Welcome to the Electronic Conference

Drought resistant soils: Optimization of soil moisture for sustainable plant production

12 November - 17 December 2004

organized by the

Land and Plant Nutrition Management Service (AGLL)
and the
Water Resources, Development and Management Service (AGLW)

INTRODUCTION

Basic concepts of the e-conference
General issues and objectives
The importance of soil moisture for crop productionhy
Factors influencing available soil moisture for crops
Some specific and potentially contradictory aspects
Sustainable improvement in soil moisture and water-use efficiency
Introducing innovatve experiences in managing soil moisture

Basic concepts of the e-conference

Water is the "lifeblood" of agricultural practice, worldwide. To minimize the impact of drought, soil needs to capture the rainwater that falls on it, store as much of that water as possible for future plant use, and allow for plant roots to penetrate and proliferate. Problems or constraints to one or several of these conditions cause soil moisture to be one of the main limiting factors for crop growth. Where natural rainfall patterns or quantities do not allow reasonably secure satisfaction of crop water requirements, the conventional answer to water deficit often has been to increase water availability through irrigation systems. However, irrigation may not be feasible or even desirable. On the other hand, building on local experiences over generations, farmers' worldwide have developed management options that can increase the soil's ability to store water for plant use and reduce vulnerability to drought. Soil can be managed in ways that reduce the need for supplemental watering and increase the sustainability of the farm. Yet, market forces and rural-urban migration in many regions have introduced changes in farming systems and practices that negatively impact on resources management and increase the risk of drought, as well as related negative environmental effects.

The principal focus of this e-conference is to strive for better rainwater use efficiency in rainfed agriculture, towards improved cropping systems and the mitigation of food, fiber and cash crop insecurity.

In the world today, approximately 40percent of food production comes from irrigated agriculture. However, most smallholder farmers in developing countries are reliant on rainfed agriculture; a practice that will continue for several reasons. Irrigation water and infrastructure is often not accessible or viable depending on the resource endowment and agricultural infrastructure. Additionally, irrigation development is commonly expensive, and sometimes economically and environmentally hazardous. As a result, investment support for irrigation has considerably declined over the last 20 years. The potential for expansion of irrigation schemes in the arid and semi-arid areas is limited, hampered by land suitability, water availability, conflicts over water-ownership among farmers (and between farmers and non-farmers). In this context, a significant contribution to food security would be made by the development of improved rainfed agriculture systems, that are affordable and sustainable, and that increase water availability to crops in dry areas where other water resources are not available or uneconomical to develop. Recognizing the importance of rainfed agriculture in food security and in particular to the majority of smallholder farmers worldwide, improved rainfed management strategies have to be developed to improve water productivity within present farming systems. Under low and variable rainfall conditions, efficient soil moisture management is a good way for improving water use efficiency. Specific runoff farming and water harvesting techniques may also be considered.

This e-conference will consider soil moisture management. It will focus on better use of rainwater in rainfed agriculture; however, technical considerations should also be considered for application in irrigated systems.

The figure 1 shows the main water fluxes in rainfed agriculture.

General issues and objectives

The goal of the e-conference is to identify, describe, discuss and promote actions that will assist farmers to improve water-use efficiency in rainfed agriculture and drought-proof their system. Generally, applicable as well as locally adapted practices that affect evaporation, rainfall interception, drainage and runoff will be considered, with their effects on soil moisture and crop transpiration. Indirect agronomic and environmental effects and externalities, (positive or negative) will also be considered, including water and matter that moves off-site, i.e. that transfers downslope and downstream.

The e-conference also aims at identifying and promoting best management options to address specific constraints, and will have a specific focus on water efficiency in arid and semi arid regions. Consideration will be given to management techniques as well as information management such as the application of meteorological forecasts and the iterative use of crop water use measures / instrumentation.

