Larry P. Wilding

Overview of vertisols: characteristics and impacts on society.

Publisher Summary Vertisols are clayey soils that shrink and swell extensively upon changing soil moisture conditions. They occur globally under various parent material and environmental conditions. Vertisols exhibit unique morphological properties such as the presence of slickensides, wedge-shaped aggregates, diapir (mukkara), and gilgai. Shrink-swell phenomena are the dominant pedogenic processes in vertisols and are attributed to changes in interparticle and intraparticle porosity with changes in moisture content. This is in contrast to the commonly invoked process of clay interlayer hydration-dehydration to explain shrink-swell phenomena. However, models proposed to explain the genesis of vertisol features have not received universal agreement. Because of their clay content, vertisols are global resources that are resilient to degradation compared to other soils. Degradation of vertisols has occurred and has been reported worldwide regardless of the parent material, environmental conditions, and level of cultural input. Vertisols are significant global resources that serve as the lifeline in subsistence agriculture because of their high productivity. Efforts toward comprehension and successful utilization are imperative for continued productivity and long-term sustainability of these resources for current and future civilizations. This chapter is based on the literature published about vertisols and includes recent developments and concepts concerning vertisols with regard to their distribution, formation, pedogenesis, and classification; their morphological, mineralogical, chemical, biological, and physical properties; and their management as a soil resource in the world.

Soil and topographic controls on runoff generation from stepped landforms in the Edwards Plateau of Central Texas

[1]xa0The rugged Hill Country of Central Texas is part of the extensive Edwards Plateau region. Significant portions of the Texas Hill Country overlie the Glen Rose Formation, which is characterized by a stair-step topography formed by the weathering of interbedded carbonate materials having different weathering susceptibilities. This process has sculpted the strata into a series of “risers” and “treads” that mimic stairways. In this paper, we document the soil hydrology within the riser–tread catena. Our results are counterintuitive in that we find the highest infiltration and deepest soils on the steep riser slopes. In addition, we find that the riser subsoils are saturated or very wet for extended periods. On the basis of these findings, we suggest that (1) groundwater recharge on these hillslopes is minimal and occurs only in highly fractured zones; (2) the water-holding capacity of the subsoils is sufficient for supporting the woody vegetation; and (3) runoff generation occurs as a combination of surface and subsurface flow, with the risers serving as sinks or recharge zones and the treads as source areas.

Detection of edaphic discontinuities with ground-penetrating radar and electromagnetic induction

Quantification of edaphic properties which may regulate the spatial distribution of vegetation is often limited by the expense and labor associated with collecting and analyzing soil samples. Here we evaluate the utility of two technologies, ground-penetrating radar (GPR) and electromagnetic induction (EMI), for rapid, extensive and non-destructive mapping of diagnostic subsurface features and soil series map unit boundaries.Strong reflectance from fine-textured, near-surface soils obscured radar signal reflectance from deeper horizons at our field test site in the Rio Grande Plains of southern Texas, USA. As a result, ground-penetrating radar did not delineate known edaphic contrasts along catena gradients. In contrast, EMI consistently distinguished boundaries of soil map units. In several instances, gradients or contrasting inclusions within map units were also identified. In addition, the location and boundary of calcic or cambic-horizon inclusions embedded within a laterally co-extensive and well-developed argillic horizon were consistently predicted. Correlations between EMI assessments of apparent conductivity (ECa) and soil properties such as CEC, pH, particle size distribution and extractable bases were low (i.e., explained<6% of the variance), or non-significant. As a result, EMI has a high prospecting utility, but cannot necessarily be used to explain the basis for edaphic contrasts.Results suggest EMI can be a cost-effective tool for soil survey and exploration applications in plant ecology. As such, it is potentially useful for rapidly locating and mapping subsurface discontinuities, thereby reducing the number of ground truth soil samples needed for accurate mapping of soil map unit boundaries. An application, addressing hypotheses proposed to explain the role of edaphic heterogeneity in regulating woody plant distribution in a savanna parkland landscape, is presented.

