Chapter 5 Mineralogy and chemistry of vertisols

Abstract

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.