horizons were deeper than in “cold” unplowed Spodosols.

AbstractWe studied 20 pedons and four composite A horizon samples in the Tuscarora Mountain Area in northern Nevada in an effort to differentiate between two morphologically similar Xerolls (Donna and Stampede series) that support strongly contrasting kinds of sagebrush (Artemisia sp.) plant communities. We found no differences between surface horizons by laboratory analyses, but there are significant differences between the soils in clay content of AB, BA, and Bt horizons and in most related properties such as linear extensibility, cation exchange capacity, and 1.5‐MPa water. Thickness of the A horizon has the highest utility as a mapping criterion for the Donna and Stampede soils. Water retention difference between 0.03 and 1.5 MPa and depth of rooting best explain the differences in kind and amount of vegetation found on the two soils.

AbstractX‐ray diffraction (XRD) studies of about 2000 soil clays show that 1500 kPa water retention, in addition to Atterberg limits, cation exchange capacity, 33 kPa water retention, volume of water retained between 33 and 1500 kPa, and linear extensibility varies with kind of soil clay and the intensity of the XRD peak of the dominant clay mineral. Hence, knowledge of soil clay mineralogy is particularly useful for predicting the behavior of soils. In standard characterization work, however, determining the property of interest is usually more efficient than determining mineralogy and then estimating the property. Determining both is not necessary if their relationship is understood. In predicting the behavior of soils, identification of the dominant clay mineral is sufficient if that mineral is smectite. If the mineral is kaolinite or clay mica, somewhat greater accuracy of prediction of properties is possible if measurements of the relative intensities of the other clay minerals in the suite are also known.

AbstractVariability in data from morphologically matched pairs of pedons was assessed to establish operational norms in sampling and to consider alternative sampling procedures.Coefficients of Variation (C.V.'s) were computed for the physical properties—percent sand, silt, clay, and 1,500 kPa moisture—and range from 9 to 40% for loess, 23 to 35% for glacial drift, 33 to 47% for alluvium and residuum, 18 to 32% for A and B horizons, and 33 to 51% for C horizons. C.V.'s were computed for the chemical properties—extractable acidity, sum of bases, cation exchange capacity (CEC), base saturation, pH, and organic carbon—and range from 12 to 50% for Alfisols, 4 to 71% for Aridisols, 6 to 61% for Entisols, 10 to 63% for Inceptisols, 9 to 46% for Mollisols, 16 to 132% for Spodosols, 10 to 100% for Ultisols, and 8 to 46% for Vertisols. Cation exchange C.V.'s are directly related to pH dependent charge.On the average, clay content for central concepts of phases of series can be estimated within ± 5% clay (95% confidence level) by 1 to 4 samples in loess soils, 2 to 8 samples in glacial drift soils, and 4 to 24 samples in alluvial and residual soils.Similar estimates of base saturation (sum of cations) within ± 10% (95% confidence level) require 2 samples for Aridisols and Vertisols; 5 for Mollisols; 10 for Alfisols, Ultisols, Entisols, and Inceptisols; and 16 for Spodosols.Vertical distribution of properties and properties of important horizons are efficiently evaluated by sampling one complete pedon plus satellite samples of important horizons from other pedons. To assess a single horizons efficiently, sample only that horizon in several pedons. Sampling of paired pedons is a good first approach technique to study soils in an area.

AbstractA recent field trip for a group of soil scientists from the western United States permitted comparison of professional soil scientists' description of dry and moist soil consistence and comparison of the descriptions of consistence with unconfined compressive strengths of dry soil.Field party members agreed fairly well with each other in the description of the consistence of moist soils (60% agreement) and dry soils (70% agreement). There was a logarithmic relation between their description of dry consistence and unconfined compressive strengths. The unconfined compression test thus seems to provide a suitable soil consistence standard. The range in unconfined compressive strengths within classes increases from the slightly hard to the very hard classes.

AbstractAlthough the clay in some moderately fine and fine‐textured Bt horizons of soils of arid and mediterranean climates of the southwestern United States is highly oriented, no distinct illuvial clay skins can be recognized. The distribution of clay skins is related to shrink‐swell potentials. Clay skins are absent in horizons having a shrink‐swell potential of more than 4% or a masepic or omnisepic plasmic fabric; they are present in equivalent horizons having low shrink‐swell potentials and an insepic or mosepic plasmic fabric. The clay content, mineralogy, and moisture regime of a Bt horizon in turn largely determine its potential to shrink and swell and hence determine its plasmic fabric.Evidence that clay illuviation has indeed taken place in these finer textured Bt horizons is based on four pairs of geographically associated soils with horizons of clay accumulation. Bt horizons of the coarser textured members of pairs have clay skins and the finer textured members do not. The distribution of biotite pseudomorphs in some of these pairs parallels the distribution of clay skins, suggesting that oriented bodies of clay can be destroyed. Clay orientation in one of the horizons was reformed experimentally to show that the highly oriented soil fabrics do not acquire their orientation by illuviation of clay.The studies further indicate that bodies of oriented clay in medium and fine‐textured B horizons have been erroneously described as clay skins.