Essentials of Soil Science: Soil Formation, Functions, use and Classification (World Reference Base, WRB) 3443010903, 9783443010904

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Essentials of Soil Science Soil formation, functions, use and classification (World Reference Base, WRB)

This concise, yet comprehensive text is ideal reading for all those looking to understand soils, their functions, their importance in terrestrial and aquatic environments and their contribution to the development of human society. It will provide a valuable resource for teachers, practitioners and students of soil science, agriculture, farming, forestry, gardening, terrestrial and aquatic ecology and environmental engineering.

ISBN 978-3-443-01090-4 www.borntraeger-cramer.com

9

783443

010904

Winfried E. H. Blum • Peter Schad • Stephen Nortcliff

Essentials of Soil Science Soil formation, functions, use and classification (World Reference Base, WRB)

Essentials of Soil Science

This book is an introduction to soil science and describes the development of soils, their characteristics and their material composition as well as their functions in terrestrial and aquatic environments. Soil functions include the delivery of goods and services for human society, such as food, clean water, and the maintenance of biodiversity. The book is illustrated with many coloured figures and tables to accompany the text and ease its understanding. Particularly, the chapter on soil classification, based on the World Reference Base for Soil Resources (WRB), includes numerous coloured pictures to facilitate understanding the characteristics of particular soil types. Chapters on soil protection and remediation as well as on soil monitoring and the history of soil sciences conclude the book together with a comprehensive alphabetical index, allowing for a quick and easy orientation about the most important terms in soil sciences.

Blum • Schad • Nortcliff

W. E. H. Blum • P. Schad • S. Nortcliff

B

BBorntraeger Science Publishers

W.E.H. Blum/P. Schad/S. Nortcliff

Essentials of Soil Science Soil formation, functions, use and classification (World Reference Base, WRB)

Essentials of Soil Science Soil formation, functions, use and classification (World reference Base, WRB) Winfried E. H. Blum Peter Schad Stephen Nortcliff

With 101 figures and 22 tables

B

Borntraeger Science Publishers, Stuttgart 2018

Winfried E. H. Blum, Peter Schad, Stephen Nortcliff: Essentials of Soil Science. Soil formation, functions, use and classification (World Reference Base, WRB)

Author’s addresses: em.Prof.Dipl.Ing.Dr.Dr.h.c.mult. Winfried E.H. Blum, University of Natural Resources and Life Sciences (BOKU), Vienna, Peter-Jordan-Str. 82, 1190 Vienna, Austria. [email protected] Dr. Peter Schad, Technical University of Munich, TUM School of Life Sciences Weihenstephan, Chair of Soil Science, Emil-Ramann-Str. 2, 85354 Freising, Germany. [email protected] Professor Stephen Nortcliff, Emeritus Professor of Soil Science, Soil Research Centre, Department of Geography and Environmental Sciences, School of Archaeology, Geography and Environmental Science, Whiteknights, PO Box 233, University of Reading, Reading, RG6 6DW, United Kingdom. s.nortcliff@reading.ac.uk

We would be pleased to receive your comments on the content of this book: [email protected] Front cover: Stagnosol from loess, Swabian Alb, Germany (Photo: P. Schad)

 

,6%1HERRNSGI --44301129 ISBN 978-3-443-01090-4 Information on this title: www.borntraeger-cramer.de/9783443010904 Classroom sets of 10 and 20 copies are available:

Order No. for 10 copies: 001201611

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© 2018 Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical photocopying, recording, or otherwise, without the prior written permission of Gebr. Borntraeger Science Publishers.

Publisher: Gebr. Borntraeger Verlagsbuchhandlung Johannesstraße 3A, 70176 Stuttgart, Germany [email protected] www.borntraeger-cramer.de P Printed on permanent paper conforming to ISO 9706-1994 Printed in Germany by Tutte Druckerei & Verlagsservice GmbH, 94121 Salzweg

Foreword “Essentials of Soil Science” is an updated English translation of the highly valued German textbook „Bodenkunde in Stichworten” (now in its 7th edition), which was conceived in 1969 as a standard text in soil science for universities, high schools and all kinds of learned institutions related to soil science and its applications, including practitioners in agriculture, forestry, landscape planning and architecture and users of soil in engineering and other areas. The book contains fundamental chapters on soil formation, physical, chemical and biological soil properties and functions, soil classification, soil use, and soil protection, which have been revised and updated in order to create an ideal introduction into soils and their use. The authors are grateful to Mrs. Elfriede Schuhbauer for her valuable help to improve the figures and thank the editor for the excellent outlay and printing of the book. Winfried E.H. Blum, Peter Schad, Stephen Nortcliff February 2017

About the authors: Winfried E.H. Blum Emeritus Professor of Soil Science at the University of Natural Resources and Life Sciences (BOKU) Vienna / Austria. Studies of natural sciences and forest engineering in Germany and France. Professor of soil science at Freiburg University/Germany, State University of Paraná in Curitiba/Brazil and BOKU University in Vienna/Austria. Member of numerous learned national and international organisations worldwide. Author of more than 800 scientific publications including books and monographs in 15 languages with numerous distinctions and awards.

Peter Schad Peter Schad is a lecturer of soil science at the Technical University of Munich (Germany). He holds a Master’s Degree in Biology and a PhD in soil science. He is the lead author of the current edition of the international soil classification system WRB. During his countless excursions worldwide, he has been able to familiarize himself with the soils of all ecozones across the globe.

Stephen Nortcliff Stephen Nortcliff is Emeritus Professor of Soil Science at Reading University, United Kingdom. He joined Reading in 1978, teaching and researching in a wide range of soil science areas. His initial research focus was in soil variability and its characterisation to assist soil survey and soil mapping. He subsequently worked on the sustainable management of soils of the Tropics.Within the United Kingdom he worked on the use of recycled organic residues in the form of compost and anaerobic digestates to reduce the requirements for inorganic fertiliser inputs. He was Secretary General of the International Union of Soil Sciences from 2002 to 2010 and continues to be actively involved in the Union’s management.

