Integrated Assessment of Soil Structural Quality Mansonia Alejandra Pulido Moncada Promoters Examination Board and Reading Committee Prof. Dr. ir. Wim Cornelis Prof. Dr. ir. Ann Verdoodt Em. Prof. Dr. ir. Donald Gabriels Department of Soil Management Department of Soil Management Faculty of Bioscience Engineering Faculty of Bioscience Engineering Ghent University Ghent University Prof. Dr. ir. Geert Baert Prof. Dr. Deyanira Lobo Department of Applied Biosciences Departamento de Edafología Faculty of Bioscience Engineering Facultad de Agronomıía Ghent University Universidad Central de Venezuela Dr. Lars Munkholm Dean Department of Agroecology Prof. Dr. Ir. Guido Van Huylenbroeck Aarhus University Faculty of Bioscience Engineering Dr. Bruce Ball Ghent University Crop and Soil Systems Research Group Rector Scotland’s Rural College Prof. Dr. Anne De Paepe Prof. Dr. ir. Eric Van Ranst Ghent University Department of Geology and Soil Science Faculty of Sciences Ghent University Prof. Dr. ir. Monica Höfte Chairman Department of Crop Protection Faculty of Bioscience Engineering Ghent University MSc. Mansonia Alejandra Pulido Moncada Integrated assessment of soil structural quality Thesis submitted in fulfilment of the requirements for the degree of Doctor (PhD) in Applied Biological Sciences: Land and Water Management Dutch translation of the title: Geïntegreerde evaluatie van de kwaliteit van de bodemstructuur Please refer to this work as: Pulido Moncada, M., 2014. Integrated assessment of soil structural quality. PhD Thesis. Ghent University, Belgium. ISBN-number: 978-90-5989-750-2 The author and the promoter give authorisation to consult and to copy parts of this work for personal use only. Every other use is subject to the copyright laws. Permission to reproduce any material contained in this work should be obtained from the author. To the most wonderful people in my life, My Family How is the soil? Is it fertile or poor? Are there trees in it or not? Do your best to bring back some of the fruit of the land. Numbers 13:20, New International Version It had been planted in good soil by abundant water so that it would produce branches, bear fruit and become a splendid vine. Ezekiel 17:8, New International Version Acknowledgments The author is grateful to the Universidad Central de Venezuela for the four-year grant from the ‘Beca Sueldo Exterior’ programme of the Consejo de Desarrollo Científico y Humanístico (CDCH). I also thank the Ghent University for its financial support, which made this research possible. Special thanks are given to the Doctoral School of the Ghent University for financial support for following a specialist course at Aarhus Universit (Denmark), transferable skills seminars and courses at Ghent University, as well as conference expenses, as part of the minimum set of activities within the Doctoral Training Programme. I am also grateful to the Scientific Research Committee (CWO) of the Faculty of Bioscience Engineering, Ghent University, for granting me to conduct an internship at the Scotland’s Rural College (UK). I am thankful to my research project supervisor for his professional guidance and valuable support and to co-supervisors, members of the examination board and reading committee, co-authors, colleagues and anonymous reviewers for their useful and constructive recommendations and amendments to the manuscripts published and chapters of this dissertation. I would also like to extend my thanks to the technicians of the laboratories of the Soil Management Department and the Department of Geology and Soil Science of Ghent University as well as those of the Instituto de Edafología of Universidad Central de Venezuela for their help in the analyses and their technical assistance. As well as those colleagues and friends who very kindly helped me during the sampling campaigns. Special thanks to my family for their love, prayers and unconditional support during this period of my life. Furthermore, many thanks go to the persons who became my friends during these four years and have blessed me with a wonderful friendship. My deepest gratitude is for God for directing each step of my life. Without Him, I do nothing. List of contents List of figures i List of tables v List of abbreviations ix Chapter 1 Introduction 1 1.1. Land degradation: deterioration of soil quality 1 1.2. Soil quality and soil quality indicators 2 1.3. Soil physical quality: deterioration of soil structure as a 2 common factor 1.4. Soil structure: Concepts and importance 3 1.5. Assessment of soil structure 5 1.5.1. Assessing aggregate stability 6 1.5.2. Assessing soil structural form 8 1.5.2.1. Methods based on soil profile evaluation 10 1.5.2.2. Methods based on topsoil examination 12 1.6. Linking and interpreting soil