Soil Clays

Linking Geology, Biology, Agriculture, and the Environment

G. Jock Churchman, Bruce Velde

• Editore: CRC Press
• Anno: 2019
• ISBN:
  9780429532245
• Edizione: 1
• Argomento: Scienze biologiche
• Categoria: Conservazione e Tutela dell'Ambiente

As the human population grows from seven billion toward an inevitable nine or 10 billion, the demands on the limited supply of soils will grow and intensify. Soils are essential for the sustenance of almost all plants and animals, including humans, but soils are virtually infinitely variable. Clays are the most reactive and interactive inorganic compounds in soils. Clays in soils often differ from pure clay minerals of geological origin. They provide a template for most of the reactive organic matter in soils. They directly affect plant nutrients, soil temperature and pH, aggregate sizes and strength, porosity and water-holding capacities.

This book aims to help improve predictions of important properties of soils through a modern understanding of their highly reactive clay minerals as they are formed and occur in soils worldwide. It examines how clays occur in soils and the role of soil clays in disparate applications including plant nutrition, soil structure, and water-holding capacity, soil quality, soil shrinkage and swelling, carbon sequestration, pollution control and remediation, medicine, forensic investigation, and deciphering human and environmental histories.

Features:

• Provides information on the conditions that lead to the formation of clay minerals in soils
• Distinguishes soil clays and types of clay minerals
• Describes clay mineral structures and their origins
• Describes occurrences and associations of clays in soil
• Details roles of clays in applications of soils
• Heavily illustrated with photos, diagrams, and electron micrographs
• Includes user-friendly description of a new method of identification

To know soil clays is to enable their use toward achieving improvements in the management of soils for enhancing their performance in one or more of their three main functions of enabling plant growth, regulating water flow to plants, and buffering environmental changes. This book provides an easily-read and extensively-illustrated description of the nature, formation, identification, occurrence and associations, measurement, reactivities, and applications of clays in soils.

1	Introduction and Definitions

The soil is the great connector of lives, the source and destination of all. It is the healer and the resurrector, by which disease passes into health, age into youth, death into life. Without proper care for it we cannot have community, because without proper care for it we can have no life.

Wendell Berry, 1934

1.1 Soil

Soil is basically the material formed at the atmosphere, rock or sediment interface on the Earth, being largely affected by plant chemical action. There is no definition of “soil” that is accepted universally. Some conclude that this must be the case. R.E White, in his textbook Principles and Practice of Soil Science states: “There is little point in giving a rigorous definition of soil because of the complexity of its make-up, and of the physical, chemical and biological forces that act on it. Nor is it necessary to do so, for soil means different things to different users” (2006, p. 4). In the same manner, Hillel (2008, p. 2) states, “A precise definition (of soil) is elusive, for what is commonly called soil is anything but a homogeneous entity. … Perhaps the best we can do at this stage is to define soil as the naturally occurring, fragmented, porous and relatively loose assemblage of mineral particles and organic matter that covers the surfaces of our planet’s terrestrial domains”, although this author then goes on to describe its formation and its fractions. Some have attempted definitions that simply describe its appearance. These include Birkeland (1974, p. 3), to whom soil is “a natural body consisting of layers or horizons of minerals and/or organic constituents of variable thickness, which differ from the parent material in their morphological, physical, chemical and mineralogical properties and their biological characteristics”. This definition, highlighting horizons as the distinguishing compositional feature of soils, is endorsed by Weaver (1989). Furthermore, Chesworth (2008, p. 629) effectively defines soils as the products of the particular processes that have given rise to horizons. Thus:

The gravitational movement of water within soil effects the downward transport of solid particles and dissolved species whereas the solar energy concentrates constituents at the surface, either indirectly, through plants, or directly, with the upward transport of water under evaporative conditions. This produces over a relatively short period (order of 10²–10³ years) a vertical differentiation that appears megascopically as a series of horizons more or less parallel to the land surface.

The World Reference Base for Soil Resources (WRB) has a definition of soil which is very similar to this horizon-based compositional definition (Canarache et al., 2006).