Table of Contents Foreword ..................................................................................................... 5 About the authors ......................................................................................... 6 1 Introduction .................................................................................... 13 1.1 Definition of soil .................................................................................. 13 1.2 Soil functions ....................................................................................... 13 1.3 Soil science .......................................................................................... 14 2 Soil components ............................................................................ 16 2.1 Mineral components ............................................................................. 2.1.1 Parent materials ................................................................................. 2.1.1.1 Rocks and technogenic materials ................................................... 2.1.1.2 Minerals ......................................................................................... 2.1.2 Transformation processes of rocks and minerals .............................. 2.1.2.1 Weathering processes ..................................................................... 2.1.2.1.1 Physical weathering processes .................................................... 2.1.2.1.2 Chemical and biological weathering processes .......................... 2.1.2.2 Formation of new minerals (neoformation) ................................... 2.1.2.2.1 Formation of clay minerals from micas ...................................... 2.1.2.2.2 Formation of clay minerals from end products of silicate weathering 2.1.2.2.3 Formation of oxides and hydroxides ........................................... 2.1.3 The products of silicate weathering .................................................. 2.1.3.1 Clay minerals ................................................................................. 2.1.3.1.1 1:1 clay minerals ......................................................................... 2.1.3.1.2 2:1 clay minerals ......................................................................... 2.1.3.1.3 2:1:1 clay minerals ...................................................................... 2.1.3.1.4 Allophanes ................................................................................... 2.1.3.2 Oxides and hydroxides ................................................................... 2.1.3.3 Water-soluble components ............................................................. 2.1.4 The mineral fractions of soils ............................................................

16 16 17 19 21 22 22 22 24 25 25 26 27 27 27 28 31 32 32 34 34

2.2 Organisms ............................................................................................ 36 2.2.1 Soil flora ............................................................................................ 36 2.2.2 Soil fauna .......................................................................................... 39 2.3 Organic components ............................................................................ 2.3.1 Sources of soil organic matter ........................................................... 2.3.2 Transformation processes of soil organic matter .............................. 2.3.2.1 Mineralization of soil organic matter ............................................. 2.3.2.2 Formation of stable soil organic matter ......................................... 2.3.3 Humic substances .............................................................................. 2.3.4 Humus forms ..................................................................................... 2.3.5 Organic matter content of soils .........................................................

40 40 41 41 42 44 45 46

8

2.3.6 Soil organic matter and the global C budget ..................................... 47 2.4 Soil water ............................................................................................. 2.4.1 Water-holding capacity of soils ......................................................... 2.4.1.1 Binding energy ............................................................................... 2.4.1.2 Water capacity ................................................................................ 2.4.1.3 Water tension .................................................................................. 2.4.2 Water movement in soils ................................................................... 2.4.2.1 Flow of liquid water ....................................................................... 2.4.2.2 Movement of water vapour within soils ........................................

48 49 49 49 50 52 52 53

2.5 Soil air .................................................................................................. 53 2.5.1 Composition of soil air ...................................................................... 54 2.5.2 Gas exchange between soil and atmosphere ..................................... 54 3 Soil characteristics ....................................................................... 55 3.1 Physical properties of soils .................................................................. 3.1.1 Particle size and texture .................................................................... 3.1.2 Soil structure ..................................................................................... 3.1.2.1 Soil pore volume ............................................................................ 3.1.2.2 Types of soil structure .................................................................... 3.1.2.3 The development of soil aggregate structure ................................. 3.1.2.3.1 Coagulation and peptization ....................................................... 3.1.2.3.2 Shrinking and swelling of soils ................................................... 3.1.2.3.3 Frost action .................................................................................. 3.1.2.3.4 The activity of soil organisms ..................................................... 3.1.3 Particle density and bulk density ...................................................... 3.1.4 Soil consistence ................................................................................. 3.1.5 Soil temperature ................................................................................ 3.1.6 Soil colour .........................................................................................

55 55 57 58 60 62 62 62 63 63 64 64 64 65

3.2 Physico-chemical properties of soils ................................................... 3.2.1 Ion exchange in soils ......................................................................... 3.2.1.1 Cation exchange ............................................................................. 3.2.1.1.1 The causes of cation exchange .................................................... 3.2.1.1.2 Cation exchange processes ......................................................... 3.2.1.1.3 The properties of the ions ............................................................ 3.2.1.1.4 The properties of the ion exchanger ............................................ 3.2.1.1.5 Ionic composition and ion concentration of the solution ............ 3.2.1.1.6 Combined effects of the various factors ..................................... 3.2.1.1.7 Theory of cation exchange in soils ............................................. 3.2.1.2 Anion exchange .............................................................................. 3.2.2 Soil pH (Soil reaction) ...................................................................... 3.2.2.1 How the soil pH is established ....................................................... 3.2.2.2 The causes and consequences of soil acidity ................................. 3.2.2.2.1 Processes poducting H+ ions in soils .......................................... 3.2.2.2.2 Loss of base cations from the soil ...............................................

66 66 67 68 70 71 71 71 72 73 73 74 76 76 77 78

9

3.2.2.2.3 pH variation in soils with depth .................................................. 3.2.2.3 Soil pH buffering ............................................................................ 3.2.2.4 Why soil pH is important ............................................................... 3.2.3 Redox dynamics in soils ...................................................................

78 78 80 80

3.3 Biological properties of soils ............................................................... 82 4 Soil formation (Pedogenesis) ....................................................... 83 4.1 Factors of soil formation ...................................................................... 4.1.1 Parent material (parent rock) of soils ................................................ 4.1.1.1 Chemical and mineral composition of parent material .................. 4.1.1.2 Structure and particle size of parent material ................................ 4.1.2 Climate .............................................................................................. 4.1.2.1 Temperature effect on soil formation ............................................. 4.1.2.2 Water (as agent of soil formation) .................................................. 4.1.2.3 Overall characterization of the climate .......................................... 4.1.3 Relief and gravity .............................................................................. 4.1.4 Biota (vegetation, animals, microorganisms) ................................... 4.1.5 The human impact on soils ............................................................... 4.1.6 Interaction of the soil-forming factors over time ..............................

83 84 84 85 85 86 86 86 87 88 89 90

4.2 Processes of soil formation .................................................................. 4.2.1 Transformation processes .................................................................. 4.2.2 Translocation processes .................................................................... 4.2.2.1 Salt, gypsum and carbonate transport ............................................ 4.2.2.2 Clay migration ............................................................................... 4.2.2.3 Transport of organic substances ..................................................... 4.2.2.4 Si, Al, Fe and Mn transport ............................................................ 4.2.2.5 Turbation (mixing processes) ......................................................... 4.2.2.6 Surface transport of soil material ................................................... 4.3 The soil profile ..................................................................................... 4.3.1 Soil properties ................................................................................... 4.3.2 Soil horizons ..................................................................................... 4.3.3 Horizon combinations and soil groups ............................................. 4.4 Soil sequences ......................................................................................

