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Soils are a vital resource in agricultural production.
This book aims to provide an overview of several topics concerning agricultural soil management. Without claiming to be exhaustive, it nevertheless provides a useful basis for understanding how agricultural soils function, and how to manage these functions within agroecosystems. Our approach to soils is multidisciplinary, ranging from natural sciences to human and social sciences.
Agricultural Soil Science also highlights the characteristics of soils in specific geographical regions (Mediterranean soils, tropical soils) and environments (urban soils). The content of this book is based on the training programs run by its authors at the higher education establishments where they teach.
At a time of ecological transition, soils must once again play a central role in the management of agroecosystems. This book sets out to play its part in making that happen.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright Page
Introduction
Toward the multifunctional usage of agricultural soils
Overview of the book
1 Tillage and Structure of Agricultural Soils
1.1. What is tillage? Why should soils be “tilled”?
1.2. Soil structure
1.3. Consequences of the different methods for tilling soil
1.4. Conclusion
1.5. References
2 The Biodiversity of Agricultural Soils
2.1. Soil organisms
2.2. Biodiversity of agricultural soils compared with other land uses
2.3. Effects of agricultural practices on soil biodiversity
2.4. Comparison of cropping systems
2.5. Soil biodiversity management – keys for promoting action
2.6. Conclusion
2.7. References
3 Spatial Variability and Mapping of Agricultural Soils
3.1. Introduction
3.2. Background: the origins of soil mapping in France
3.3. The digital age, from paper maps to spatialized databases: the available information for soil mapping
3.4. Some of the general concepts and methods of soil mapping
3.5. Examples of uses of soil maps
3.6. References
4 Runoff and Soil Erosion
4.1. Preliminary considerations
4.2. The essential processes at work on agricultural land
4.3. Impact of agricultural activities on processes
4.4. Toward integrated watershed management
4.5. The art of avoiding recipes
4.6. References
5 Soils and Agricultural Land Property: Legal Aspects of Sectoral and Functional Protection
5.1. Soil, a physical environment like no other?
5.2. Soils are covered by the law as a biological environment
5.3. Soils are covered by the law as a property right
5.4. Toward a global protection of physical environments?
6 Methods for Estimating the Agronomic Value of Soils
6.1. Why evaluate the agronomic value or the cultural suitability of soils?
6.2. Soil functions for agricultural production
6.3. Intrinsic properties and characteristics of soils
6.4. Extrinsic factors
6.5. Evaluation of agricultural values
6.6. Examples of observation and classification methods
6.7. Conclusion
6.8. References
7 The Concept of Ecosystem Services and Its Application to Soil: Between Promises and Reality
7.1. Introduction
7.2. A short history of Ecosystem Services: from inception to the international institutionalization of this concept
7.3. Definition and conceptualization of Ecosystem Services
7.4. Classification of Ecosystem Services
7.5. The biophysical quantification of Ecosystem Services
7.6. Mapping Ecosystem Services
7.7. Applying the concept of Ecosystem Services to the soil component of ecosystems
7.8. Conclusion
7.9. Acknowledgments
7.10. References
8 Mediterranean Agricultural Soils
8.1. Introduction
8.2. Fersiallitic red soils
8.3. Vertisols
8.4. Soils with limestone accumulations or Calcarisols
8.5. Salisols and Sodisols
8.6. Conclusion
8.7. References
9 Tropical Soils and Sustainable Management
9.1. Introduction
9.2. Tropical pedogenesis: dominated by geochemical weathering
9.3. Typical profile and differentiation of horizons and pedological units
9.4. Principal characteristics, diversity of tropical soils
9.5. Properties and agronomic consequences
9.6. The sustainable management of soils with high geochemical weathering: the role of organic matter in Ferralsols
9.7. Conclusion
9.8. References
10 Urban Agricultural Soil
10.1. Introduction
10.2. Redeploying urban agriculture
10.3. Urban soils: the basis of urban agriculture
10.4. Managing ecosystem services provided by urban soils
10.5. Toward decision-making tools
10.6. Conclusion
10.7. References
List of Authors
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1. Average biomass and species richness (known or estimated) of soil o...
Chapter 3
Table 3.1. Examples of the recommended densities of soil measurements based on...
Table 3.2. Impact of the number of points used to calibrate the prediction rel...
Chapter 6
Table 6.1. Relationships between the functions, properties and characteristics...
Table 6.2. Example of a five-class grid for the evaluation and scoring of cons...
Table 6.3. Example of a scoring grid for the constraint of the concentration c...
Table 6.4. Example of a hierarchical classification model. Classification into...
Chapter 7
Table 7.1. Classification of biological production services, regulating servic...
Chapter 8
Table 8.1. Equation for the hydrolysis of primary feldspar minerals (Pédro 198...
Table 8.2. Equivalence between Fersialsols and other denominations (Bresson 20...
Table 8.3. Main methods of iron extraction, their dosage and interpretations (...
Table 8.4. Characteristics of color and the minerology of iron oxides for “ter...
Table 8.5. Colors and forms of iron between Rendosols and Fersialsols from the...
Table 8.6. Examples of Vertisols (FAO 2015) in Sicily (“Detritus” and “Clay 5”...
Table 8.7. The equivalence between the names for Salsodic solums (AFES 2008)
Table 8.8. Examples of salsodic solums in Languedoc, in the Orb delta (Sérigna...
Chapter 9
Table 9.1. Specific area and charges of the chief minerals present in the soil...
List of Illustrations
Chapter 1
Figure 1.1. Image showing the tillage process in ancient Egypt at the tomb of ...
Figure 1.2. Mechanized plowing, Grignon plateau, 2002. Crop residues (corn) lo...
Figure 1.3. Left: profile of forest soil (Luvic Cambisol, Grignon park – soil ...
Figure 1.4. Expansion effect of plowing on the soil surface layer. The average...
Figure 1.5. Shrinkage curves of three soils (1: clay-rich soil – Vertisol; 2: ...