Nonetheless, most of the definitions that have been proposed include statements about both its composition and its functions with some putting more emphasis on its composition, or morphology, whereas others emphasise its functions, or properties. Some also include statements about the origins of soil. In 1975, the US Soil Survey Staff included the following as defining features of soils (Fanning and Fanning, 1989):

Natural bodies on the Earth’s surface

• Contain living matter
• Support or are capable of supporting plants out of doors
• Have air or shallow water as an upper limit
• At their margins, grade to deep water or rock or ice
• Include horizons that differ widely as a result of interactions through time of climate, living components, parent materials and relief, etc.
• Normally have the lower limit of biological activity as their lower limit

To its inclusion of horizons and its ability to support rooted land plants, Schaetzl and Anderson (2005, p. 20–22) add that soils comprise solids, liquids and gases, are “essential to life through recycling of nutrients, carbon and oxygen” and are “nonrenewable in human timescales”. In the 14th edition of their textbook, Brady and Weil (2008) defined soils by their main functions, i.e. as a medium for plant growth, as a regulator of water supplies, as a modifier of the atmosphere and as a habitat for soil organisms. Together with Canarache et al. (2006), these authors also rightly observed that soils are also defined as an engineering medium, but that particular application is beyond our concern here. Churchman (2010a) deduced from the literature that studies of soils are uniquely concerned with horizons, aggregates and distinctive colloidal material. Churchman and Lowe (2012) make note of the character of soils as “the most complex ecosystem on earth” and “a biological habitat and critically important repository for genes”, as well as a provider of “ecosystem services” and as “natural capital”.

Undoubtedly, however, an unequivocal, universally accepted definition of soils remains as a work in progress according to a recent contribution by Certini and Ugolini (2013). These particular authors widen the definition of soils to include the possibility that they occur on other planets. Their newly proposed definition requires no requirement for plants in their formation. In contrast, we believe that something of the essential nature of soils is derived from their processes of formation (see next Section). These almost always involve plants, except in extreme conditions on Earth such as the dry valleys of Antarctica and deserts. With these exceptions, it is consistent with the action of plants on weathering, including physical weathering, that we can say there is no soil without clay (or <2 µm particles).

1.2 The Origin of Soils and Clays in Geological Time

In order to be able to appreciate differences between soil clays and those from other ‘geological’ origins, it helps to realise that soils are relative late-comers in the geological record, and also that weathering occurred and clay minerals were formed before there were any soils on Earth. Recently, the development of new minerals has come to be seen as an evolutionary process. Following an era of planetary accretion prior to 4.5 billion years (4.5 Ga) ago, volcanism occurred, with associated processes, followed by the formation of granites and pegmatites, until the inception of plate tectonics before 3 Ga led to subduction of a range of materials in a water-rich environment (Hazen and Ferry, 2010). New types of minerals appeared at each stage. Weathering occurred and modelling of possible pathways using irreversible thermodynamics shows that the conditions, which were reductive owing to low oxygen levels, could have led to some clay minerals, including kaolinite Al₂Si₂O₅(OH)₄, among just a few other minerals (Sverjensky and Lee, 2010).

However, as Hazen and Ferry (2010, p. 11) describe it, “the situation changed in a geological instant”. They are describing the “Great Oxidation Event” involving the onset of an oxygen-rich atmosphere, which began about 2.4 Ga BP. This led to a profusion of new types of minerals formed as hydrated, oxidised products of previously existing minerals. Most known mineral species were formed following that event (Sverjensky and Lee, 2010). However, the onset of soils only occurred as a result of the advent of vascular land plants in the Silurian period about 440 million years BP. (Knoll and James, 1987). While lichens colonising rocks had been effecting some weathering before this time, it was the emergence of higher (vascular) plants with deep roots that led to the establishment of soils (Verboom and Pate, 2006).