91 91 91 91 92 92 92 94 95 95 95 96 99 99

5 Soil Classification ......................................................................... 101 5.1 Soil classification systems .................................................................... 101 5.1.1 5.1.2 5.1.3 5.1.4

Factor systems ................................................................................... Property-based systems ..................................................................... Combined systems ............................................................................ The International Soil Classification System WRB ..........................

101 102 103 103

5.2 The major soils of the world ................................................................. 104 5.2.1 Soils with initial to intermediate soil formation ............................... 105

10

5.2.2 5.2.3 5.2.4 5.2.5 5.2.6 5.2.7 5.2.8 5.2.9

Soils with reducing conditions .......................................................... Soils with relatively higher clay contents in the subsoil ................... Soils with thick dark mineral topsoil horizons ................................. Soils of arid and semi-arid regions ................................................... Strongly weathered tropical soils ...................................................... Soils typical for polar and boreal zones ............................................ Soils with specific physical properties .............................................. Anthropogenic soils ..........................................................................

107 109 111 113 116 117 119 121

5.3 Regional distribution of soils ............................................................... 122 5.3.1 How soils are associated with each other ......................................... 122 5.3.2 Soil maps ........................................................................................... 126 6 Soils in the environment .............................................................. 129 6.1 Humans and the environment .............................................................. 129 6.2 Six major soil functions ....................................................................... 6.2.1 Ecological functions of soils ............................................................. 6.2.1.1 Agricultural and forest production function .................................... 6.2.1.2 Filtering, buffering and transformer functions ............................... 6.2.1.3 Gene protection and gene reserve function .................................... 6.2.2 Technical-industrial, socio-economic and cultural functions of soils 6.2.2.1 Infrastructure function ................................................................... 6.2.2.2 Raw material supply function ........................................................ 6.2.2.3 Cultural function ............................................................................

129 129 129 129 130 131 131 131 131

6.3 Competing soil functions as a key to understanding soil conservation 131 6.4 Threats to soil functions – soil loss and soil pollution ......................... 6.4.1 Soil development and history of land use ........................................ 6.4.2 Soil loss caused by infrastructural measures .................................... 6.4.3 Soil pollution ..................................................................................... 6.4.3.1 Air pollutants .................................................................................. 6.4.3.1.1 Soil acidification .......................................................................... 6.4.3.1.2 Soil pollution by toxic substances ............................................... 6.4.3.1.3 Soil pollution by radionuclides ................................................... 6.4.3.2 Pollution of surface waters and groundwater ................................. 6.4.3.3 Impacts from agriculture, forestry and waste management ........... 6.4.3.3.1 Physical soil deterioration ........................................................... 6.4.3.3.2 Chemical and biological pollution .............................................. 6.4.4 Summary: soil loss, soil pollution ..................................................... 6.5 Soil protection ...................................................................................... 6.5.1 Evaluation of soil loss and soil pollution .......................................... 6.5.2 Principles of soil protection .............................................................. 6.5.3 Operational measures for soil protection .......................................... 6.5.4 European soil protection strategy ......................................................

133 133 134 136 137 138 139 139 139 139 139 139 141 141 141 142 143 144

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7 Soil as a plant-growing medium .................................................. 146 7.1 Soil fertility .......................................................................................... 146 7.2 The root zone ....................................................................................... 147 7.3 Water, air and heat supply .................................................................... 147 7.4 Nutrient supply ..................................................................................... 7.4.1 Nutrients ............................................................................................ 7.4.2 Binding forms of the nutrients .......................................................... 7.4.3 Nutrient availability .......................................................................... 7.4.4 Identification of the status of supply ................................................. 7.5 Soil group and soil fertility, soil evaluation .........................................

149 149 150 151 152 153

8 Soil information systems............................................................... 8.1 Soil inventory and soil mapping .......................................................... 8.2 Soil monitoring .................................................................................... 8.3 Soil information systems in the network of environmental monitoring

156 156 156 157

9 History of soil science .................................................................. 158 References ................................................................................................... 161 Subject index ............................................................................................... 165

1.1 Definition of soil

1

13

Introduction

1.1 Definition of soil Soil is a surface component of the Earth that developed from geological materials and dead biomass. Soils may be divided into horizons and, together with the air, water and living organisms contained in them, all soils together are known as the pedosphere. Substances and energy from the atmosphere result in the constant alteration of inorganic and organic materials. The solid matter, air and water form a dynamic three-phase system. In contrast to rocks, plants and animals, soils are not discrete natural objects, but defined by complex relationships with the lithosphere, hydrosphere, atmosphere and biosphere (Figure 1). The pedosphere is the continuum of soils at the surface of the Earth. The composition of soils changes from place to place at different intensities. This variation is taken into account when we recognize the basic soil unit or individual as the pedon. A pedon is defined as the smallest, three-dimensional unit at the surface of the Earth that is considered a soil. Its area extends on the surface between 1–10 m2 and has a depth normally between 0.5–2 m (Fig. 1). The two-dimensional exposure of the pedon in a soil pit to depth is called a soil profile (Fig. 1).

1.2 Soil functions Soils provide a range of functions: three ecological functions and three technical, industrial, socio-economic and cultural functions (Figure 2): – biomass production (6.2.1.1) – the filtering, transformation and buffering functions at the interface of the soil with atmosphere, hydrosphere and biosphere (6.2.1.2)

Figure 1: Arbitrary division of a section from the pedosphere into pedons, left: idealized pedon with soil profile.

14

1 Introduction

– – – –

the function of biodiversity, maintaining the complex ecological functions relating plants, animals and microorganisms (6.2.1.3) the function of providing the base for industry, infrastructure and the space to allow humans to fully experience life (6.2.2.1) a source of basic raw materials such as sand, gravel and peat (6.2.2.2) protection of the cultural heritage (6.2.2.3)

human health air

biomass production (e.g. food)

culture surface water soil

biodiversity

groundwater

Figure 2: Soil functions for humans and the environment.

