Soil Taxonomic Systems Research Articles

Redefining the fragipan to improve field recognition and land use relevance

AbstractThe fragipan subsoil horizon can be a significant limitation to land use and management because of its ability to perch water, restrict root growth, and reduce available water storage. Recognition of the presence of a fragipan is used to describe some diagnostic horizons (mollic, umbric, cambic) and to identify the limits of the particle size control section when classifying a soil according to the U.S. soil taxonomic system. While soil scientists often disagree on whether fragipan diagnostic criteria are met, they will typically agree that the horizon has fragic soil properties. To potentially improve field identification agreement, we propose that the fragipan be redefined to meet criteria more like fragic soil properties. We also propose to include some current criteria for the fragipan denoting pedogenesis, slaking, and HCl reaction. In addition, we propose a new requirement be added for redoximorphic features above the fragipan as additional evidence of its restrictive nature. Changing the fragipan's definition will do little to alter soil land use interpretations given most interpretations are based indirectly off the effects of the fragipan. Analysis of existing series indicates that our proposal should not remove series now classified with a fragipan but would eliminate some ambiguities in field identification. Whether the fragipan's spatial extent would increase or decrease is dependent on regional mapping efforts.

Spodosols in Brazil: distribution, characteristics and diagnostic attributes of spodic horizons

Spodosols are reported in specific environments of Brazil, such as sandy materials in the Amazon and Pantanal basin, sandbanks in the coastal fringes and coastal tableland areas, and a few in high-altitude mountains with quartz-rich rocky substrates. This study describes variations in characteristics and attributes of spodic horizons from soils classified as Spodosols in Brazilian pedoenvironments. Data were obtained from the literature, comprising landscape and location, morphological characteristics, physical and chemical attributes of Spodosol profiles, classified by the Brazilian Soil Classification System (SiBCS). The data grouped according to the pedoenvironments demonstrated the predominance of the sand fraction in the spodic horizons. The finer texture influences the organic carbon and sum of bases accumulation. Soils from the Pantanal basin with high pH values and low organic carbon content differ from the central concept of Spodosols and the podzolisation process. The aluminium and iron extractable by selective dissolution indicate higher participation of aluminium in detriment to iron. The establishment of quantitative values in the SiBCS for attributes associated with the podzolisation process is important to correlate with international soil taxonomic systems and global databases.

Measuring functional pedodiversity using spectroscopic information

Pedodiversity studies greatly depend on the discretization of the soil continuum via various soil classification standards. Soil taxonomic systems are constructed considering soil properties and their organization in the soil profile. Therefore, it seems logical that the pedodiversity of an area could be calculated by considering the simultaneous variation of multiple soil properties within that area. With the aim of creating a bridge between these two lines of thinking, this study presents the development of two new indices of pedodiversity (HULLdiv and EIGENdiv) which consider continuous variables as input. The soil input information tested was a) Visible and Near Infrared (Vis-NIR) reflectance values and b) values of five soil properties predicted from Vis-NIR spectra. Both indices were employed to measure the pedodiversity at different extents in two perpendicular transects containing 27 (North to South) and 22 (East to West) locations respectively in New South Wales (NSW), Australia. Each location considered natural and intervened land use. The indices successfully represented the pedodiversity of the area of study, however they were different depending on the input soil information i.e., raw Vis-NIR and predicted properties. HULLdiv was affected by extreme soil observations and as a result its discrimination power between areas with different soil diversity was inferior. On the other hand, EIGENdiv represented well the pedodiversity of an area, despite the extreme observations. The results showed good agreement with conventional methods for assessing pedodiversity i.e., Shannon's and Simpson's indices. However, the new indices were more discriminating by being able to better represent the landuse effect. This study represents the first attempt to measure pedodiversity in a continuous way using Vis-NIR information.

