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The importance of soil organic matter

Key to drought-resistant soil and sustained food production

FAO SOILS BULLETIN

80

ISSN 0253-2050

Cover photographs: Background: Honduras. FAO/18884/G. Bizzarri. Inset: Honduras. FAO/18907/G. Bizzarri.

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The importance of soil organic matter

Key to drought-resistant soil and sustained food and production

by Alexandra Bot FAO Consultant and

Jos? Benites

FAO Land and Plant Nutrition Management Service

FAO SOILS BULLETIN

80

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: copyright@

? FAO 2005

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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

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