1.3 Soil science Soil science is a discipline of natural sciences and includes aspects of physics, chemistry, biology, geosciences (e.g. geology, mineralogy and geography) and agronomic and technical sciences (e.g. agriculture and forestry) with a focus on research and teaching about soils.

The specific areas of soil science deal with: – – – – –

Soil inventory (the documentation and mapping of soils) Soil processes (the chemical, physical and biological processes in soils) Soil genesis (pedogenesis, soil formation, the nature and development of soils and their components) Soil classification (the classification of soils based on soil genesis, distribution or function) Soil ecology (the study of soils as the environment for plants, animals, microorganisms and people).

1.3 Soil science

15

Applied Soil Science: Economic evaluation of the soil for agriculture, forestry or gardening with the assessment of soil productivity; the establishment of the ability and capacity of the soil as a medium for filtering, buffering or transforming added materials; soils as the habitat of organisms and a reservoir of genes. Soil science is strongly connected with archaeological and paleontological topics. Knowledge about soils is essential in order to extract materials from soils (e.g. clay, sand and gravel) and for construction purposes (e.g. pipelines for water and gas; civil engineering and sealing). Soil science also helps answer forensic questions (e.g. was there a dead human body lying on a soil and for how long?).

16

2

2 Soil components

Soil components

Soils are made up of four basic components: – mineral components (including primary and secondary minerals, amorphous components and water-soluble salts) – organic components (dead biomass and the products of their decomposition; living biota such as animals, plants and microorganisms are, strictly speaking, not part of the soil) – water – air The relative proportions of these components vary depending on the environmental context of the soil and its use (Figure 3).

Figure 3: Schematic composition of a grassland soil (% by volume).

2.1 Mineral components Mineral matter is the dominant solid component of most soils (with the exception of peat and other organic soils). The mineral matter forms a solid/firm substrate between the plant roots and provides them with nutrients.

2.1.1 Parent materials The basic constituents of the soil are derived by weathering and transport from consolidated and unconsolidated rocks of the surface of the Earth. In urban and peri-urban environments, they may develop from artificial materials such as building waste, including bricks and concrete.

2.1 Mineral components

17

Table 1: Overview of the most important igneous rocks and their composition.

plutonic volcanic rocks rocks

quartz ortho- plagio- amphi- micas clase clase boles, pyroxenes

% granite

rhyolite

diorite

andesite porphyrite

gabbro

basalt diabase

%

20-30 30-50

clay Sands may have saturated water conductivities in excess of 5 cm per hour, whereas clays commonly have conductivities of less than 0.5 cm per hour.

2.5 Soil air

53

An example of an ecologically optimal permeability is a loamy soil with good structure and a conductivity on the order of 2 cm per hour. Unsaturated flow: As a soil dries, its largest pores are emptied of water first, so that water is confined to finer pores and finally to films of bound water. In a saturated sandy soil with all pores water-filled, flow rates may be as high as 7 cm/h; when only pores of diameter loam > silt > clay.

3.1 Physical properties of soils

3

55

Soil characteristics

Soil constituents – mineral, organic and organo-mineral materials, water and air – are not just mixed together haphazardly, but form an organized soil body (pedon → Figure 1) of definite structure and distinctive physical and chemical properties. These properties whilst in part resulting from the individual constituents are specific to the integrated soil system – the pedosystem. The resulting properties of the different soils determine their responses to the environment and to soil use (see 4.1, 6, 7).

3.1 Physical properties of soils 3.1.1 Particle size and texture Particle size and soil texture: composition of primary and secondary minerals (2.1) of different particle size, dependent on particle size of the parent material and degree of weathering. Particles ≥2 mm are called coarse fragments and make up the coarse fraction. Particles 200 mm to vertical dimensions

3.1 Physical properties of soils

single grain

prismatic

massive

granular

columnar

61

blocky

platy

Figure 33: Scheme of the most important types of soil structure.

The following structures form usually larger elements: prismatic: vertical dimensions > horizontal dimensions; sharp edges columnar: like prismatic, but with rounded tops wedge-shaped: elliptical, interlocking lenses that terminate in acute angles. The types of soil structure are shown schematically in Figure 33. The macrostructure of a soil is easily visible with the naked eye, however, in thin sections under a microscope, it is possible to recognize the microstructure. Using micromorphological examination methods it is possible to determine the orientations of soil constituents, for example of clay at aggregate surfaces: clay coatings are the result of clay illuviation (4.2.2.2) and stress cutans (argillans) are the result of reorientation of clay particles which are compressed by swelling and shrinking (4.2.2.5). The effect of soil structure on porosity Single grain structures have larger pores than massive structures, which often feature many fine pores. Granular structure mitigates the effects of particle-size distribution, increasing the number of fine and medium-sized pores in sands and increasing the number of coarse pores in clays. Segregation structures have both fine and medium-sized pores within the aggregates, but also very coarse pores and cavities between the aggregates. Optimal pore volume and pore-size distribution are provided by a well aggregated soil structure, particularly granular structure.

62

3 Soil characteristics

3.1.2.3 The development of soil aggregate structure The development of aggregate structure types is controlled by the presence of the following agents: mineral and organic colloids, which allow coagulation and peptization, the soil organisms and roots of higher plants and CaCO3. Aggregate structure also depends on the processes/forces of aggregation (cohesion, adhesion) and segregation (shrinkage of colloids by water loss and pressures exerted by freezing water). 3.1.2.3.1 Coagulation and peptization

Mineral (clay minerals, oxides, hydroxides) and organic (humic substances, litter substances) colloids, in the presence of water, may occur in a sol state (dispersed or peptized) or a gel state (flocculated or coagulated). The sol state (where individual particles are dispersed in the medium) is favoured by a high specific surface (Table 13), where sedimentation is delayed by frictional resistance, Brownian motion, mutual repulsion of particles of same (positive or negative) charge, hydration of colloids and their adsorbed ions (hydration shells of the individual particles). Clay minerals are pulled apart if monovalent cations with large hydration shells dominate. Furthermore, the absence of soluble salts allows the water molecules to accumulate between clay minerals. Coagulation (change to the gel state) is favoured by drying (withdrawal of the medium of dispersion), but also in water by the attraction of particles as a result of opposite electrical charges and van der Waals forces. Coagulation of clay minerals is favoured by the presence of di- and trivalent cations with small hydration shells. The presence of soluble salts (salinity) in the pore solution attracts water molecules and withdraws them from the surroundings of the clay minerals. Re-peptization is possible for most soil colloids (reversible colloids: clay minerals and organic substances) by addition of water or peptising (monovalent) ions (Na > K; NO3 ≈ Cl); some oxides and hydroxides can, however, not be re-peptized (irreversible colloids). Flocculated colloids form weak coagulates, the precursors of aggregates (3.1.2.2); some (but not absolute) desiccation and pressure may result in a massive structure, especially in the subsoils of certain soil groups. 3.1.2.3.2 Shrinking and swelling of soils

