Chapter 2
Concepts and impacts of conservation agriculture

"Leaving crop residues on the soil surface is like using a sombrero; it conserves the sweat, and keeps the head cool." A small-scale farmer, Costa Rica.

Concepts

Under forest, the great production and cycling of foliage results in much biological activity, humus formation, and hence a dark coloured topsoil. Because of the great numbers of insects and worms there are large pores, which allow water infiltration. In contrast, under annual crops, leaf production is much less, the biomass is largely removed, the soil is tilled several times each year and so is much drier. Consequently, less food and moisture are available for earthworms and other insects, and their habitat is repeatedly disturbed or destroyed.

Where topsoil has been eroded, and soil layers of poorer quality for root growth have become exposed, it is essential to rehabilitate and restore the soil to bring it up to good productive capacity for the next crop or pasture. Failing this, a spiral of degradation is set in motion as a result of the reduced vegetative cover and biomass production and reduced soil and water retention. Thus the quality of the soil that is left behind should be of even greater concern than the quantity and quality of that which has been lost.

Farmers need to create favourable conditions for soil life and should manage organic matter so as to create a fertile soil in which healthy plants can develop. In tropical rainfed agriculture, in which poor farmers generally suffer from decreasing soil fertility and declining soil water dynamics, the restoration of soil organic matter is essential for the stabilisation of production.

However, this cannot be accomplished by merely incorporating organic matter into the soil, as under tropical conditions, the degradation process is too fast to allow any medium or long-term improvement of soil properties. Moreover, incorporation implies tilling the soil, which accelerates organic matter breakdown and destroys soil structure and organisms.

The primary need is to feed soil organisms (bacteria, fungi, earthworms, etc.) and to regulate their living conditions, while protecting them from chemical and mechanical impacts. For example, shallow tillage, ridge-tillage, or zero-tillage and surface management of crop residues has often led to increases in earthworm activity compared to areas where deep tillage is practised.

Providing a permanent or semi-permanent soil cover (growing crops, crop residues or mulch) provides food for soil organisms, protects the soil from the destructive forces of rain, wind and sun, improves water infiltration, reduces soil moisture loss, and regulates the soil microclimate (Plate 3).

This practice should be accompanied by others related to conservation agriculture, which intend to minimise soil disturbance and protect and nourish the soil life, such as:

• reducing or eliminating tillage operations;
• practising crop rotations;
• using fertilizers as appropriate;
• relying on integrated pest and weed management.

Plate 3
Smallholder coffee farmers covering the soil with straw to preserve moisture, Malawi

[A. Conti/FAO/17732]

The benefits of conservation agriculture include agro-environmental features (Box 3). Nutrient losses may be minimised through the appropriate use of deep-rooting cover crops that recycle nutrients leached from the topsoil, moisture management, and improved collection, storage and application of wastes from crops, livestock and the household (food wastes). Nutrients that are harvested and removed may be replaced through symbiotic nitrogen fixation, organic matter from elsewhere, or the complementary use of fertilizers and feed supplements.

Box 3: Agro-environmental features of conservation agriculture Soil loss does not exceed rates of soil formation; Soil fertility and soil structure are maintained or enhanced; Biodiversity is maintained or enhanced; Downstream effects of run-off or leaching do not impair water quality; Rainfall is managed to avoid excessive runoff; Emissions of greenhouse gases are reduced; Food production levels are maintained or enhanced; Environmental stewardship is engendered amongst rural communities and producers of all types, ensuring continuity of sound land management.

Pest management can also benefit from conservation practices that enhance biological activity and diversity, and hence competitors and predators, as well as alternative sources of food. For instance, most nematode species (especially the pathogens) can be significantly reduced by application of organic matter, which stimulates the action of several species of fungi attacking nematodes and their eggs.

Several key concepts and terms used in this report are described in Annex 1.

Changing mentalities

The current concept shift from soil being a thin layer of material at the outside of the lithosphere immediately below the atmosphere to a living entity that has dynamics of root growth and soil fauna, temperature, moisture and oxidation-reduction, has profound significance for ecological study and practical management. Nutrients that are lost from the soil by crop production, erosion and leaching need to be replaced and the availability of all nutrients needs to be optimised. The broader focus of conservation agriculture embraces not only the nutrient content of soils but also their structure and biological status, which are determinants of sustained productivity.