Figure 1.6. Changes in bulk density of the tilled horizon of a remolded loamy ...
Figure 1.7. Cross-section of the surface of a loamy soil with a slaked crust (...
Figure 1.8. Soil compaction curve as determined by the Proctor test (according...
Figure 1.9. Tillage tools: (a) front cultivator; (b) rotary harrow; (c) subsoi...
Figure 1.10. Crop profile of a plot where corn has been sown (Grignon, France)...
Figure 1.11. The three internal structural states of the Profil cultural: (a) ...
Figure 1.12. Hydraulic properties of a silt loam soil seeded with corn (April ...
Figure 1.13. Temporal dynamics of the saturated hydraulic conductivity at diff...
Figure 1.14. Soybeans grown using direct seeding in grain straw residues on a ...
Figure 1.15. Block of soil extracted with a garden spade from a plot using con...
Figure 1.16. Relative difference in organic C content between no-till (AC) and...
Chapter 2
Figure 2.1. Classification of soil organisms by size (adapted from Tibbett et ...
Figure 2.2. Impact of repeated applications of different types of OWPs in the ...
Figure 2.3. Contributions of different indicators to biological activity and b...
Figure 2.4. Mineralization of isoproturon and glyphosate in soils cultivated u...
Chapter 3
Figure 3.1. Approach and use of soil mapping.
Figure 3.2. The “GEOSOL” online mapping tool from the BDAT on the Gis Sol webs...
Figure 3.3. The notion of scale in the different stages of the cartographic pr...
Figure 3.4. Different sampling strategies for soil surveys at the scale of an ...
Figure 3.5. The most common spatial media types (geometries) for soil data in ...
Figure 3.6. Examples of format change. (a) Original points format. (b) Destina...
Figure 3.7. IDW interpolation using the inverse of the distances. (a) Ze the e...
Figure 3.8. Example of the adjustment of a variographic model: the clay conten...
Figure 3.9. Tessellation by Thiessen polygons. (a) Probe points used to start ...
Figure 3.10. Spatial aggregation and disaggregation of the number of auger hol...
Figure 3.11. Drone-assisted zoning in precision agriculture based on (a) diffe...
Figure 3.12. The Sensefly eBee drone, used during the AgroParisTech study
Figure 3.13. The approach used for calculating maps of soil surface properties...
Figure 3.14. General operating diagram for the INRAE AgroParisTech Cadastre_NH...
Chapter 4
Figure 4.1. Adaptation of Hjulström’s (1935) diagram
Figure 4.2. Illustration of the different ways in which erosion types may comb...
Chapter 6
Figure 6.1. Texture class triangle (Aisne soil map triangle or Jamagne triangl...
Figure 6.2. Values of maximum available water content (MAWC) of fine earth in ...
Figure 6.3. Relationship between texture, structural stability, and primary co...
Figure 6.4. Assimilation diagram of mineral elements as a function of pH (from...
Figure 6.5. Examples of agricultural layouts. Left image: vineyard on a very s...
Figure 6.6. Five-class scoring system for texture and the structural stability...
Figure 6.7. Progress made by TypTerres (November 2022) (RMT Sols et territoire...
Chapter 7
Figure 7.1. Representation of the two main families of definitions of Ecosyste...
Figure 7.2. Conceptualizing Ecosystem Services through a “cascade model”. Exam...
Figure 7.3. Decision-tree to choose a mapping strategy for Ecosystem Services ...
Chapter 8
Figure 8.1. The order of weatherability of the main minerals (Goldich 1938; Le...
Figure 8.2. Size of iron oxides for “terra rossa”-type Fersialsols in Sardinia...
Figure 8.3. Pellicular weathering (adapted from Lamouroux 1967). The notations...
Figure 8.4. Chronosequence of fersiallitic soils in the Rhone Valley (Bornand ...
Figure 8.5. Wine-growing Fersialsol formed from fluvio-glacial pebbles from th...
Figure 8.6. Altitudinalclimosequence for soils with calcite accumulations in n...
Figure 8.7. Soils with CaCO3 accumulations in northeastern Morocco (Millot et ...
Figure 8.8. Lateral shifts between the various types of limestone accumulation...
Figure 8.9. Evaporative sequence around an endorheic depression (based on the ...
Figures 8.10. Things to be vigilant about with respect to irrigation in the Me...
Chapter 9
Figure 9.1. General, simplified sequence of the weathering of minerals in soil...
Figure 9.2. Formation of a Ferralsol (adapted from Lepsch 2011).
Figure 9.3. Profiles of Rhodic Ferralsol, rich in kaolinite, located in Paraca...
Figure 9.4. The relation between soil pH (measured in 0.01 M CaCl2) and the ex...
Chapter 10
Figure 10.1. Photos of the microfarm run by Planète Lilas association, in the ...
Figure 10.2. Illustrations of the different forms of urban agriculture. (a) Th...
Figure 10.3. Diversity of urban soils (pseudo-natural vegetated soil, construc...
Figure 10.4. Urban agriculture sites are often located in direct proximity to ...
Figure 10.5. Urban soils, especially those used for agriculture, have higher t...
Figure 10.6. Springtails (soil mesofauna, i.e. smaller than 2 mm) play a role ...
Guide
Cover Page
Table of Contents
Title Page
Copyright Page
Introduction
Begin Reading
List of Authors
Index
WILEY END USER LICENSE AGREEMENT
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SCIENCES
Agronomy and Food Science, Field Directors – Jack Legrand and Gilles Trystram
Agronomy, Subject Heads – Yves Coquet, Marianne Le Bail, Jean Roger-Estrade
Agricultural Soil Science
Sustainable Management of Agricultural Soils
Coordinated by
Yves Coquet
Joël Michelin
First published 2025 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
www.iste.co.uk
John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA
www.wiley.com
© ISTE Ltd 2025The rights of Yves Coquet and Joël Michelin to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.