Rapid and frequent cycles of strong drying and rewetting cause soil shrinking and swelling and result in the formation of cracks and fissures (Figure 34). The nature and extent of these cracks and fissures depend on the clay content (in particular the 2:1 expanding minerals) and on the species of the ions adsorbed to the clays. High clay content and adsorbed Na+ ions result in a coarse pattern of cracks, lower clay content and Ca2+ ions produce finer patterns (Figure 34). There is a continuum of intermediate forms between these extremes, primarily dependent on the fraction and type of clay minerals and secondarily on ions present. The presence of organic substances reduces crack development by shrinkage. Depending on clay content and the ions present, swelling and shrinking with strong vertical cleavage produce a prismatic structure (high clay content plus dominance of

3.1 Physical properties of soils

63

Figure 34: Shrink-swell cracks of marine deposits with high clay and high Na contents and of loess with low clay and high Ca contents.

Ca) or a columnar structure (high clay content plus dominance of Na and Mg); with cleavage in all directions, blocky structures result (medium clay content and Ca the dominant ion), a common feature of loamy and silty soils, particularly in the subsoil, but also in topsoils of low organic matter content and low biological activity. Platy structures form, where shrinkage and swelling occurs with predominantly horizontal cleavage, often present in compressed soils. 3.1.2.3.3 Frost action

As in physical weathering of rocks, the increase of the volume of water on freezing (volume increase of 9%) and subsequent growth of ice crystals affects both soils with massive structure and soils with aggregates and causes them to break down into a fine blocky structure (in extreme cases, frost shattering may occur); in fine-textured subsoils, horizontal ice lamellae may be formed which results in platy structures. Where in a permafrost soil, the topsoil thaws over a frozen subsoil, the water may pool in the topsoil and cause the dispersion of the aggregates resulting in a massive or a single grain structure, especially if the aggregates had been small due to frost shattering. Where ice lenses form beneath coarse fragments, these fragments may eventually be lifted up to the soil surface leading to a patterned ground, typical of permafrost areas. 3.1.2.3.4 The activity of soil organisms

Granular structure is formed only in topsoils with a significant amount of organic matter and high biological activity. Large and small mineral particles and organic substances are held together in fine aggregates by the physical forces of cohesion, by fungal hyphae and bacterial colonies, by mixing by passage through the gut of soil animals, by the binding effect of excretions and by the root hairs of higher plants. This leads to heterogeneous, porous aggregates of high stability. Characteristic of a good granular structure is a high proportion of clay-humus complexes (organo-mineral associations; 2.3.2). As biological activity is a characteristic of mull type humus, granular structure is a characteristic feature of this humus form; in the moder humus form, the structure is coarser (often subangular blocky), and aggregates may be even absent in mor humus forms (2.3.4). The development of granular structure may be supported by a high content of CaCO3, which acts as a cementing and coating agent.

64

3 Soil characteristics

3.1.3 Particle density and bulk density Particle density: The particle density is mass per unit volume excluding pore spaces. The particle density of the mineral matter of soils ranges from 2.60–2.75 kg dm–3 with a mean of 2.65 (the density of quartz); the particle density of organic substances is variable and often around 1.4 kg dm–3. Because of the different combinations of minerals and organic substances the particle density varies, particularly with organic matter content, but for a ‘normal’ topsoil of moderate organic matter content, the values will range between 2.4 and 2.65. Bulk density (BD) is the density of a dry soil including the pore spaces. BD of dry mineral soils varies from 1.1–1.8 kg dm–3, most mineral soils have BDs in the range of 1.3–1.5 while organic soils (peat) may have a BD of 0.15. The bulk density of loose, humus-rich topsoils is lower than that of denser subsoils with little organic matter. The bulk density is, e.g., required for calculating the weight of a soil per unit area to a given depth. For example, a layer of soil, 10 cm thick, with a BD of 1.3 kg dm–3 weighs 130 kg m–2 or 1300 t ha–1.

3.1.4 Soil consistence Consistence refers to the degree and kind of cohesion and adhesion of the soil mass. It describes the strength and stability (mechanical strength; packing density) of soil aggregates and fragments and thus the resistance of the soil to penetration, deformation and rupture under an applied stress. Soil consistence is very important in respect to rootability, soil workability and resistance against soil erosion. Consistence is the sum effect of particle-size distribution, organic matter content, and type and stability of soil structure, at a given water content. The evaluation of consistence includes: Rupture resistance: the strength of soil to withstand an applied stress Manner of failure: the rate of change and the physical condition soil attains when subjected to compression Stickiness: the capacity of soil to adhere to other objects Plasticity: the degree to which reworked soil can be permanently deformed without rupturing Penetration resistance: the ability of soil in a confined state to resist penetration by a rigid object of specified size Consistence is principally determined by inner soil surfaces of organic matter and clay and the water content.