Chapter 5 Mineralogy and chemistry of vertisols

Summary Mineralogy and chemistry are important aspects of Vertisols. Their great diversity in terms of mineralogical and chemical properties makes it difficult to develop, adapt and transfer technology from one region to another. Factors of soil formation: (1) parent material, (2) climate, (3) topography, (4) vegetation and (5) time are the primary aspects and the basis for utilization and management of Vertisols, and vertic intergrades. Vertisols may develop from a variety of parent materials. The latter, associated with environmental conditions provide generally a high fine-clay content (high external surface area) and often a high base status. Vertisols also occur under a wide range of climatic conditions. Climate is important for weathering processes and governs the duration and intensity of dry-wet cycles necessary for the shrink-swell behavior. Topography and vegetation do not influence regional distribution but have an important impact on soil moisture regimes and hydrology, i.e. water distribution and availability, leaching potential, soil depth, mineralogy, etc. The influence of time is dependent on factors cited above, particularly the type of parent material and climatic conditions. Under certain conditions a few hundred years are sufficient to develop a Vertisol. The five soil forming factors are complex and interdependent. Therefore, the comprehension of formation and genesis of Vertisols requires that the prediction of their behavior based on their properties by soil scientists and others be site specific since more than one pathway may lead to Vertisol formation. A large spectrum of minerals originating from inheritance, transformation or neoformation occur in Vertisols. Kaolinite, illite, smectite and hydroxy-interlayered smectite (HIS) are the phases reported as abundant in Vertisols throughout the world. Clay minerals must be small in size and have high surface area. Vertisol smectites, in particular, may be iron-rich, have a high layer charge and be thermodynamically more stable than smectites from geological origins. Many minerals other than phyllosilicates occur in Vertisols; their presence strongly depends on the origin of the Vertisol and the past and present environmental conditions. These minerals influence Vertisols physical and chemical properties. Vertisols may be either acid, neutral or alkaline in reaction; this impacts use and management interpretations. Cationic exchange capacity generally ranges between 20 and 45 cmol kg -1 (soil) and is attributed to organic carbon, clay content and the type of minerals present. Exchangeable cations reflect pH conditions. Aluminum, magnesium and exchangeable acidity when acid, calcium and magnesium in proportion on the exchange sites. Cation and anion behavior are of agronomic and environmental interest. Potassium and ammonium are subject to strong retention in the presence of micas, vermiculite, high-charge smectites and phyllosilicates interstratified with these components. Phosphorus is generally limiting due to its low content in parent material of most Vertisols and its high propensity to sorption on mineral surfaces. Nitrate and sulfate are mobile and may pollute groundwater. Organic matter content ranges between 5 and lOOgkg -1 depending on many factors. The amount and type of organic matter are involved in clay organic complexes from molecular to macroscopic levels. Inappropriate management practices such as continuous cultivation, enhanced salinity, etc., have a negative impact on aggregate stability and lead to progressive structural degradation of Vertisols. Shrink-swell phenomena result from the interactions among mineralogical, physical and chemical properties of Vertisols. Major volume changes occur under normal soil conditions due to the modification in microstructure, pore volume and water content. Interparticle and intraparticle porosity of the microstructure are largely responsible for the shrink-swell phenomena in soils. The popular beliefs of expansion/collapse of the interlayer space of clay minerals and diffuse double layer have a slight influence under very specific conditions. The present chapter attempted to discuss an exhaustive review on mineralogical and chemical properties of vertic soils (Vertisols and vertic intergrades) around the world. Some regions of the world, however, have not been described or widely reported. Efforts in that direction should be done in a near future. Also, as mentioned in our discussion, further research related to mineralogical and chemical properties of Vertisols should consider: o particle size classification of phyllosilicates in relation with clay reactivity, shrink-swell behavior and other properties; behavior, e.g. dispersion, sorption, desorption, retention, etc., of cations and anions on different phyllosilicates; pedobiology, location of organic matter and properties of clay organic complexes of Vertisols derived from different parent material, environmental conditions and land utilization; prediction and control of shrink-swell behavior of vertic soils; stress accumulation-relaxation on shrinking-swelling processes; chemical behavior of clay-water systems saturated with more than one cation and in different electrolyte concentrations; resihency, physical and chemical regeneration of degraded Vertisols.

Formation of a cultivated Spodosol in East-Central Finland

Karkeille hietamaille syntyneet podsolit ovat Suomen kehittyneimpia maannoksia. Niita tutkimalla saadaan uutta tietoa taman koko pohjoisella havumetsavyohykkeellayleisen maannostyypin kehittymiseen johtaneista prosesseista, joista edelleenkin vallitsee erilaisia kasityksia. Taman tutkimuksen kohteena oli Sotkamossa karkealla hietamaalla oleva noin 50 vuotta viljelty maa, joka on ollut kuivillaan noin 10 700 vuotta. Muokkauskerroksen alapuolella oli huuhtoutumiskerros (valkomaa), joka sisalsi lahes pelkkaa kvartsihiekkaa. Sen alapuolella oli noin 10 cm paksu rautapalsi eli iskostunut horisontti, johon ylempaa orgaanisina kompleksiyhdisteina huuhtoutuneet rauta ja alumiini ovat saostuneet. Mikroskoopilla voidaan nahda, miten nama saostuneet ainesosat peittavat kvartsihiekan jyvaset ja sitovat ne yhteen. Tassa horisontissa oli erittain runsaasti heikosti kiteytynytta rautaoksidia, joka on uutettavissa ammoniumoksalaattiliuoksella, kun taas valkomaassa tallaista rautaa oli erittain vahan. Rikastumiskerroksen alumiinista valtaosa oli pyrofosfaattiin uuttuvassa, oletettavasti orgaanisen aineksen sitomassa muodossa, mika viittaa aineiden kulkeutuneen tahan horisonttiin nimenomaan kelaatteina eika epaorgaanisina kolloideina. Syva kynto on nostanut valkomaata ja kappaleita rikastumiskerroksen iskostumasta myos muokkauskerrokseen. Rikastumiskerroksen alapuolella kvartsihiekkajyvasten pinnoilla ei ollut paljonkaan rautasaostumia, mutta mikroskoopilla nakyi runsaasti rapautumatonta biotiittia. Rikastumiskerroksen rauta lienee suureksi osaksi peraisin juuri biotiitista, joka on kokonaisuudessaan rapautunut pintamaasta. Vahemman biotiittia sisaltaviin maihin ei todennakoisesti kehity nain vahvaa rikastumiskerrosta maan pienemman rautapitoisuuden takia. Podsoloituminen on luultavasti pysahtynyt sen jalkeen, kun maa on otettu viljelyyn ja sen pintaosien pH on kalkituksen seurauksena noussut.