Library of Congress Control Number: 2024950286
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78945-185-6
Introduction
Yves COQUET and Joël MICHELIN
UMR ECOSYS, AgroParisTech, INRAE, Université Paris Saclay, Palaiseau, France
Toward the multifunctional usage of agricultural soils
Historically, the study of soils has been inseparable from that of agriculture. Backed by a utilitarian approach and the scientific renewal of the 18th century, its initial focus was on understanding the determining factors of agricultural soil fertility on the basis of its two founding disciplines: soil chemistry and agricultural hydraulics. The end of the 19th century saw the emergence of a naturalistic approach to soils with the birth of pedology. These two methods of soil study continued until the end of the 20th century, particularly in the teachings of the agricultural colleges in France. During the second half of the 20th century, the emergence of several environmental challenges brought to light the fact that soils have other functions as well as ensuring sufficient plant production. Today, soil is at the heart of new challenges, such as climate regulation, pollution control, water flow regulation, waste management and the maintenance of biodiversity. The broad-reaching nature of these issues has led soil science to evolve from a disciplinary approach divided into subdisciplines (soil chemistry, soil biology, pedology, etc.) to a truly trans-disciplinary approach, examining the interrelations between the different functions of the soil and drawing on other disciplines (ecology, economics, sociology, etc.). This evolution is reflected in an increased integration and globalization of knowledge regarding soils, supported by the use of systemic approaches and increasingly effective computer software.
In recent decades, training on soils in agricultural higher education programs have changed significantly, from an approach essentially centered on plant production capabilities (in agriculture and forestry) – heir to 19th century agricultural chemistry – to a multifunctional approach to soils. This paradigm shift owes a great deal to the work of ecologists, and in particular to the work carried out during the Millennium Ecosystem Assessment at the very beginning of the 21st century. Beyond the multifunctional approach of soils, this work has highlighted the many ecosystem services provided by soils, in addition to agricultural, forest and pasture production services. For the past 20 years, the evolution of soil-related training in higher education has followed this dynamic, particularly at AgroParisTech, France, with greater integration with other disciplines, more attention paid to functions other than ones related to agriculture and forestry production, and a more prominent role given to the assessment of ecosystem services provided by soils. The goal of this book is to illustrate this evolution, focusing on a limited number of themes.
Overview of the book
Chapter 1, addressing soil tillage and its impact on the structure of the soil, demonstrates the role played by plowing and the consequences of its elimination in the context of conservation agriculture. Chapter 2 focuses on the impacts of agricultural practices on soil biodiversity and the mechanisms for managing this biodiversity. Chapter 3 concerns the study of the spatial variability and the mapping of agricultural soils, with an update on the information available on soils in France and around the world, as well as examinations on the use of relatively recent tools such as drones. Chapter 4 is devoted to soil erosion in connection with agricultural activities and the techniques for remedying it through integrated watershed management. Chapter 5 presents the legal aspects of soil protection and agricultural land. Chapter 6 concerns the methods for estimating the agronomic value of soils, while Chapter 7 looks at the application of the notion of ecosystem services to soils. The last three chapters are devoted to specific types of soils: Mediterranean agricultural soils (Chapter 8), tropical soils (Chapter 9) and urban agricultural soils (Chapter 10). The chapters that comprise this work cover only a small part of the entirety of the topics relating to agricultural soil management. Rather, the goal is to demonstrate how some of these issues are now addressed within the framework of higher education as it is practiced today, and in particular at AgroParisTech and at Paris-Saclay University, where the majority of the contributors teach. We hope that these elements can be useful to all readers who wish to bring their knowledge up-to-date on the topics covered here, or simply to become familiar with them.
1Tillage and Structure of Agricultural Soils
Yves COQUET1 and Lionel ALLETTO2
1UMR ECOSYS, AgroParisTech, INRAE, Université Paris Saclay, Palaiseau, France
2UMR AGIR, INRAE, Université de Toulouse, Castanet Tolosan, France
1.1. What is tillage? Why should soils be “tilled”?
Soil tillage likely dates back to the same time as sedentarization of human societies during the Neolithic age, as the need arose to transform the natural environment to allow for agricultural activities. The first stage of this settlement process consisted of clearing or deforesting a given location, often accompanied by the burning of vegetation – a process which still occurs today in the form of “slash-and-burn” practices. Thus, it is conceivable that a major concern for Neolithic farmers was to maintain the gains derived from slash-and-burn techniques, namely, a “clean” plot – a term still used today by many farmers, referring to a plot where no vegetation grows other than what has been planted for cultivation. This brings to light one of the primary functions of tillage: to fight against weeds.
After clearing comes the sowing of the crops, and then the second function of tillage: the burying of seeds. Historically, the plow was used primarily to sow seeds (Sewell 1919; Sigaut 1977). The first tools for soil tillage were made from wood, some of which featured very simple designs, such as the use of simple sticks for burying seeds or the well-known African daba. These appeared at the same time as agriculture: the first appearance of ox-drawn plows dates back to 3000 BCE in Egypt and Mesopotamia. The first iron tools date back to the Roman Empire (around 200 BCE). Over time, the plow was gradually improved by adding elements such as wheels and moldboards, until the advent of the tractor around the middle of the 19th century, which increased the farmer’s capacity to work the soil tenfold.
Figure 1.1.Image showing the tillage process in ancient Egypt at the tomb of Sennedjem, 13th century BCE, Deir el Medineh, Egypt (source: The Yorck Project (2002)).
In today’s highly mechanized agriculture, tillage is no longer used to sow seeds in the ground. Instead, it is used to prepare the soil to facilitate the installation of the crop, burying the seeds directly in the prepared soil using such techniques being carried out by tools specially designed for sowing (seeders). The function of burying weeds continues, but also concerns the organic materials present on the surface of the soil, whether from the previous harvest – thus referred to as “crop residues”, such as grain stubble if it is not exported in the form of straw – or exogenous inputs, such as manure, slurry, composts and other organic waste products. Tillage makes it possible to incorporate these organic materials by mixing them over the full breadth of the surface layer of the soil being tilled, and thus to increase the organic matter content of this layer.