3.1.5 Soil temperature Soil temperature reflects the heat energy content of a particular soil body. Temperature is particularly important for the germination and growth of higher plants, the

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activity of soil organisms (2.2), weathering processes (2.1.2.1), decomposition processes (2.3.2), structural development of soils (3.1.2.3), soil water budget (2.4) and soil air budget (2.5). Soil temperature depends on heat input and output to the soil, its specific heat and heat conductivity. Heat input is almost exclusively from the sun. The intensity of the incident radiation depends on the geographic latitude, the time of year and time of day, the weather, the aspect and inclination of the soil surface, soil colour (3.1.6), and the plant cover. There are minor heat inputs from geothermal sources and from exothermic reactions (oxidation) during weathering, SOM decomposition and root respiration. Heat loss occurs by radiation from the soil surface and by evaporation of soil water. How much heat is lost depends on the time of year, the time of day, the plant cover, the soil colour and the water content. Heat capacity is the product of specific heat and bulk density. Specific heat is measured in joules (J) and denotes the heat energy required to raise the temperature of one gram of soil by one Kelvin (K) or degree Celsius (°C). Specific heat of water is 1 J, of air 0.24 J, of soil mineral particles ~ 0.2 J and of organic matter ~ 0.4 J. Consequently, the specific heat of a soil is chiefly controlled by its water content. Heat conductivity is the heat energy in J transferred in 1 s across a distance of 1 cm over an area of 1 cm2 at a temperature gradient of 1 K. The heat conductivity of a soil is strongly influenced by the air content as the heat conductivity of air is only 1/20 that of water and 1/60 that of solid soil material. Effectively, air is a thermal insulator. The heat budget (input and output), heat capacity, and heat conductivity result in distinctive soil temperature curves which depend on the time of year and the time of day, and feature temperature maxima in summer and the middle of the day. The temperature of surface soil layers varies considerably more than in the respective subsoils, and there is a marked time lag between surface and subsurface temperature changes. Moist soils (high heat capacity) warm up more slowly than dry soils do, and consequently they cool more slowly also.

3.1.6 Soil colour Soil colour is widely used to identify soils (for example Chernozems, Cambisols, Podzols) and soil horizons (for example humus-rich horizons, mottled horizons, bleached horizons). Soil colour influences soil temperature, as dark soils absorb more solar radiation. Colour is described by: Hue: spectral composition, for example reddish brown, yellowish brown Value: extent of black and white, for example dark brown, light brown Chroma: colour saturation, for example strong brown. Precise colour description is achieved by using standard colour charts, e.g. that of Munsell, in both moist and dry soils.

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3 Soil characteristics

Soil colour is mainly caused and determined by soil organic matter and compounds of Fe and Mn. Organic matter darkens soil colour in proportion to its content and its degree of decomposition and produces black, brownish black or grey soil colours. Oxidized Fe and Mn compounds yield distinctive red, orange, brown, blackish brown or brownish black soil colours (Table 7). Reduced Fe compounds (silicates, carbonates, phosphates, sulfides) give soils greenish, yellowish, bluish or grey to black colours (3.2.3 and 4.2.2.4). Light soil colours indicate the absence of organic matter and coloured Fe and Mn compounds in soils, where the mineralogy is dominated by quartz (colourless) and pale-coloured primary and secondary silicates (Table 4). Because moist soils reflect less light than dry soils, moist soils appear generally darker and their colours are more intense.

3.2 Physico-chemical properties of soils The physico-chemical properties of soils comprise ion exchange, soil pH (soil reaction) and redox dynamics.

3.2.1 Ion exchange in soils Mineral and organic particles with high specific surface, such as clay minerals and humic substances (and to a lesser extent, oxides), are able to adsorb molecules (H2O, N2, O2, CO2, NH3, SO2) and both anions and cations on their surfaces. Adsorption of H2O has been treated in 2.4.1.1. Adsorption of gaseous molecules is not relevant except for the adsorption of air on strongly decomposed peats (5.2.7), which then turn hydrophobic and ‘puffy’. The adsorption of ions, however, is of great significance for soil reaction (3.2.2), soil structure (3.1.2), soil forming processes (4.2) and soil fertility (7.4). Exchange sites for ions are provided by soil particles, mostly Ca2+ > Na+ Hydration: All ions in aqueous solution are surrounded by a hydration shell of oriented H2O dipoles (2.4.1.1). Smaller ions of equal valence, attract more H2O molecules and hence have thicker hydration shells than larger ions, because the central positive charge is closer to the H2O dipoles. Thus, the hydrated ionic radius (ion plus hydration shell) decreases with increasing ionic radius: Ca2+ < Mg2+; K+ < Na+ Ions of smaller hydrated radius have a stronger adsorption affinity and are more strongly bound because their positive charges are less attenuated by the hydration shell and consequently more strongly attracted by negatively charged sites on the exchanger. Considering the above, adsorption affinity and bonding to the exchanger decrease in the following order; Al3+ > Ca2+ > Mg2+ > K+ > Na+ The adsorption affinity of H+ is similar to that of K+ because H+ combines with 1 molecule of H2O to form H3O+ which is of similar size as the K+ ion (3.2.2). 3.2.1.1.4 The properties of the ion exchanger

Exchangers of high charge density bind polyvalent ions more strongly than monovalent ions; those with permanent negative charge in the tetrahedral sheets (e.g. vermiculite) adsorb cations more strongly than those with charge situated in the octahedral sheets (e.g. smectite). Electrostatic forces are more powerful in the interlayers than on the surfaces and broken edges of clay minerals as they are two-sided, and polyvalent ions are retained more strongly in the interlayer positions. Expanding 2:1 minerals with a high tetrahedral excess charge (mica derivatives – transitional clay minerals, vermiculite and smectite from illite) contract as soon as K+ (and similar-sized NH4+) ions enter the six-oxygen rings of the mineral surface (Figure 5 right) and are fixed there (see contractability of clay minerals in Table 6, also 2.1.3.1.2 and 7.4.3). The fixation of K+ and NH4+ is a special and specific aspect of the selectivity of the exchanger. 3.2.1.1.5 Ionic composition and ion concentration of the solution

As the equivalent concentration of an ion in solution increases (i.e. offering more ions), the proportion of this ion on the exchanger also increases. The exchange follows a logarithmic course and reflects the adsorption affinities of the cations present (3.2.1.1.3 and Figure 38). If such a solution is diluted with ionic composition unchanged, polyvalent ions are adsorbed preferentially compared to monovalent ions, and vice versa if the solution gets more concentrated (Figure 39).

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% saturation at the exchanger

100 Ca++ Mg++ K+ 50

Na+

Figure 38: Exchange curves for increasing supply with Na, K, Mg, Ca ions at an exchanger occupied with NH4 ions (after 0 increasing concentration in the soil solution (volume unchanged) Schachtschabel).

% saturation at the exchanger

100 Ca++

50 K+ 0

increasing volume of the soil solution (ion composition unchanged)

Figure 39: Exchange curves at an exchanger occupied with Ca and K ions, depending on the concentration of the solution, with ionic composition unchanged (after Schachtschabel).