Deep-rooted higher plants brought about an acceleration in the rate of production of soils through concomitant processes of mineral weathering and the deposition of plant products, including exudates and litter, to produce organic matter following processing by microorganisms (Lambers et al., 2009). Roots of these higher plants, and associated microbes, have co-evolved with soils (Verboom and Pate, 2006). There is much evidence available, especially in semi-arid environments in Australia (Verboom and Pate, 2006), to show that higher plants and microorganisms can play a proactive role in “bioengineering” pedogenic processes for their own benefit. Their effect can be both physical and chemical/biochemical. Among physical effects, roots create macropores and fragment primary minerals (e.g., Calvaruso et al., 2009). Among biochemical effects, metal-chelating root exudates and the activity of microorganisms are key bioengineering agents (Verboom and Pate, 2006). Thus, vertical channels and pores, ideal for transporting water to depths, may become lined with Fe or Si, and surface layers may be rendered hydrophobic, which, together with the creation of hardpans and texture contrast, or of pavements from compounds of Al, Si, Ca and Fe and also of carbonates from Ca, enable retention of water at depth for use by roots. The various concentrations of Al, Fe, Si and/or secondary carbonates also enable sequestration of concentrations of phosphorus for later supply of this essential nutrient to plants via various means, including mycorrhizal exchange of carbon for P and other forms of microbial ‘mining’.

Vascular deep-rooted plants also played an important physical part in the sustained development of soils. Their deep roots into the soils they helped to create enabled these soils to withstand erosive forces, at least for considerable periods of time, and hence remain in place for a relatively long time. As well, fungi and root hairs confer stability to soils through their adhesion to soil particles, while various chemicals in root exudates, including phenolics, but especially polysaccharides, enhance processes of aggregation of the particles, as do cycles of wetting and drying (Hinsinger et al., 2009).

1.3 Weathering as the Origin of (Most) Soils

Weathering literally occurs when minerals (‘primary minerals’) in or from rocks are exposed to the weather at Earth–surface conditions. These minerals have developed in rocks by crystallisation out of magma as it cooled, or else have been deposited through recycling, hence sedimentation, with possible metamorphism from increases in temperature and/or pressure. Changes occur because minerals of igneous, metamorphic and even sedimentary origin are out of thermodynamic equilibrium with the environment in which they now reside. As expressed by Kittrick (1967, p. 315), “Fundamentally a mineral is a package for its elements. It will persist in nature only as long as it is the most stable package for those elements in its environment”. For minerals of igneous origin, Figure 1.1). Goldich’s series is the exact inverse of a classic series devised by Bowen in 1922 to denote the relative order in which minerals crystallised out of magma on cooling (Bowen, 1922). For example, since mineral A which crystallised from magma at a higher temperature than mineral B was thereby more out of equilibrium with conditions at the Earth’s surface than mineral B, mineral A would therefore be more vulnerable to breakdown by weathering than mineral B. Essentially, it is ‘last in (to igneous rocks), first out (to breakdown on weathering)’.

FIGURE 1.1 Stability series for the common primary minerals (after Goldich, 1938) and volcanic glass (not part of the Goldich series). Basaltic and other glasses, and olivines are normally the first phases altered by weathering (Wolff-Boenisch et al., 2004). (From Churchman, G.J., and D.J. Lowe, 2012, Alteration formation and occurrence of minerals in soils, p. 20.1–20.72, in P.M. Huang, Y. Li, and M.E. Sumner (eds.), Handbook of Soil Sciences: Properties and Processes, 2nd edn. CRC Press, Boca Raton.)

Given the aforementioned considerations we will adopt the view that soils, and the clay materials found in them, have an intimate relationship wit...

Indice dei contenuti

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Preface
• Authors
• Chapter 1 Introduction and Definitions
• Chapter 2 Soil Clays
• Chapter 3 Geology
• Chapter 4 Primary Minerals and Their Alteration by Weathering
• Chapter 5 Driving Forces of Alteration
• Chapter 6 Chemistry of Alteration by Weathering
• Chapter 7 Formation of Clays in the Soil Zone of Alteration
• Chapter 8 Nature and Origin of Surface Soil Clays
• Plates
• Chapter 9 The Importance of Climate in the Formation of Soil Clays
• Chapter 10 Associations of Soil Clays
• Chapter 11 Occurrence and Extraction of Soil Clays
• Chapter 12 Identification and Quantification of Clay Minerals in Soils
• Chapter 13 Surfaces, Surface Reactions and Particle Size Effects
• Chapter 14 Role of Soil Clays in Agriculture, the Environment and Society
• Chapter 15 Summary
• Annex: Simplified Methods for the Interpretation of X-Ray Diffraction Diagrams of Soil Clay Assemblages
• Index