Finally, the fourth and final function of tillage is to modify the soil structure. Through their mechanical action on the soil, soil tillage tools increase and/or modify soil porosity. This porosity allows for better rooting of the plants and a better infiltration and circulation of the water in the soil. Tillage is a very effective technique for increasing the porosity of the soil by fragmenting the soil into structural elements (i.e. clods and aggregates) of various sizes. When these structural elements are too large in relation to the size of the planted seeds, the farmer may have to carry out “surface techniques”, consisting of an additional tilling at a level shallower than the plowing. This creates a “finer” soil structure, consisting of smaller structural elements, of sizes suitable for the seeds that have been sown. This is referred to as a “seedbed”. The objective of these tilling operations is to obtain a good soil structure (“a good tilth” as referred to by Kuipers (1963)), which will allow for the optimal development of the crop.
Figure 1.2.Mechanized plowing, Grignon plateau, 2002. Crop residues (corn) located on the surface of the soil are buried by tillage (photo credit: C. Coutadeur)
1.2. Soil structure
Soil structure describes the way in which the different particles that constitute the solid phase of the soil are arranged in physical space. These particles, whether of a mineral or organic nature, can vary greatly in size, ranging in scales from micrometers or smaller (as in the case of clays) to decimeters (coarse, “block”-type elements). The porosity of the soil is the space located between the soil particles. The size of the porosity can also vary to a large extent. The soil particles are connected to each other through forces of various different kinds. In particular, the bonds between mineral particles and organic particles play an important role in the determination and stability of the soil structure. Only part of the soil structure is visible to the naked eye (visual resolution is around 0.1 mm). This is the soil structure that is described by soil scientists or agronomists based on observation pits.
Figure 1.3.Left: profile of forest soil (Luvic Cambisol, Grignon park – soil thickness: 1 m) (photo credit: L.M. Bresson). Right: types of soil structure (1: granular; 2: crumb; 3: angular blocky; 4: subangular blocky; 5: prismatic; 6: columnar; 7: wedgelike; 8: platy) (source: Baize and Jabiol 2011).
The structure of natural soils is the result of their pedogenesis (soil formation process), which involves mechanisms of rock weathering, in conjunction with the climate, the topography and the living organisms that are present. Through cultivation, the farmer modifies the natural structure of the soil (Or et al. 2021) and, above all, gives it a much more marked temporal dynamic. The search for an “optimal” soil structure for a given crop or a given cultivation system has led to the question of how to objectify this structure, particularly by seeking to “measure” this structure. This has been a problem for a long time, and to this day, a satisfactory answer has not been found.
1.2.1. Porosity and bulk density
As mentioned earlier, one of the objectives of plowing is to increase the porosity of the soil. This porosity, P (as a fraction of volume), corresponds to the volume of pores contained within a given volume of soil. When a plow is passed, it cuts and fragments the soil into clods (structural elements larger than 2 cm) and fine earth (structural elements of a size less than or equal to 2 cm). The result is a “fluffy” soil surface level, with a high roughness (Figure 1.2) and whose average surface level is slightly higher than the initial average surface level.
Figure 1.4.Expansion effect of plowing on the soil surface layer. The average altitude of the soil surface (red dashed line) increases compared to the bottom of the plowed area (black solid line). The bulk density, ρb, of the plowed layer decreases.
The porosity of the surface layer is therefore increased by plowing. One indirect and very common way of measuring soil porosity consists of taking a known volume of soil (generally a cylinder) and measuring its dry mass (which is its mass after being dried in an oven at 105°C). The bulk density of the soil ρb (in kg.dm−3) can therefore be obtained by:
where M is the dry mass of the soil sample and V, the volume of the sampling cylinder. If the density of the soil solid phase ρs is known, the porosity can then be calculated as:
The density of the solid phase of a soil can be measured in the laboratory using a pycnometer, but for most soils, if they are not too organic, we can use the value of 2.65 kg.dm−3, which is the same as that of quartz or calcite. Therefore, a soil with a bulk density of 1.5 kg.dm−3 will have a porosity of about 43%. When plowing a 25-cm thick soil layer with an initial bulk density of 1.5 kg.dm−3, this results in a layer with a bulk density of 1.25 kg.dm−3; the height of the average soil surface will increase by 5 cm and the porosity by 10 percentage points (increasing from approximately 43% to 53%). Similarly, any compaction of the soil, whether by mechanical devices or trampling by animals, may under certain conditions, cause a local lowering of the height of the soil surface and an increase in the bulk density of the soil.
In fact, bulk density has been and remains an effective measurement of soil structure. However, it is a very crude measurement that provides little information. Indeed, two soils of the same bulk density can have very different pore size distributions. The bulk density does not make it possible to determine whether the porosity consists of large or small pores (Gupta et al. 1989), nor does it offer any information about their level of connectivity, which plays a major role in the circulation of fluids in the soil.
1.2.2. Soil structuring mechanisms
Since natural soils have a structure, we will distinguish between natural mechanisms and anthropogenic mechanisms, all of which lead to changes in the soil structure. The natural mechanisms are linked to the action of the climate and that of living beings. These include the physical processes of freezing–thawing and shrinkage–swelling, as well as phenomena such as slaking and hardsetting. The processes that involve biological agents are aggregation and bioturbation (including root growth). Mechanisms of anthropogenic origin include tillage and settlement by agricultural machinery. Settlement can also occur due to the trampling of animals and can be therefore natural in origin. However, this phenomenon is often accentuated by human activity (by increasing the load of animals per hectare, or by the introduction of ungulates, as has occurred on the Australian continent).