3.2.1.1.6 Combined effects of the various factors on the cation composition of the exchanger

The majority of soils in humid temperate climate, retain Ca2+ and Mg2+ ions more strongly than K+ and Na+ ions (Figure 35). Particularly Na+ ions are strongly leached, which is important, e.g. for the desalination of salt marshes and for preventing Na enrichment where Na-rich fertilizers and deicing salts are used. Exchange processes in soils not only depend on the addition and removal of cations but also on variations of soil water content, which significantly controls the availability of mono- and divalent cations (Figure 39) to plants. (7.4.3). This is especially relevant in regions with wet and dry seasons. The ratio of adsorbed ions varies with the type of exchanger. Ca2+ and Mg2+ ions are relatively strongly retained on organic exchangers while K+ and NH4+ ions are held particularly well by illites, transitional clay minerals and vermiculites. This preference results in differences in the plant availability of nutrients.

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3.2.1.1.7 Theory of cation exchange in soils

Above (selectivity coefficients 3.2.1.1.2) we provided in broad terms how cations behave in soils. Considerable work was undertaken to explain the fundamental principles of cation exchange and a number of models were developed. Some of these, using the Donnan theory or the theory of a diffuse double layer, have been shown to be valid for ideal systems (manufactured exchangers), but are unable to explain cation exchange processes in natural soils accurately due to the following exchange anomalies. Polyfunctionality: different properties of the exchange sites at clay minerals (outer surfaces, broken edges and interlayer sites); various effects of permanent charge in the octahedra and tetrahedra; and differences between the mineral Si-OH and Al-OH groups and the COOH and OH groups of organic compounds. Hysteresis effect (Greek = lagging after): Caused by partial disequilibrium and delayed equilibration at interlayer sites on the clay minerals and at internal sites on humic substances, ions already adsorbed are preferentially retained, protecting them from being exchanged for ions from the solution. Structural instability: Some clay minerals (transitional clay minerals, vermiculites and smectites, derived from illite) contract in the presence of K+ and NH4+ ions (2.1.3.1.2, 3.2.1.1.4) which modifies their ion exchange characteristics (Figure 11 and Figure 12).

3.2.1.2 Anion exchange Soils exchange anions to a smaller extent than cations, because positively charged sites are far less abundant than negatively charged sites. However, there are positively charged sites in clay minerals, (hydr-)oxides and humic substances. Clay minerals: At lower pH values (acidic conditions: higher H+ concentration) H+ ions may attach to Al-OH sites (AlOH + H+ = AlOH2+) creating positive charges that attract anions. Al-OH groups are found at the broken edges of all clay minerals (Figure 36) and at the octahedral outer surfaces of 1:1 clay minerals, allophanes and imogolites. Hydroxides and oxides of iron and aluminium (and, to a lesser extent, other metals) behave similarly in that FeOH and AlOH groups, as well as (especially in strongly crystallized oxides) FeO and AlO groups, can add H+ ions. Humic substances: NH and NH2 groups (Figure 23) can attach H+ ions giving them a positive charge. NH2+ and NH3+ groups are able to adsorb and thus exchange anions. As the positive charge increases with decreasing pH, anion exchange capacity is, similar to CEC, pH-dependent, but anion exchange capacity increases with decreasing pH (3.2.1.1.1). Exchangeable anions: Important exchangeable anions are PO43–, SO42–, NO3– and Cl– ions, but also organic anions (for example acetate and citrate ions) and, to a lesser extent, molybdate and borate ions (MoO42–, B(OH)4–). Potentially harmful substances (6.4.3) such as As, F and Se may be adsorbed as arsenate, fluoride and selenate ions (AsO43–, F–, SeO42–).

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As is the case for cations, the adsorption affinity and bonding strength of anions depend on hydration state and valence and decrease in the order: PO43– > SO42– > NO3– > Cl– PO43– ions are stable only under alkaline conditions, in neutral and acidic soils, phosphate speciates as HPO42– and H2PO4– ions, but their exchange behaviour is more or less the same as that of PO43– ions. The polyvalent ions PO43– and SO42– are adsorbed in significant amounts, whilst the monovalent ions NO3– and Cl– are very weakly retained and remain almost entirely in the soil solution where they are potentially leached. SO42– ions are appreciably adsorbed only in acidic soils at pH values below 5.5. Anion exchange capacity (for phosphate ions) of most soils in temperate regions varies between 0.5 and 2 cmolc kg–1, depending on the contents of clay minerals, (hydr-)oxides and organic matter. Tropical soils with high kaolinite and (hydr-)oxide contents, may have an AEC as high as 20 cmolc kg–1. Similarly, high AEC values also prevail in soils with high allophane and imogolite contents. PO43– adsorption may be irreversible because it forms very strong bonds; this process is called phosphate fixation. PO43– may also form Ca, Al and Fe phosphates. In all these forms, P availability to plants is difficult (7.4.3).

3.2.2 Soil pH (Soil reaction) pH is a measure of the hydrogen ion concentration (cH+) in the soil solution; known as the soil reaction; acidic, neutral or alkaline (basic). The pH is the negative logarithm (base 10) of the H+ ion concentration (Table 14). pH = –log cH+ Table 14: Relationship between H+ ( mol l–1) concentration and pH (measured in CaCl2) and the descriptive terms applied to soils of different pH values.

cH+ [mol/l]

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

pH (CaCl2)

2

3

4

5

6

7

8

9

very strongly

strongly

extremely

medium weakly

weakly strongly

extremely

the soil is acidic

neutral 'neutral range'

alkaline alkalin

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75

In aqueous solution, H+ ions are usually hydrated with 1 molecule of H2O forming the oxonium ion H3O+ or with 4 molecules to form the hydronium ion H(H2O)4+. The strict definition of pH is the negative logarithm of the activity of the H+ ions in the solution, which, in the compositional range of soils, may be replaced by its concentration. The real pH is measured in deionized water. But for better reproducibility, pH measurements are usually made in suspensions of a dilute electrolyte (1 M KCl or 0.01M CaCl2). In soils where cation exchange dominates, this yields pH values about 0.3 to 1.0 pH units lower than those measured in water because H+ ions adsorbed to ion exchangers are exchanged by K+ or Ca2+. In soils where anion exchange is dominant, the pH measured in water is slightly lower than that measured in KCl or CaCl2. Table 14 lists the descriptive terms applied to soils of different pH. Soil acidity: acidic reaction of soils. Soil alkalinity (or basicity): alkaline or basic reaction of soils. The pH of soils of humid regions normally is between 3 and 8, most commonly 5–6.5, consequently, acidic soils pose a more frequent problem than alkaline soils. Alkalinity is a serious problem in arid regions, where soils may be saline and/or sodic (5.2.5). H+ ions in the soil solution indicate the actual (active) acidity. The exchangeable (potential) acidity also includes H+ and Al3+ ions adsorbed to the exchangers (expressed in cmolc kg–1 soil) which add to the active acidity as soon as H+ and Al3+ ions are exchanged into the solution. (It should be noted that Al3+ ions only appear at low pH ( diorite > granite On sedimentary rocks: limestone > silicate-rich sediments > quartz sand

