I S S N 0 2 5 3 - FAO 2 0 8 5 0 0 SOILS 80 BULLETIN FAO SOILS BULLETIN 80 The importance of soil organic matter The importance of Key to drought-resistant soil soil organic matter T h e and sustained food production i Key to drought-resistant soil m p o r and sustained food production t a n c e o Soil organic matter – the product of on-site biological f s o decomposition – affects the chemical and physical i l o r properties of the soil and its overall health. Its composition g a n and breakdown rate affect: the soil structure and porosity; i c the water infiltration rate and moisture holding capacity m a t t of soils; the diversity and biological activity of soil e r organisms; and plant nutrient availability. This Soils — K e Bulletin concentrates on the organic matter dynamics y t o of cropping soils and discusses the circumstances that d r deplete organic matter and their negative outcomes. o u g It then moves on to more proactive solutions. It reviews h t - r a “basket” of practices in order to show how they can e s i s t increase organic matter content and discusses the land a n t and cropping benefits that then accrue. s o i l a n d s u s t a i n e d f o o d p r o d u c t i o n ISBN 92-5-105366-9 ISSN 0253-2050 F 978 9 2 5 1 0 5 3 6 6 9 A TC/M/A0100E/1/10.05/1400 O Cover photographs: Background: Honduras. FAO/18884/G. Bizzarri. Inset: Honduras. FAO/18907/G. Bizzarri. Copies of FAO publications can be requested from: SALES AND MARKETING GROUP Information Division Food and Agriculture Organization of the United Nations Viale delle Terme di Caracalla 00100 Rome, Italy E-mail: [email protected] Fax: (+39) 06 57053360 Web site: http://www.fao.org FAO SOILS The importance of BULLETIN soil organic matter 80 Key to drought-resistant soil and sustained food and production by Alexandra Bot FAO Consultant and José Benites FAO Land and Plant Nutrition Management Service FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 2005 The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. ISBN 92-5-105366-9 All rights reserved. Reproduction and dissemination of material in this information product for educational or other non-commercial purposes are authorized without any prior written permission from the copyright holders provided the source is fully acknowledged. Reproduction of material in this information product for resale or other commercial purposes is prohibited without written permission of the copyright holders. Applications for such permission should be addressed to: Chief Publishing Management Service Information Division FAO Viale delle Terme di Caracalla, 00100 Rome, Italy or by e-mail to: [email protected] © FAO 2005 iii Contents Acknowledgements viii Preface ix List of acronyms xi 1. Introduction 1 2. Organic matter decomposition and the soil food web 5 Soil organic matter 5 The soil food web 5 Decomposition process 7 Non-humic substances: significance and function 8 Compounds and function of humus 8 3. Natural factors influencing the amount of organic matter 11 Temperature 11 Soil moisture and water saturation 12 Soil texture 12 Topography 13 Salinity and acidity 13 Vegetation and biomass production 13 4. Practices that influence the amount of organic matter 15 Human interventions that influence soil organic matter 15 Practices that decrease soil organic matter 15 Decrease in biomass production 15 Replacement of perennial vegetation 15 Replacement of mixed vegetation with monoculture of crops and pastures 16 High harvest index 16 Use of bare fallow 17 Decrease in organic matter supply 17 Burning of natural vegetation and crop residues 17 Overgrazing 17 Removal of crop residues 17 Increased decomposition rates 18 Tillage practices 18 Drainage 19 Fertilizer and pesticide use 19 Practices that increase soil organic matter 19 Increased biomass production 20 Increased water availability for plants: water harvesting and irrigation 20 Balanced fertilization 21 Cover crops 21 Improved vegetative stands 22 iv Agroforestry and alley cropping 23 Reforestation and afforestation 26 Regeneration of natural vegetation 27 Increased organic matter supply 27 Protection from fire 27 Crop residue management 27 Utilizing forage by grazing rather than by harvesting 28 Integrated pest management 28 Applying animal manure or other carbon-rich wastes 28 Compost 29 Mulch or permanent soil cover 29 Decreased decomposition rates 30 Reduced or zero tillage 30 5. Creating drought-resistant soil 35 Effect of soil organic matter on soil properties 35 Inefficient use of rainwater 35 Increased soil moisture 35 Reduced soil erosion and improved water quality 38 6. Key factors in