1.2.2.1. The freezing–thawing cycle
Freeze–thaw cycles are a very effective way of changing the soil structure from large structural elements to smaller structural elements (crumbling). This is a phenomenon that is well known to farmers who, in regions where frost is significant, make use of it to refine the plowing they carry out at the beginning of winter on plots whose soil is particularly rich in clay. This allows energy to be saved for the preparation of the seedbed for spring crops.
The fragmentation that occurs in freeze–thaw cycles is due to the significant expansion of water in the soil when it freezes. This is incommensurable with the thermal expansion coefficients of the other constituents of the soil. When water shifts to a solid state, its volume increases by 9%, implying a linear expansion coefficient of around 3%, while by contrast this coefficient is around 10−6 m.m−1.K−1 for the mineral constituents of the soil and 10−4 m.m−1.K−1 for liquid water. Thus, the expansion of water when it freezes creates very significant stresses inside the pore space of the soil, and leads to ruptures within the structural elements, which then fragment into smaller elements.
The effectiveness of the freeze–thaw cycles decreases rapidly with depth, in parallel with changes in temperature. Their fragmentation effect hardly exceeds the depth of tillage in temperate climates. The freeze–thaw cycles play a prominent role in the fragmentation process, more than the seasonal changes in soil temperature (the depth at which water pipes are buried to prevent freezing is much greater than that of the soil fragmentation effect).
1.2.2.2. Shrink–swell phenomena
The other family of processes that have significant effects in modifying the soil structure are shrink–swell phenomena. When a soil dries out, its solid-phase particles may move closer to each other due to the effects of capillary forces, which become more and more intense when it is dried. This is known as soil “shrinkage”. If the solid particles cannot move relative to each other (such as in the case of sandy soils), it is considered to be a “rigid” soil; otherwise, the soil is considered “deformable” (e.g. in the case of clay-rich Vertisol-type soils). Conversely, when the soil becomes moistened again, its volume may increase. This is referred to as the “swelling” of the soil.
Figure 1.5.Shrinkage curves of three soils (1: clay-rich soil – Vertisol; 2: loamy soil – Luvisol; 3: sandy soil – Ferralsol) (from Coquet 1996)
Figure 1.5 shows the linear shrinkage curve of three types of soil: a Vertisol (from New Caledonia), a Luvisol (from Grignon, France) and a Ferralsol (from Senegal). These shrinkage curves were obtained by measuring the variation in diameter of a cylindrical soil sample while it was dried. The Vertisol, which contains swelling clays, has a linear shrinkage coefficient on the order of 17%, the Luvisol has a coefficient on the order of 5%, while the Ferralsol can be considered to be a rigid soil (linear shrinkage coefficient of <0.2%). When it dries, a Vertisol presents dramatic cracks, which close when it is re-humidified. Considerable pressure forces can appear along the faces of these closed cracks, generating slickensides that are typical of Vertisols (see item 7 in Figure 1.3). The magnitude of the shrink–swell phenomenon is much lower for loamy soils (such as Luvisols), but can nevertheless give rise to blocky or prismatic structures (3–5 in Figure 1.3). Since the changes in water content may affect great soil depths (particularly under forests), the shrink–swell changes concern the entire profile of the soil, unlike the freeze–thaw changes.
Figure 1.6.Changes in bulk density of the tilled horizon of a remolded loamy soil during shrink–swell cycles for different initial bulk densities (as given by Kuznetsova and Danilova 1988)
1.2.2.3. Slaking and hardsetting
In some particular cases, the moistening of a soil can lead not to swelling, but rather to a reduction in the soil’s porosity, and therefore its apparent volume. This reduction is particularly striking in the case of the “hardsetting” of the soil. This hardsetting phenomenon is well known in the case of irrigation of salty clay soils. The abundance of sodium on the exchangeable complex of clays combined with the dilution occurring due to irrigation water leads to the dispersion of clay particles and a total loss of the soil structure. This phenomenon can also be observed to a lesser extent in the case of predominantly loamy soils (Bresson and Moran 2004) or even sandy soils (Lamotte et al. 1997) under specific conditions. The result of this hardsetting is a soil without any macrostructure, appearing as a continuous volume, which is difficult for water to penetrate.
Figure 1.7.Cross-section of the surface of a loamy soil with a slaked crust (photo credit: C. Roth).
Slaking also causes the soil structure to disappear, but over a very limited depth of soil (up to a few millimeters), at the surface of the soil, under the effects of rain. This phenomenon is characteristic of loamy soils with a low clay content (<15%) and organic matter content. In the phenomenon of slaking, several mechanisms can play a role. These include the physical impact of raindrops when they fall on the surface of the soil, the bursting of the aggregates when the water suddenly penetrates them under the effects of capillary forces where air is unable to escape freely from the aggregates (dead-end pores), and the dispersion of clay particles.
In the case of slaking as well as in the case of hardsetting, the tillage of the soil must be accompanied by other corrective actions (such as the addition of organic matter to combat slaking or calcium, in the form of carbonates or sulfates, to avoid hardsetting) if it is desired for the soil to “recover” an adequate structure that will be relatively stable over time.
1.2.2.4. Aggregation and bioturbation
Living beings play a decisive role in the structuring of soils. Aggregation refers to the phenomenon in which the soil attains a structure that is visible to the naked eye, where it is possible to distinguish “aggregates”, or structural elements, that vary in size (from a few tens of micrometers to a few millimeters), within which organic matter is closely bound to the mineral matter. The bonds between clay minerals and organic molecules, in particular those secreted by soil microorganisms, are particularly important in determining soil aggregation. For its part, bioturbation refers to the mixing effect of the solid phase of the soil resulting from the activities of the soil fauna, particularly earthworms.
In practice, it is difficult to distinguish aggregation and bioturbation effects, as the two phenomena frequently occur simultaneously, particularly near the surface of soils. Apart from castings from earthworms, particularly those left on the surface of the soils by anecic worms, bioturbation effects are rather visible in soil micromorphology, that is, through observing thin soil slides with a microscope, which allows for the identification of “papules” (bioturbation remains) within the soil (Sauzet et al. 2016). Vegetation also plays a very important role in the structuring of the soils, both through the mechanical actions due to the growth of the roots in the soil, as well as by stimulating the microbial processes by the contribution of organic materials by the roots (exudates or root decomposition).