The e-conference will give an opportunity to people from different backgrounds and areas of expertise to contribute to a common knowledge base and understanding, to provide options for better soil moisture management in rainfed agriculture. Practical experiences and references, and in-field examples of rainwater use efficiencies (measured and derived) will be preferred to academic considerations. Beyond the knowledge aspects, the e-conference also aims at sharing experiences and practical information and information gathering techniques among a wide community of people involved in sustainable improvement of water-use efficiency and drought mitigation. Thus, discussions on conditions for innovations appropriations will not be avoided.

The importance of soil moisture for crop production

A well-managed maize crop may use 400 to 750 mm of water depending on climatic conditions. This corresponds to 4000 to 7500 cubic metres of water per hectare over a growing season. Equivalents for sorghum are 20percent less, and for pearl millet 20percent less again. In these scenarios, the amount of water falling from the sky or stored in a reservoir is not the only factor affecting potential water scarcity. Erratic rainfall patterns, the shortening of rainy seasons, unreliable onset of rains, heavy and intense storms, etc. - all increase the risk for crop production. While little can be done to control rainfall patterns, soil moisture is the water buffer between rainfall regime and crop requirements. Adapted crop and soil water management aims to establish and maintain the buffer of soil water throughout the cropping season.

Water in the soil also transports nutrients from the soil to the plant through the rooting system. Thus, an adequate soil moisture regime will allow root growth and proliferation in the soil, and efficient extraction of soil nutrient resources and hence healthy crop growth.

At the field scale, rainwater is often wasted through evaporation, weed transpiration, deep percolation and runoff. An average of 70 to 85 percent of rainfall can be lost in this way in the semi-arid savannas of sub-Saharan Africa, for example.

Soil moisture has also to be considered throughout the crop cycle. So, sowing dates, sowing densities, crop cycle duration, cover crops, intercrop management, rotations, etc. may induce important changes in overall water use efficiency. As an example, over-consumtion of water at the beginning of the crop cycle will induce less available water later in the cycle (flowering stage), in a phase during which future yield is being determined.

Factors influencing available soil moisture for crops

The availability of soil moisture for crops is principally determined from two groups of factors, which interact at various space and time scales. The first group comprises intrinsic, mainly physical characteristics (soil depth, texture, bulk density, slope etc.), and the specific physical properties derived from composites of these, such as surface water infiltration, available water capacity and hydraulic conductivity through the soil profile. The second group depends on and strongly interacts with land management, and comprises mainly agricultural practices that influence soil structure, organic matter content, rooting depth, soil fauna and microflora, etc.

The available soil moisture regime at any time both before and during the cropping season will depend on both groups of factors.
The following table summarizes the overall situation:

Biomass is a key factor in regard to crop productivity and restoration of soil organic matter, which directly influences soil moisture holding capacity. It interacts at several different levels between intrinsic soil properties, derived (or consequential) soil physical properties and can be practically influenced and managed by farmers. Biomass production is strongly linked to crop transpiration. To achieve improved crop-water productivity, the soil and crops have to be managed with a view to maximize the ratio of crop transpiration to soil evaporation. At a secondary level, crop water productivity is also positively correlated with increased biomass production as a result of both increases in soil organic matter levels and root proliferation that directly improve water infiltration, within soil water distribution and soil water retention. Farmers indirectly manage these concepts when they try to maximize crop production under conditions of water scarcity. They tend to adopt practices aimed at limiting runoff, deep percolation and evaporation from the soil surface.

The e-conference will consider a range of practices that favour and even enhance the capacity of the soil to resist drought.

Some specific and potentially contradictory aspects

High infiltration rates favour better water storage for plants and recharge of groundwater supplies while limiting runoff and reducing the risk of soil erosion with possible damage to crops and off-site transport of sediments with associated nutrient and pesticide loads. Infiltration improves with crop cover, both standing stubble and living cover crops, with associated reductions in raindrop impact and soil sealing. However, under certain conditions, and depending on soil type and soil structure, too much infiltration may produce over-drainage, leading to loss of nutrients and potential rise of saline groundwater.

- From the point of view of a whole watershed, runoff water may not be lost, but might be used on other fields, or flow into rivers, for possible use downstream, if specific investments are made for water storage and conveyance.

