Optimizing Soil Moisture for Plant Production

The significance of soil porosity

FAO SOILS BULLETIN 79

by
Francis Shaxson
and
Richard Barber
FAO Consultants
Land and Plant Nutrition Management Service



Table of Contents



FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Rome, 2003

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 92-5-104944-0
ISSN 0253-2050

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© FAO 2003


Table of Contents


PREFACE

ACKNOWLEDGEMENTS

ACRONYMS

GLOSSARY OF SOIL MOISTURE TERMS

LIST OF BACKGROUND DOCUMENTS (AVAILABLE ON CD-ROM)

1. INTRODUCTION

A way forward

2. HYDROLOGY, SOIL ARCHITECTURE AND WATER MOVEMENT

The hydrological cycle
Catchments and watersheds
Soil architecture and the importance of pore spaces in soils
Soil water movement

Infiltration of rainwater into soil
Percolation of rainwater through soil
Loss of water vapour from soils
Water movements into and through a plant
Water stress - nutrient interactions
Causes of restricted rooting
Indicators of restricted rooting

3. RAINWATER, LAND PRODUCTIVITY AND DROUGHT

Rainwater for improving yields

Deteriorating water supply
Indicators of deteriorating water supply

Soil productivity and soil erosion

Soil productivity
Soil erosion
Plant-damaging drought
Making droughts worse
Shortening the duration of drought

Changing the perspective on saving soils
Care about roots, soil organisms and water

4. MINIMIZING WATER STRESS AND IMPROVING WATER RESOURCES

Improving restricted rainfall infiltration

Improving the infiltration capacity of the soil surface
Using surface residue covers to increase infiltration and reduce runoff
Mechanisms by which surface residue covers enhance rainwater infiltration
Advantages of surface residue covers
Constraints to using surface residue covers
The amount of residues needed
Conditions favouring the adoption of surface residue covers
Fallowing under cover crops or natural vegetation
Temporary closure of grazing lands and subsequent protection
Importance of forest protection for water infiltration
Increasing the period for infiltration by detaining runoff with physical structures

Contour field operations

Constraints of surface irregularities formed by contour field operations
Conditions favouring the adoption of contour field operations
Narrowly spaced contour planting ridges and tied ridges
Advantages of narrowly spaced contour ridges and tied ridges
Constraints of narrowly spaced contour ridges and tied ridges
Conditions favouring the adoption of narrowly spaced contour ridges and tied ridges
Impermeable and permeable contour barriers at discrete intervals downslope
Conditions favourable for adoption of impermeable cross-slope barriers for water conservation
Permeable cross-slope barriers
Bench-type terraces
Deep tillage to increase subsoil porosity and permeability

Reducing water losses from evaporation and excessive transpiration

Minimizing evaporation from the soil surface
Reducing excessive transpiration
Weed control
Windbreaks
Conditions favouring the adoption of windbreaks
Shade

Reducing rainwater drainage beyond the rooting zone

Soils without restricted rooting
Increasing available water capacity of soil
Dry planting
Improving plant nutrition for early root development
Introducing deep-rooting crops

Improving soils with restricted rooting

Conditions favouring the adoption of biological methods
Mechanical solutions to physical root restriction
Mechanical disruption of shallow root-restricting layers
Mechanical disruption of moderately deep root-restricting layers
Mechanical disruption of very deep root-restricting layers in the subsoil
Conditions favouring the adoption of mechanical methods
Chemical solutions to restricted root growth

Maximizing usefulness of low and erratic rainfall

Match land use to soil characteristics
Use of drought-resistant and drought-escaping crops and varieties
Increase crop water use efficiency
Selecting water-efficient crops
Adjusting plant population to expected rainfall
Applying fertilizers
Weed control
Seed priming
Early planting
Accumulate moisture from one season to the next
Water harvesting
Zaï pits or Tassa
Half moons (demi-lunes)
Contour stone lines
Contour earth ridges and bunds
Retention ditches
Retention pits
Retention basins
Farm ponds
Floodwater harvesting and water spreading

Collaborative stakeholder participation

Need for a facilitator
Need to tackle root causes
Participatory identification and prioritization of soil water problems
Participatory identification of the root causes
Participatory identification of possible solutions for testing
Participatory selection of possible solutions for testing
Participatory testing and evaluation of possible solutions

5. CONSERVATION AGRICULTURE

Improving soil conditions

Residue-based zero tillage in Brazil and Paraguay

History
Implementing conservation agriculture
Effects of conservation agriculture

Effects on crop yields
Effects on soil moisture
Effects on some other soil health indicators
Effects on erosion and runoff
Effects on catchment hydrology
Effects of zero tillage systems on farm economics

Observations about residue-based zero tillage systems in Latin America
Constraints of conservation agriculture and some approaches to overcome them

REFERENCES

FAO TECHNICAL PAPERS

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