1.2.2.5. Compaction
With the development of mechanized agriculture, the machines used for field work (tractors, combines, trailers for harvesting, etc.) have continuously improved, with the machines gaining in size and weight. This has given rise to an increase in the risk of soil compaction (or settlement), or even effects seen in the subsoil (in the agronomic sense, meaning the soil layer located under the tilled layer). Some authors consider that the increase in soil compaction may be one of the explanations for the stagnation in the yields observed in Europe since the end of the 1990s (Keller et al. 2019).
Figure 1.8.Soil compaction curve as determined by the Proctor test (according to D. Tessier)
Soil compaction depends on the mechanical stresses applied to it (pressure, shear), as well as on its own mechanical characteristics (shear strength and deformation capacity), which are in turn dependent on the properties and the state of the soils at the time when they are subjected to the stresses. In general, a sandy soil is much less sensitive to compaction than a loamy or clay-rich soil (O’Sullivan and Ball 1993; Unger and Kaspar 1994).
The soil water status at the time the stresses are applied is an essential factor in the compaction. A dry clay soil will be insensitive to compaction, while it will be much more sensitive if it is wet. In geotechnics, where a civil engineer seeks to obtain soils that are as stable as possible (and therefore as compacted as possible), the Proctor test makes it possible to define the optimal water content, which will enable the maximum bulk density to be achieved during compaction (Figure 1.8). This is exactly what farmers seek to avoid when they work their fields with agricultural machinery.
Soil compaction can also occur due to trampling by some of the heavier animals. Cattle and equine animals can generate stress levels of several hundred kPa under their hooves, stresses which are equivalent to (or greater than) those exerted by the heaviest machines in mechanized agriculture.
1.2.2.6. Soil tillage
The phenomena of slaking, hardsetting and above all compaction lead to a decrease in soil porosity and to a change in soil structure, which can pose a problem for farmers. When a slaked top layer is dry, this can form an impenetrable obstacle for sprouting plants, and thus prevent crops from emerging. For its part, soil compaction can seriously limit the rooting of the crop (Unger and Kaspar 1994). In order to recover a structural state of the soil surface layer that is favorable to the development of crops, farmers carry out tillage, which can be done in several stages using different tools.
The deepest type of tillage (with the exception of subsoiling) is plowing (Figure 1.2). This type of plowing has various objectives (see section 1.1), including that of recreating porosity inside the soil (Figure 1.4). In Europe, plowing is generally carried out using a moldboard plow (Figure 1.2), which makes it possible to cut a strip of soil and rotate it from 130° to 160°. By contrast, plowing in North America is carried out using a disc plow, which is less efficient than the moldboard plow in rotating the soil, but more efficient in mixing it. Currently, the depth of plowing in Europe is on the order of 20–25 cm, but in the past, it was possible for significantly deeper plowing to be carried out (of up to 35 cm or more). A fairly deep tillage can be carried out using tined tools, such as cultivators, which are used in particular for post-harvest stubble removal of grain straw. Rotary cultivators make it possible to obtain a more homogeneous relief of the soil surface than a cultivator with fixed tines.
Figure 1.9.Tillage tools: (a) front cultivator; (b) rotary harrow; (c) subsoiler (photo credits: Pottinger, Quivogne).
In general, the soil structure created by plowing is not suitable for seeding a crop immediately after plowing. As indicated previously (section 1.2.2), the freeze–thaw and wet–dry cycles can contribute significantly to the fragmentation of large structural elements (clods) created by plowing and to a “finer” structure being obtained (that is to say, one consisting of smaller structural elements). An alternative (or complementary) solution to the effects of the climate consists of carrying out a “resumption” of the plowing using another tool that works at a shallower depth, which will make it possible to obtain a finer and above all more homogeneous structure. Various different tools, such as pulverisers (or “cover-crops”) or harrows (fixed, alternating, or rotating), make it possible to effectively fragment the upper part of the plowed layer. This surface tillage of the soil can be completed using a roller, which will “recompact” the soil surface and thus promote better contact between the soil and the seeds that have been planted. The degree of fragmentation and settlement of the seedbed will depend on the size of the seed; for instance, a corn seedling will not need a seedbed as thin as a rapeseed seedling.
Certain tools, such as the cultivator (rotary harrow), are designed to remedy slaking when it appears. This is a superficial tillage (2–3 cm) that seeks to fragment the slaked crust and recreate the circulation of fluids at the surface of the soil. This tillage can also allow sprouts to rise if the slaked crust has formed immediately after seeding.
The last major category of tillage tools has the explicit objective of counteracting soil compaction. This category includes decompactors, which plow the soil to the tillage depth and can be used to decompact the soil after a harvest carried out in bad conditions (wet soil), which has led to soil compaction, and subsoilers, deeper tools, whose objective is to decompact the soil under the plowed layer.
As this section makes clear, there are a wide variety of tillage tools, which have only briefly been touched upon here. These tools are typical of highly capitalistic agriculture, since all such tools have a high cost and require large amounts of energy to be used. They are one of the factors that explain the tremendous productivity gains made by agriculture during the 19th and 20th centuries.
1.2.3. A method for characterizing the structure of cultivated soils: the “Profil cultural”
We have seen that soil structure can be quantified in a rudimentary fashion by measuring the soil’s bulk density (section 1.2.1). This measurement is generally carried out on a volume of soil that barely exceeds 1 dm3 and cannot account for the spatial and temporal variability of the structure of cultivated soils without multiplying the measurements over time and in space (Strudley et al. 2008). The original idea that the agronomist Hubert Manichon had for studying the structure of cultivated soils while preparing his thesis (Manichon 1982) was to observe the soil, using the methods of a soil scientist, but focusing on the effects of the different tools used during crop succession (Manichon 1987). The observation was carried out from a pit that was excavated perpendicular to the direction of movement of agricultural machinery on the plot, wide enough to include, at a minimum, the greatest working width of the machinery and a depth greater than the maximum depth of the soil to be tilled.