sustained food production 41 Increased plant productivity 41 Increased fertilizer efficiency 41 Reduced waterlogging 42 Increased yields 42 Reduced herbicide and pesticide use 44 Increased biodiversity 45 Resilience 46 7. The role of conservation agriculture in organic matter deposition and carbon sequestration 47 Principles of conservation agriculture 47 Organic matter deposition 47 Increased carbon sequestration 48 8. Conclusions 51 Bibliography 53 Annexes 1. Soil organisms 61 2. Effects of organic matter on soil properties 71 v List of boxes 1. Some functions of a healthy soil ecosystem 6 2. Humic substances retain nutrients available on demand for plants 9 3. Ways to increase organic matter contents of soils 20 4. Planting pits 23 5. Examples of agroforestry systems worldwide 24 6. Farmers' perceptions of the Quezungual system: benefits and disadvantages 25 7. Mulching in the highlands of northern Thailand 31 8. Does improved organic matter management pay? 43 9. Effect of different tillage practices on scarab beetle-grub holes and their volumes 45 vi List of figures 1. Components of soil organic matter and their functions 1 2. Carbon cycle 5 3. Fluctuations in microbial biomass at different stages of crop development in conventional agriculture compared with systems with residue retention and high organic matter input 7 4. Closed cycle 11 5. Composition of leaves and roots of leguminous and grass species 14 6. Open cycle system 15 7. Composition of soil macrofauna under primary forest, fallow, kudzu and grass vegetation 16 8. Evaluation of the organic matter content of a soil in Paraná 20 9. Reduction of dry matter of different cover crops 22 10. Population size of root nodule bacteria with different crop rotations 32 11. Live and dead earthworm numbers per square metre at 0–15 cm of soil depth sampled immediately after treatment 33 12. Effect of amount of soil cover on rainwater runoff and infiltration 36 13. Water infiltration under different types of management 36 14. Quantity of water stored in the soil under conventional tillage and conservation agriculture 36 15. Effect of different soil covers on in-soil storage of water 37 16. Soil loss due to water erosion for different maize cropping systems 38 17. Development of water turbidity rates in the catchment area of Rio do Campo, Paraná, 1982–1999 39 18. Temperature fluctuations at a soil depth of 3 cm in a cotton crop with and without a soil cover of mucuna 39 19. Effect of organic matter on soil pH 42 20. Maize yield under conventional tillage and direct sowing, with and without 90 kg of N fertilizer 44 21. Herbicide costs in different production systems in Lempira Sur, Honduras 44 22. Organic matter content of a soil under different tillage management 48 23. Soil carbon concentration at various soil depths affected by management system 48 24. Effect of crop residue burning once every two years on soil carbon stock 48 25. Estimation of emission and sequestration of CO2 (total over 8 years) under different maize production systems with cover crops under direct sowing compared with natural vegetation in southern Brazil 49 26. Potential for carbon sequestration in long-term experiment in southern Brazil 50 vii List of plates 1. Crop residues added to the soil are decomposed by soil macrofauna and micro-organisms, increasing the organic matter content of the soil. 1 2. Termites create their own living conditions near their preferred food sources. Inside the colony life is highly organized. 6 3. Severe soil erosion removes the potential energy source for soil microbes, resulting in the death of the microbial population and thus of the soil itself. 15 4. Clearance of primary forests often leads to rapid mineralization of organic matter. This sandy soil used to sustain a tropical forest. 16 5. The burning of residues is a common practice for clearing land, both in slash-and-burn systems and in more intensive agricultural production systems. 17 6. Animal at pasture in the area around Asmara. 17 7. Removed crop residues cannot serve as food for soil organisms. 18 8. Intensive soil tillage makes the land vulnerable to erosive processes, as the organic matter is lost through increased oxidation in the soil, the upper subsoil is compact, and the loosened topsoil can more readily wash away. 19 9. Hollows are dug in the ground to gather water during the rainy season. 21 10. Half-moons around newly planted Acacia seedlings catch and retain rainwater. 23 11. Agroforestry plot with suboptimal stand of maize because of shading by trees. 24 12. Example of the “Quezungual” system, an indigenous agroforestry system.