- A cover of crop residues limits evaporation, reduces raindrop impact, enhances soil life and biodiversity, and as a result the diversity of soil pore sizes and breakdown of organic materials to soil organic matter (which improves water infiltration and storage). However, large amounts of crop residues might also increase interception and evaporation of rainfall before crops can take up the water, especially where rainfall is light.

- Tillage creates an artificial soil structure and channels in the soil, and a rough soil surface. It also destroys weeds and ensures an adequate surface soil structure for sowing. Tillage also prepares adequate seedbed conditions for early root development and may speed up warming of surface soil for seed germination in cooler climates. However, all these effects are temporary. Tillage also modifies and destroys biological soil pores and soil life and stimulates loss of organic matter through rapid decomposition. Continuous tillage, especially in moist to wet soil, may also induce the formation of hardpans, reducing the effective soil depth and available water holding capacity by physically restricting root proliferation.

- Crop rotations, intercropping and multi-cropping help in combating weeds, pests and diseases. Crop rotations can enhance soil life and biodiversity and the diversity of interconnected soil pores as well as soil nutrient availability through alternating rooting depth. However, crop rotations must be carefully planned for an optimal use of soil water. Cover crops, not grown for commercial harvest, should not use the soil water necessary to grow a subsequent cash crop.

The above, selected examples show that soil management practices cannot be defined as good or bad in isolation, but that they should be considered in the context of a specific farming system. An important aim of the e-conference should be to compile theoretical and generic criteria for different contexts, but also to collect relevant actual case studies where contradictory effects are evidenced and resolutions achieved.

Sustainable improvement in soil moisture and water-use efficiency

Little, if anything, can be done to control rainfall or soil texture. Under certain conditions, irrigation or water-harvesting technologies may be introduced. Nonetheless, in most contexts the only potential to modify and improve soil moisture use efficiencies is through adapted agricultural practices and systems.

It is important that farmers are provided the means to analyze potential changes in water use efficiencies associated with changes in their farming practice. More than visual observations, such as change infiltration after rainfall on covered and uncovered land, are required. Prospective vision of the consequences of changes on their living conditions is also needed. For example, decisions on the fate of crop residues can have implications of water use efficiencies. Residues may be used to feed animals, or sold, or incorporated in the soil, or burnt, and therefore there might not be material for use as soil cover.

The efficiency of the technical support not only depends on knowledge, but also on exchanges and confidence between farmers and extensionists. This implies that farmers should be fully involved in developing and initiating the monitoring systems; potential benefits, problems and risks should be correctly assessed; and priority should be given to changes that meet the farmers' objectives.

Other support may also be required, such as training for farmers in the use of new tools, machinery, seeds (for cover crops, rotation crops, improved seeds), or improved weather forecasts. A favourable legislative and political context should promote the adoption and spread of improved, sustainable crop, land and soil moisture management. Nevertheless, it also may have undesired side effects which finally undermine it. Regulation, incentives, infrastructure and facilitating policies in general may be necessary to support appropriate practices or ban inappropriate ones. However, these aspects will not be considered within the framework of the present e-conference.

Introducing innovative experiences in managing soil moisture

Research impacts the productivity of farming systems by generating new technologies which, if appropriate to farmers' circumstances, will be rapidly adopted. A research/change agent centred process, usually referred to as a Transfer of Technology approach, is typically characterized as a top-down process where researchers develop the innovation, change agents promote its use, and farmers either adopt or reject the innovation.

In contrast, Participatory Assistance is a farmer/farm-centered process that seeks to ameliorate economic and environmental factors that may influence the behaviour of researchers, change agents and farmers during the development process. Regarding innovation processes, the latter approaches will be considered.

Some farmers, extensionists, and researchers worldwide have been working to develop farming systems with a better use of rainwater, particularly through soil moisture management. The present e-conference proposes to be a forum for presenting case studies, and exchanging experiences on practical aspects, which may concern specific success (or disaster) stories. Technical aspects as well as adoption (or non adoption) by farmers will be considered, and used as bases for discussing generic aspects for good soil moisture management. Examples are available through the background documents, and your participation may enrich this database.