Figure 1.10.Crop profile of a plot where corn has been sown (Grignon, France). H1: seedbed; H5: plowed layer not taken up by surface tillage; P1: soil under the tillage. L2: Path of the wheels of the tractor pulling the harrow for the creation of the seedbed; L3: area not affected by the wheel paths (as given by Desbourdes-Coutadeur 2002)
A compartmentalization of the soil profile is first carried out, taking into consideration a vertical compartmentalization according to the depths of the different tillage tools. We may distinguish certain elements such as (given in order from the surface) the seedbed, the plowed layer not modified by superficial tillage, the plow pan, and the soil that is not tilled. Additionally, it also considers a lateral compartmentalization on the basis of the location of the paths of machines’ wheels, which can generate soil compaction. The soil structure within each compartment is then described according to a purely morphological approach. The large structural elements (clods) are described geometrically and according to their “internal state”. A state noted as “Γ” is distinguished, representing a soil with a macroporosity clearly visible to the naked eye, as well as a condition noted as “Δ”, corresponding to a compacted soil without macroporosity, and a state as “Φ”, corresponding to a state Δ transformed by the shrink–swell and freeze–thaw phenomena and with a more advanced state of cracking. Structural elements with a size of less than 2 cm are classified as “fine earth”.
Figure 1.11.The three internal structural states of the Profil cultural: (a) state Γ, (b) state Δ and (c) state Φ (as given by Desbourdes-Coutadeur 2002).
The description of the soil profile that is tilled according to the Profil cultural method makes it possible to situate the heterogeneity of the soil structure observed in relation to the various cropping actions (plowing, sowing, introduction of inputs, harvesting, etc.) and thus to carry out a diagnosis on the cultivation system and its impact on the soil structure. The proportion of state Δ has proven to be an interesting indicator for qualifying the structure of an agricultural soil, given the adverse impact of compaction on the development of crops (Roger-Estrade et al. 2000). It should be noted that the Profil cultural method, initially developed for cultivation systems that include plowing, has been adapted for systems that exclude plowing (Boizard et al. 2017). A number of methods that are “lighter” to implement have been developed from the Profil cultural method, in particular, the spade test, and more recently, the 3D profile method (Tomis et al. 2019).
1.3. Consequences of the different methods for tilling soil
In general, it is extremely difficult to demonstrate the systematic effects of different tillage practices. This can be explained by, on the one hand, the great diversity of tillage tools and the methods for using them, and, on the other hand, the diverse array of pedoclimatic situations – the same tillage tool can be used identically and have different effects depending on soil texture, its water status at the time when the tool is used, or its organic matter content. In addition, these effects can vary broadly in space and in time (Strudley et al. 2008; Alletto et al. 2010), making it difficult to clearly demonstrate their effects through measurements, which must take into account such differences. The “Profil cultural” method is an elegant possibility for giving consideration, at least partially, to the spatial component of these variations (see section 1.2.3).
For the rest of this chapter, we will narrow our focus to two main methods of tillage, which are widely studied in the literature and whose major trends can be identified: first, cultivation systems in which the tillage is achieved with a moldboard plow – characteristic of “conventional” agriculture as practiced in France; and second, “no-till” systems, which are typical of so-called “conservation” agriculture. It should be noted that conservation agriculture is characterized not merely by tillage-free agriculture, but also includes the permanent presence of a soil cover (whether in the form of crops or their residues) and a more elaborate crop rotation than in conventional agriculture (featuring in particular a diversification of the species that are cultivated).
1.3.1. Systems that include tillage (“conventional” agriculture)
In section 1.1, we made reference to the different functions of tillage. One of these is to increase soil porosity. This increase in porosity represents a decrease in the soil’s bulk density (section 1.2.1), but what are the functional components of this porosity? Figure 1.12 shows the water retention characteristics of the soil (a), which represents the volumetric water content of the soil in relation to the matric potential of the water, expressed in pF, and the hydraulic conductivity characteristics of the soil (b), which relates its hydraulic conductivity to the matric potential. The pF is the base-10 logarithm of the absolute value of the matric potential, expressed in hPa. The matric potential of water in the soil quantifies the energy with which water is retained by the soil. This is a negative value, with the value of zero representing free water. The lower the matric potential is (and thus the more positive the pF is), the higher the strength of the water retention by the soil.
In the case of the soil studied (silt loam soil in Grignon), the tillage increases the water retention of the soil; at a given pF value, excluding the values at 1 and 4.2, the volumetric water content of the tilled soil is higher than that of the non-tilled soil (under the tilled layer). On the other hand, there is hardly any difference between the tilled layer not affected by the surface tillage (in this case, carried out using a rotary harrow) and the seedbed. At 4.2 pF, the permanent wilting point of the plants, the water content of the tilled soil is identical to that of the non-tilled soil. On the other hand, the water content at the field capacity, which can be located between 2 and 2.5 pF, is higher in the tilled soil than in the non-tilled soil. As a result, the tillage has a slightly positive effect on the reserve of water usable by the plants, arising from the increase in porosity that results from the plowing.