Islands of biogeodiversity in arid lands on a polygons map study: Detecting scale invariance patterns from natural resources maps

Many maps (geology, hydrology, soil, vegetation, etc.) are created to inventory natural resources. Each of these resources is mapped using a unique set of criteria, including scales and taxonomies. Past research indicates that comparing results of related maps (e.g., soil and geology maps) may aid in identifying mapping deficiencies. Therefore, this study was undertaken in Almeria Province, Spain to (i) compare the underlying map structures of soil and vegetation maps and (ii) investigate if a vegetation map can provide useful soil information that was not shown on a soil map. Soil and vegetation maps were imported into ArcGIS 10.1 for spatial analysis, and results then exported to Microsoft Excel worksheets for statistical analyses to evaluate fits to linear and power law regression models. Vegetative units were grouped according to the driving forces that determined their presence or absence: (i) climatophilous (ii) lithologic-climate; and (iii) edaphophylous. The rank abundance plots for both the soil and vegetation maps conformed to Willis or Hollow Curves, meaning the underlying structures of both maps were the same. Edaphophylous map units, which represent 58.5% of the vegetation units in the study area, did not show a good correlation with the soil map. Further investigation revealed that 87% of the edaphohygrophilous units were found in ramblas, ephemeral riverbeds that are not typically classified and mapped as soils in modern systems, even though they meet the definition of soil given by the most commonly used and most modern soil taxonomic systems. Furthermore, these edaphophylous map units tend to be islands of biodiversity that are threatened by anthropogenic activity in the region. Therefore, this study revealed areas that need to be revisited and studied pedologically. The vegetation mapped in these areas and the soils that support it are key components of the earth's critical zone that must be studied, understood, and preserved.

Lower Cretaceous paleosols and paleoclimate in Sichuan Basin, China

Abundant Lower Cretaceous (Berriasian–Hauterivian) paleosols have been recognized in the Sichuan Basin, along with the preserved pedogenetic features, e.g., soil horizons, soil structure, root traces and pedogenic nodules. Chemical, geochemical and mineralogical analyses were used to examine the paleosols. These paleosols were classified as Entisols, Inceptisols, Aridisols and Alfisols in terms of the modern soil taxonomic system. Early Cretaceous paleoprecipitation and paleotemperature in the Sichuan Basin were estimated from the degree of chemical weathering for non-calcareous paleosols, and from the depth to the calcic horizon and stable oxygen isotopic composition of pedogenic carbonates in calcareous paleosols, respectively. A temperate semi-arid climate generally prevailed in the Sichuan Basin as a part of the South China Block (SCB) and was controlled by subtropical high-pressure and a rain-shadow effect because the humid air masses from the Paleo-Pacific were impeded by the highlands of the South China Block. Further, several intervals of sub-humid paleoclimate occurred due to strengthened monsoonal circulation in the Early Cretaceous. Using the paleosol barometer, the paleoatmospheric CO2 levels of the Early Cretaceous are estimated to range from ∼120 to ∼520 ppmv, with a mean of 305 ppmv. Regional temperature is generally coupled with atmospheric CO2 concentration and is roughly consistent with the sea level fluctuation.

Evaluation of geochemical associations as a screening tool for identifying anthropogenic trace metal contamination

Geochemical association plots are used as a screening tool for environmental site assessments and use empirical log-log relationships between total trace metal concentrations and concentrations of a major (i.e., reference) soil metal constituent, such as iron (Fe), to discern sites with naturally elevated trace metal levels from sites with anthropogenic contamination. Log-log relationships have been consistently observed between trace metal and reference metal concentrations and are often considered constant. Consequently, we used a regional geochemistry data set to evaluate background trace metal/Fe log-log associations across soils with highly diverse composition. Our results indicate that, although geochemical associations may be proportional, they significantly differ across predominant United States Department of Agriculture (USDA) soil orders. This suggests that highly complex interactions between soil-forming factors and variable secondary clay mineral composition affect the ratio of trace metals to Fe concentrations in soils. Also, intra-order variability in trace metal/Fe ratios generally ranged multiple orders of magnitude which suggest that the order level of the USDA soil taxonomic system is insufficient to reasonably classify background trace metal concentrations. Consequently, geochemical association plots are a useful screening tool for environmental site assessments, but ubiquitous application of generic background metal data sets could result in erroneous conclusions. Because significantly different ratios were observed across predominant USDA soil orders, an agglomerative clustering technique was used to elucidate hierarchical patterns of association. We present these results as a mechanism to aid environmental assessors in screening candidate background metal data sets for their applicability to site-specific soil composition; although site-specific background metal data should be utilized if ample pristine reference sites with similar (i.e., sub-order) soil composition can be identified and sampled.