In many cases this may require a combination of changes in tillage and soil management practices, crop rotations and planting times, soil conservation measures, the strategic use of organic materials and the appropriate use of inorganic fertilizers to match farmers' combinations of crops, land, availability of organic materials and market opportunities.

An improved approach to the integrated and sustainable use of natural resources requires a paradigm centred on the user's role and the significance of the soil's biological and architectural dynamics, both at and below the surface, as much as on the increased synergy between local, internal and external forces. As farmers use management skills and better knowledge to work more closely with the biological world, they will often find ways to reduce purchases of external inputs.

With a new emphasis on conservation agriculture has come a reawakening of interest in soil organic matter. Some issues, such as soil fertility (thus food security), water storage, compaction, and erosion are directly related to soil organic matter. Others, such as disease and insect pest infestations, may be indirectly related to it. Thus, the build-up and maintenance of the soil biota and good levels of organic matter in soils are of critical importance.

The adoption of conservation agriculture requires the opening up of dense and compacted soils as well as an opening of minds and innovative thinking. In fact, almost all of the past limitations to change in Brazil have been overcome with positive and creative thinking. During 1998 and 1999, 140 extensionists were trained from seven different states. The training courses were a great success and completely turned around the attitudes of extension services to zero tillage, paving the way for collaboration in more pilot projects with small farmers and leading to considerable benefits to the small farm sector.

Agricultural science generally has poorly understood, overlooked or ignored indigenous knowledge and traditional approaches. Soil conservation staff commonly have focused on what they have seen as technically desirable solutions to problems of erosion and runoff. Extension agencies often find it difficult to learn from farmers and rural people. Few systemic processes exist for enhanced two-way feedback on performance. An examination of the situation in tropical Brazil identified several reasons for the delay in research attention to farmers' practices (Box 4).

Box 4: Reasons for the slow research response to zero tillage in Brazil prior to 1995 Rejection of farmer-based experience (practices not proven statistically) Resistance to the costs and effort of change Research was on-station Research generally not system-oriented Researchers not in close contact with farmers Rewards to researchers depended on publications rather than on farmer impact Little or no farmer control over research priorities Priority to feed urban populations makes decision-makers risk-averse. Misapprehension that zero tillage would only be appropriate for large farmers

In this case, the resistance to change of researchers, academics and advisers was much greater than that of farmers. The farmer saw immediate benefits over and above the cost of change, while the professionals saw a significant cost in the effort of change but failed to foresee the economic benefits accruing to this extra effort. Farmers, they felt, needed to be motivated by non-financial stimuli, which they believed would take much longer.

Combating land degradation and improvement of land productivity

The control of soil erosion and establishing permissible amounts of soil loss have long been principal foci in addressing land degradation and aiming at increasing and stabilising agricultural production.

In contrast to this narrow approach to soil-related problems, it is being increasingly recognized that land and its soil components should be looked upon as a living resource to be nurtured and used in sustainable and responsible ways. The definition of land is implicit in the following quotation:

"For a land use system to be sustainable requires, first, that it should meet the needs of farmers and other land users; and, secondly, that it should achieve conservation of the whole range of natural resources, including climate, water, soils, landforms, forests and pastures." (Young, 1998)

In the past, soil conservation has been advocated as a necessary starting point to raise crop yields. Soil erosion has conventionally been perceived as one of the main causes of land degradation and the main reason for declining yields in tropical regions. Based on these assumptions, conservation measures were directed at three main components:

• physical works to catch, guide and prevent damage by run-off (Plate 4);
• pressures to stop people from deforesting the area and to reduce the number of grazing animals;
• planning of different land uses according to Land Use Capability Classifications, based on the assessment of different degrees of erosion hazards.