Figure 1.12.Hydraulic properties of a silt loam soil seeded with corn (April 1999). (a): water retention characteristics; (b): hydraulic conductivity characteristics close to saturation (according to Desbourdes-Coutadeur 2002)
Figure 1.13.Temporal dynamics of the saturated hydraulic conductivity at different depths for two types of cultivation systems (Conv: conventional agriculture with plowing; CA: conservation agriculture without plowing) applied to two soils (boulbènes soils, with a rich topsoil) from the Southwest of France (according to Alletto et al. 2022)
The effect of tillage on hydraulic conductivity (Figure 1.12(b)) is different from its effect on water retention. On average, the hydraulic conductivity of the seedbed is eight times higher than that of the plowed layer and three times higher than that of the non-tilled soil, except at saturation (zero matric potential). For the plowed layer, the hydraulic conductivity is three times lower than that of the non-tilled soil. In fact, plowing tends to reduce the continuity of the macropores present in the non-tilled soil (such as holes from roots, earthworms, etc.), which strongly impacts the ability of the soil to conduct water when the soil is close to saturation (pF 1 represents a matric potential of −1 kPa). The superficial tillage carried out for the seedbed production fragments and homogenizes the tilled soil, making it possible to attain a good hydraulic conductivity close to saturation, which will facilitate the infiltration of water and exchanges with the atmosphere.
The effects of tillage on the porosity and on the hydraulic properties of soils most often have a transient effect, which can last from a few weeks to a few months, depending on the characteristics of the soil, the climate and agricultural practices. In the case of the boulbène soils from Southwest France (Figure 1.13), which are loamy soils whose structure is particularly unstable, the increase in hydraulic conductivity of the soil arising from plowing is clearly visible in the spring, but quickly fades from the summer and is completely gone by the fall.
Tillage, when carried out in relatively humid conditions, can lead to a compaction of the soil located just below the plowing depth, over a thickness of a few centimeters. This layer, called the “plow pan”, results both from the compaction under the tractor tires located at the bottom of the hollows opened by the plow (see the tire on the right in Figure 1.2) and from a smoothing effect at the level of the plow(s). This plow pan generally has a much higher bulk density and penetration resistance than non-tilled soil (Jeřábek et al. 2017) and can generate subsurface flows on steep slopes (Bertolino et al. 2010).
1.3.2. No-till systems (“conservation” agriculture)
Conservation agriculture (the term implied to refer to “soil conservation”) is based on the desire to protect soils from damage caused by erosion, whether from water or wind. Historically, it was initially developed in the United States and in South America. It has proven to be particularly well suited to tropical soils, which are often poor in organic matter and have low structural stability. In addition, the low content of swelling clays in these soils makes it difficult to naturally recover a favorable structure after compaction. Therefore, the compaction of these soils tends to persist and generate intense runoff, and consequently, water erosion.
Cultivation systems in conservation agriculture imply a lack of tillage (thus the name “no-till”) or a very limited tillage (tillage only on the seeded rows) and/or a simplified tillage (with the use of a single tool, such as a seeder adapted for direct seeding). They also imply a permanent covering of the soil by crop residues or service plants and diversified cash crops (Scopel et al. 2013; Palm et al. 2014). The presence of a permanent ground cover makes it possible to greatly reduce the risks of erosion. This soil cover often combines a mulch, consisting of the residues of the previous crop, with the crop in place, whether it is a service crop (such as an intermediate nitrate trap crop) or a cash crop (Figure 1.14).
By creating a rough soil surface, it allows runoff water to be slowed down. In addition, no-till techniques allow for the settlement and development of macrofauna, particularly earthworms, as well as the establishment of root galleries, which will last over time (Figure 1.15). This results in an increased porosity that is biological in origin and presents a much better connectivity than in the case of plowed soils. Therefore, although the bulk density of the soil is generally higher in conservation agriculture than in conventional agriculture, the infiltration capacity of the soil using this technique is often significantly greater (Figure 1.13).
Figure 1.14.Soybeans grown using direct seeding in grain straw residues on a plot using conservation agriculture in Gers (photo: L. Alletto)
On the other hand, no-till techniques lead to a very different distribution of organic matter between the two systems. As plowing results in the homogenization of the tilled layer’s organic carbon content, the soil surface content is generally lower than in a non-plowing situation, where the crop residues are not buried (Figure 1.16). Therefore, the soil surface’s organic carbon content is generally greater in conservation agriculture than in conventional agriculture (Figure 1.15), which gives better structural stability to the soil surface and therefore better resistance to the erosive effect of runoff.
Figure 1.15.Block of soil extracted with a garden spade from a plot using conservation agriculture. The coloring gradient of organic matter and the macroporosity created by biological activity are clearly visible (photo: L. Alletto).
Figure 1.16.Relative difference in organic C content between no-till (AC) and plowing (CONV) for 237 pairs of plots in 47 sites around the world (according to Angers and Eriksen-Hamel 2008).
During the conversion of a cultivation system from conventional agriculture to conservation agriculture, a progressive densification of the soil layer initially tilled can be observed. The term “densification” is used instead of compaction, because this process is observed without any settlement taking place due to agricultural machinery or by the trampling of animals. The agricultural performance of the plots converted to conservation agriculture is often lower and more variable than before conversion during a so-called “transition” period, whose duration varies depending on the type of soil, its initial situation and the agricultural practices implemented. In certain situations, the densification of the soil that occurs in conservation agriculture may require tillage for cracking the soil (using subsoiling, for example, see section 1.2.2.6) in order to recreate porosity, particularly at greater depths.
1.4. Conclusion
Tillage has always been an important component of agricultural activity. Today, given the need to reduce the impact of agriculture on the climate, the evolution of agricultural systems toward techniques with lower energy-consumption appears to be a positive development. From this point of view, the reduction of tillage is notable. By foregoing plowing operations, 15–50 L of diesel per hectare can be saved. But there are also drawbacks to no-till techniques, especially in terms of weed management. In many cases, techniques that do not use plowing give rise to difficulties in controlling weeds, which can no longer be buried. This results in an increased use of herbicides, particularly glyphosate, which has the advantage of being a very effective total herbicide whose agronomic effect is short-lived, and which is used on plots where weed growth is out of hand. Insofar as the European Union wishes to drastically reduce the use of pesticides in agriculture, it is to be expected that this will give rise to a clash that will have to be resolved in the future by agricultural research, which is more grounded in biological regulations and a better understanding of the relationships between soils and plants.
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