A comparison of three mathematical models of response to applied nitrogen using lettuce

Modern fertilization recommendation must optimize crop yield and quality and minimize chances of negative environmental effects due to over fertilization. Data from fertilizer studies can be fitted to several mathematical models to help determine optimum fertilizer rates, but resulting recommendations can vary depending on the model chosen. In this study, lettuce (Lactuca sativa L.) was used as a case study vegetable crop to compare models for estimating fertilizer N requirements. Field studies were conducted to measure yield response to applied N. The area was located between 25 ,21 E longitude and 51 ,38´ N latitude in the North of ´ Varamin city, (Tehran province, Iran) in the alluvial plain of Varamin. Soils family were fine, mixed, active, thermic, typic haplocambids based on Soil Taxonomic system. Plants were grown in Central Research Station of Varamin and received five rates of N (0.0, 150, 200, 250 and 300 kg ha ) as a urea in split applications. Data 1 for plant fresh mass and N uptake were recorded. The logistic model described the data for cultivar quite well, with correlation coefficients of 0.90 and above. Logistic, linear-plateau and quadratic models were compared for the field data. Coefficients for the linear-plateau model were derived from the logistic model. All three models for lettuce production were compared graphically and analytically. The model coefficients were then used to make improved estimates of fertilizer recommendations for field production of lettuce.

Problems arising from fixed-depth assessment of deeply weathered sandy soils

Practicality dictates a limit to the depth of routine soil examination, but for some deeply weathered soils the prescribed depth is not sufficient to establish a full genetic context. Failure to account for all related soil features on a landscape by ignoring features below a fixed depth (e.g., 2 m) can result in (i) inconsistent genetic interpretations and (ii) an inaccurate and disjointed depiction of soils on the landscape. The upper boundary of some B horizons in sandy soils undulates above and below 2-m depth on the same landform. Considering only the upper 2 m of such soils yields a truncated, “tip of the iceberg” impression. Some Quartzipsamments on older surfaces have horizons below 2 m that are contiguous with B horizons of spatially associated Ultisols or Spodosols. They hence are well-developed soils rather than undeveloped “A–C” soils, with an illuvial layer below the depth of consideration. An E horizon of an Ultisol or Spodosol does not change to a C horizon as the illuvial horizon dips below an arbitrary depth (2 m) and as the Order changes to Entisol. Quartzipsamments, however, are often described in such settings as A–C soils. Soil taxonomic systems can be validly applied with fixed depth maxima as long as full-context genetic interpretations are made prior to and independently of the classification. It is not necessary to bore routinely below conventional limits, since deep horizons can generally be inferred from (i) periodically encountering them at shallower depths and (ii) observing them regionally in deep excavations.

Revisiting the definitions of gypsic and petrogypsic horizons in Soil Taxonomy and World Reference Base for Soil Resources

Gypsic and petrogypsic horizons occur in large areas of arid and semiarid regions of the world. The occurrence of gypsum in soils is considered a key feature by most soil classification and mapping systems that have coined specific names for these soils and horizons. However, the current methodology for description and definition of gypsic and petrogypsic horizons in the two most popular soil taxonomic systems “Soil Taxonomy” and “World Reference Base for Soil Resources” (WRB) does not address sufficiently the advances in knowledge of the constitution, genesis, and behaviour of gypseous horizons. Some of their basic statements, like the presence of secondary gypsum or the degree of cementation, are often ambiguous in the field. Further, the definitions of gypsic and petrogypsic horizons should not be interlocked, each definition should be based on field characteristics linked with microscopic and hydric properties that control the durability and the life-supporting capability of gypseous soils. The avoiding of confusion between gypsum-rich and calcium carbonate-rich horizons when grouping soil taxa or diagnostic horizons by means of soil-forming processes is stressed.