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3.2.2.2.1 Processes poducting H+ ions in soils

H+ ions are produced by the following processes: a. CO2 production as a result of respiration of soil organisms and plant roots and the reaction of CO2 with water CO2 + H2O ←→ H2CO3 ←→ HCO3– + H+ High CO2 contents in the soil air (2.5.1) markedly reduce the pH, if the soil is poorly buffered (3.2.2.3 and Table 15). Table 15: pH of water in equilibrium with air of varying CO2 concentrations. CO2 (volume %)

pH

Atmosphere

0.04

5.6

Soil air (1)

0.4

5.2

Soil air (2)

1.0

5.0

Soil air (3)

10.0

4.5

b. H+ release by plant roots when they take up cation nutrients (3.2.2.2.2). In natural soils, the resulting drop in pH is minimal, because the mineralization of the plant litter returns the cations to the soil and consumes the released H+ ions. This is part of the process of nutrient cycling. In cultivated soils, biomass is removed by harvesting instead of being mineralized at the site. The H+ ions are consequently not consumed and may cause the soil pH to decline markedly, unless manure, basic fertilizers or lime are applied. (Plants also take up anion nutrients and add OH– or HCO3– to the soil. But most plants take up more cations than anions, and therefore the result is an increase in H+ concentration in the soil.) c. Decomposition of organic matter produces acidic humic substances (2.3.3). This process lowers the soil pH more strongly than CO2 production, and pH values of 3 and below may be reached in some soils such as Podzols and many Histosols. d. Oxidation of S and N and formation of H2SO4 and HNO3 by weathering (2.1.2.1.2) and/or biological oxidation (2.2.1); under extreme conditions, e.g. by oxidation of sulfides, which have accumulated in marine muds, pH may drop below 2.5 (acid sulfate soils). H+ ions are also produced by oxidation of reduced Fe and Mn compounds (3.2.3). e. Input by precipitation. Pollutants such as SO2 and NO2, from industrial sources or burning biomass, dissolve in rainwater and cause ‘acid rain’ (6.4.3.1). The mean pH of rainwater in industrial areas is often 3.0–4.5, whereas rainwater in unpolluted regions, which contains dissolved CO2 only, has a pH of 5.6 (Table 15). f. Fertilization with acidic fertilizers such as superphosphate or ammonium sulfate.

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3.2.2.2.2 Loss of base cations from the soil

The production of H+ ions results in the adsorption of H+ and Al3+ to the exchange sites and the concomitant release of adsorbed Ca2+, Mg2+, K+ and Na+ ions, which may subsequently be leached from the soil by percolating water. Leaching of cations only occurs together with corresponding anions like NO3–, SO42– or organic anions. Loss of cations is more extensive in highly permeable soils in regions of high precipitation. Cation loss may be countered, e.g. by applying basic fertilizers to the soil which provide OH– ions to neutralize the H+ ions and supply Ca2+ and Mg2+ and other base cations (raising base saturation), and by liming with Ca(OH)2, CaO (CaO + H2O → Ca(OH)2) and CaCO3 (CaCO3 + CO2 + H2O → Ca(HCO3)2). 3.2.2.2.3 pH variation in soils with depth

As the production of H+ ions (3.2.2.2.1) is greater in humus-rich topsoils compared to deeper soil layers (4.3.2), the pH tends to increase with depth. (This is not the case in arid climates or on well limed soils.) The more basic the soil parent material, the steeper is the pH gradient, for example in a Cambisol on glacial marls, pH increases from topsoil to parent material from 5.6 to 7.6 and in a Podzol on glacial outwash sands from 3.8 to 4.3.

3.2.2.3 Soil pH buffering Buffering is the ability to resist changes in pH when H+ ions (or OH– ions) are added (3.2.2.2). Buffering H+ ions means that they are chemically transformed into a non-dissociated form. Buffering mechanismes vary with soil characteristics and materials added (Table 16). In soils, there are 4 important buffer systems: 1. Carbonate buffer: In carbonate-containing soils, calcite (CaCO3) is converted to soluble Ca(HCO3)2 (Table 16). This reaction continues until all the CaCO3 in the fine earth is consumed. The buffering takes place in the pH range from 8 to around 6.5. 2. Buffering by variable charge at the exchangers: Humic substances, clay minerals and (hydr-)oxides adsorb alkaline earth ions and alkali ions (Ca2+, Mg2+, K+, Na+), if the pH is medium acidic or higher. These ions may be replaced by H+ ions, which adsorb onto functional groups on the exchange surfaces. Cations released from the exchange surfaces may be leached from the soil thus causing severe loss of nutrients. 3. Buffering by silicate weathering: Primary silicates may be weathered by protolysis and secondary minerals (2.1.2.1.2, 2.1.2.2.2) are formed. H+ ions may also attack layer silicates, especially those central atoms that result from isomorphous substitution. This attack releases Al3+ ions from the tetrahedral layers or Mg2+ ions from the octahedral layers into the soil solution, from which these may be adsorbed to other exchange sites or, in the case of the Al, directly form Al hydroxy complexes in the interlayer space of developing clay minerals, which then will be chlorites. Silicate buffering is ecologically significant, because cation exchangers are dissolved and nutrients are released. On the other hand, the released Al3+ ions can be toxic for some agricultural plants.

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4. Buffering by weathering of oxides and hydroxides: Buffering occurs by protonation of OH or O sites in Al and Fe (hydr-)oxides. While Al (hydr-)oxides buffer pH between 5 and 3, the protonation reaction of iron oxides and hydroxides may result in pH values