structural quality using an 15 integrated framework 1.7. General and specific objectives 16 1.8. Outline of the dissertation 17 Chapter 2 Selection of study sites 19 2.1. Introduction 19 2.2. Selection of study sites and soils 20 2.2.1. Tropical environment: central-northern part of Venezuela 21 2.2.2. Temperate environment: Flanders Region of Belgium 26 Part I Soil physical quality assessment based on aggregate stability 29 Chapter 3 The influence of wet sieving methods on soil physical quality 31 assessment 3.1. Introduction 31 3.2. Materials and Methods 32 3.2.1. Soils description and soil sampling 32 3.2.2. Saturated hydraulic conductivity, soil water release curve 33 and soil bulk density 3.2.3. Aggregate stability 33 3.2.4. Structural stability index 36 3.2.5. Statistical data analysis 36 3.3. Results 36 3.3.1. Comparison of methods for measuring aggregate stability 36 3.3.2. Association of aggregate stability with other soil physical 42 quality indicators 3.4. Discussion 44 3.5. Conclusions 47 Chapter 4 Soil organic matter and its fractions as indicators of soil structural 49 stability 4.1. Introduction 49 4.2. Materials and Methods 50 4.2.1. Site description and soil sampling 50 4.2.2. Soil organic matter analysis 50 4.2.3. Chemical fractionation of soil organic matter 51 4.2.4. Physical fractionation of soil organic matter 51 4.2.5. Clay mineralogy analysis 52 4.2.6. Aggregate stability determination 53 4.2.7. Statistical analyses 53 4.3. Results 53 4.3.1. Chemical characterization of soil organic matter 53 4.3.2. Physical fractionation of soil organic matter 54 4.3.3. Clay mineralogy related to soil organic matter fractions 55 dynamic 4.3.4. Soil organic matter and its interaction with soil aggregate 55 stability 4.4. Discussion 57 4.4.1. Distribution of soil organic matter over different fractions 57 4.4.2. Relationship between soil organic matter and aggregate 58 stability 4.5. Conclusion 61 Chapter 5 Is the mineral composition of the clay fraction of aggregates an 63 indicator of aggregate stability? 5.1. Introduction 63 5.2. Materials and Methods 64 5.2.1. Study site and data collection 64 5.2.2. Aggregate size fractionation 65 5.2.3. Extraction of the clay fraction and the X-ray diffraction 65 analysis 5.2.4. Data analysis 66 5.3. Results 66 5.3.1. Aggregate size distribution in the studied soils 66 5.3.2. Mineral particle size distribution and organic carbon 67 concentration within the aggregate sizes 5.3.3. Mineral composition of clay fraction among aggregate 69 sizes 5.3.4. Mineral composition of clay fraction among aggregate 76 fractions obtained by two different aggregate stability methods 5.4. Discussion 77 5.4.1. Does the mineralogical composition of the clay fraction 77 vary among aggregate sizes? 5.4.2. Are the results of mineral distribution of the clay fraction 80 among the aggregate sizes influenced by the method applied for aggregate stability determination? 5.4.3. Are mineralogical indicators capable of evaluating 83 aggregate stability as a key aspect of structural quality? 5.5. Conclusion 83 Part II The use of visual examination methods for assessing soil structural 85 quality Chapter 6 The performance of visual examination methods in ‘tropical’ soils 87 6.1. Introduction 87 6.2. Materials and Methods 88 6.2.1. Soil sampling 88 6.2.2. Visual soil structure assessment 88 6.2.3. Soil physical analysis 94 6.2.4. Data analysis 94 6.3. Results 94 6.3.1. Soil structural quality as evaluated by different visual field 94 assessment 6.3.2. Overall assessment of each soil 100 6.3.3. Relationships between the visual field assessment scores 101 and indicators of soil physical quality 6.4. Discussions 104 6.4.1. Comparison of soil quality classification 104 6.4.2. Validity of the methods based on their relationships with 105 soil physical properties of soil structure 6.4.3. Adjustment of the visual assessments for ‘tropical’ soils 106 6.5. Conclusions 107 Chapter 7 Validation of morphological approaches for assessing soil structural 109 quality 7.1. Introduction 109 7.2. Materials and Methods 111 7.2.1. Soil sampling 111 7.2.2. Soil physical and hydraulic properties for assessing soil 111 structural quality 7.2.3. Visual examination of the soil structural quality 113 7.2.4. Visual soil structural quality assessment based on type of 113 aggregates 7.2.5. Visual soil structural quality assessment based on 113 aggregate stability 7.2.6. Data analysis 114 7.3. Results 115 7.3.1. Comparison of soil structural quality evaluated by visual 115 examinations and by soil physical properties 7.3.2. Comparison of soil structural quality evaluated by visual 117 type of aggregates index and by water flow 7.3.3. Comparison of soil structural quality evaluated by water 119 aggregate stability 7.3.4.