The role of soil-forming processes in the definition of taxa in Soil Taxonomy and the World Soil Reference Base

Modern soil taxonomic systems, including Soil Taxonomy (ST) and the World Reference Base (WRB) for Soil Resources, classify soils using diagnostic horizons, properties, and materials. Although these systems are based on genetic principles, the approaches used have de-emphasized the role of soil processes in soil taxonomic systems. Meanwhile, a consideration of soil processes is important for understanding the genetic underpinnings of modern soil taxonomic systems and developing quantitative models of pedogenic systems. Seventeen generalized soil-forming processes are identified, briefly discussed, and linked to soil taxa and diagnostic horizons, properties, and materials in ST and the WRB. The processes are illustrated in simple diagrams and include: (1) argilluviation, (2) biological enrichment of base cations, (3) andisolization, (4) paludization, (5) gleization, (6) melanization, (7) ferrallitization, (8) podzolization, (9) base cation leaching, (10) vertization, (11) cryoturbation, (12) salinization, (13) calcification, (14), solonization, (15) solodization, (16) silicification, and (17) anthrosolization. The implications of soil-forming processes on present and future soil classification systems and pedogenic models are discussed.

Subaqueous Soils Pedogenesis in a Submersed Environment

Proposals for the inclusion of permanently submersed materials in soil taxonomic systems have periodically been put forth since some time in the mid 1800s. The proposals were largely conceptual in nature, relying more on subjective reasoning rather than analytical and field data. Advances in computer and global positioning technology, and the continuing development of the discipline of pedology provided the opportunity to examine shallow water estuarine sediments within a pedological framework. Morphological and analytical data from 85 1.5‐ to 2.0‐m profiles from Sinepuxent Bay, Maryland, indicate that the four pedogenic processes of additions, losses, transformations, and transfers outlined in the generalized theory of soil genesis are active in a subaqueous environment. The evidence of pedogenic processes includes the addition of biogenic calcium carbonate (shells), organic fragments, and organic matter; the loss of organic matter and surficial material; the transfer of oxygen through diffusion and bioturbation processes; and the transformation of humic substances and sulfur (sulfidization). The change in the concept of estuarine substrates from sediment to soil has significant ramifications for pedologists, ecologists, estuarine researchers, and government agencies involved in soil resource inventory and estuarine restoration programs. Application of these concepts could help further our understanding of the relationships between subaqueous soil distribution and submersed aquatic vegetation (SAV); clam, scallop, and oyster habitat; and dredge sites with potential acid‐sulfate weathering. This work resulted in a change to the definition of soil in Soil Taxonomy to include subaqueous soils that are capable of or presently support rooted SAV.

Properties of Some Soils of the Upper Amazon Basin of Peru

AbstractSeveral representative soil profiles of the Peruvian Amazon jungle were collected in the vicinity of Yurimaguas and Iquitos and characterized in the laboratory in order to classify them according to the US soil taxonomic system. The most extensive well‐drained soils are Typic Paleudults; the poorly‐drained ones are Typic Tropaqualfs or Tropaquepts. The Paleudults were predominantly of kaolinitic mineralogy with small quantities of 2:1 minerals present. The poorly‐drained soils were predominantly montmorillonitic or of mixed mineralogy. Where coarsetextured parent materials occur in localized areas. Tropaquods and Haplorthods are found. High base status Haplaquolls are found along the margins of the Amazon and Napo Rivers. The base status of the Paleudults is not as low as in the main soils of the South American savannas. The base status of the more poorly‐drained soils is higher. Evidence from surrounding countries suggests that these soils are very extensive in the Upper Amazon Basin of South America, outside the influence of the Brazilian and Guyana shields. These areas have been previously mapped as Oxisols or their equivalents.

Use of Physical Methods to Expand Soil Survey Interpretations of Soil Drainage Conditions

AbstractClassification systems of soil drainage conditions are used in the soil survey program to characterize hydrodynamics of soils in the field and these data are used for soil survey interpretations. Estimates of soil permeability and internal soil drainage, based mainly on inferences made from morphological features, and an estimate of runoff are used in the Soil Survey Manual to determine the placement of a soil in one of seven drainage classes. Different soil moisture regimes are defined in the new soil taxonomic system in terms of measurements made in situ. These methods do not provide the quantitative data needed for predicting what soil behavior will be under environmental conditions that have changed temporarily or permanently. Complex numerical procedures and simple approximate physical methods, both requiring hydraulic conductivity and moisture retention data, can be used to calculate hydrodynamic soil behavior for a variety of simplified boundary conditions. Two approximate methods were applied to four Wisconsin soils in which measurements of hydraulic conductivity had been made in situ. This work revealed that two soils that had been placed previously in different drainage classes on the basis of differences in soil mottling, actually had comparable hydrologic properties.