Plate 4
The use of tied ridges to catch and guide run-off and prevent damage to the crops

[FAO]

Experience has shown that none of the recommended physical and institutional anti-erosion methods was widely adopted by the smallholder farmers of tropical regions. Since conserving soil does not by itself raise yields, and is not the farmers' overriding concern (while improving productivity may be), it is advisable to emphasise those practices of good soil and crop management that have, positive effects on conservation. This insight has led to a switch from stopping erosion to assisting farmers to achieve a more conservation-effective, higher and more stable production.

Case studies where these insights have been applied show that it is technically feasible and economically profitable to develop intensive production systems in the tropics while improving the quality of the natural resources and protecting the environment. This requires a focus on the management of biological resources together with related hydrological and nutrient cycling functions, complemented where necessary, with physical works (contour ridging, conservation banks or terracing) as appropriate on steep slopes (Box 5).

BOX 5: Conservation structures and practices in Southern Brazil Large areas of arable land in Southern Brazil suffered from erosion to such an extent that the very livelihood of the farmers was being endangered. Initial efforts to contain the damage by the implementation of conservation works such as terracing did not prove effective. As research studies developed, scientists confirmed that the erosion problem was due to the way the land between terrace banks was managed. Even if the terraces were well constructed, the rate of rainwater infiltration was progressively reduced due to excessive soil movement and compaction. The technique presented as a solution when used as an isolated practice, i.e. the construction of terraces, accentuated rather than alleviated the problem (Mielniczuk, personal comm.). This resulted in the revival of the ancient practice of green manuring. Firstly with the clear objective of erosion control, which later developed into what could be defined as good soil management. More important than using physical barriers to control runoff, which is responsible for only 5 percent of erosion, research showed that the ideal solution is to maintain soils covered as much of the time as possible with growing plants or crop residues. By avoiding the detachment of soil particles by raindrop impact, which accounts for 95 percent of erosion, soil losses are avoided and at the same time the soil can be cultivated in conditions similar to those found in forests (FAO, 2000). This was accompanied by the emergence of new systems of land preparation such as minimum tillage and direct sowing techniques as alternatives to the conventional practices introduced from temperate climates. Depending on the crop to be sown, the area of soil to be disturbed is limited to a narrow strip, between 10 and 50 cm wide. In this strip, the vegetative cover is partially incorporated and the soil surface is still 60-80 percent protected from raindrop impact and the sun's rays. Direct sowing consists of the elimination of ploughing or soil disturbance using traditional equipment such as the plough or cultivator. Direct sowing is practised through a cover of crop residues or in a narrow partially cleared strip.

Similarly, especially in the arid and semi-arid tropics, it is opportune to emphasise with farmers the management of rainwater as a productive resource rather than merely as a means of saving soil. Achieving better infiltration and in-soil storage of rainwater when these have been limitations, while favouring agricultural production, automatically also reduces soil and water movement and transport. In this regard, to enhance water availability and retain soil productivity it is important to consider those practices which promote rainfall capture in the soil before considering those which aim to control run-off - they are complementary in a sequence, and are not competing alternatives.

In areas of high rainfall and tendency to soil water-logging, conservation of water and soil requires careful management of soil structure and the vegetative cover to enhance infiltration and maintain above-ground and internal drainage.

Facilitating farmers to improve their land care - land husbandry - thus provides a more effective response than efforts to combat erosion alone. It specifically recognizes farmers' desire to raise yields and incomes as they stabilise or reverse resource depletion. It also provides opportunities for governments to harmonise certain national objectives (better management of natural resources and development of sustainable agriculture) with major objectives of farm families (secure livelihoods). However, this approach requires many adjustments in common thinking (Hinchcliffe et al., 1995).

Socio-economic advantages

The adoption of conservation agriculture practices by farmers often shows increased yields (double or even triple sometimes), which can be seen by farmers and measured, as for example in Figure 1 and Box 6.

Figure 1

Production increase of maize and sorghum under the Quesungual system (J. Hellin, 1998)

BOX 6: Farmers' benefits - Lempira (Honduras) In Lempira (Honduras), farmers moved from a traditional slash and burn system to the Quesungual system: conservation agriculture with an agroforestry component. An economic analysis of this transition showed that during the first two years maize and sorghum yields are about equal to those obtained with the traditional slash and burn system. From the third year, however, their yields increase, in addition to which the plot provides the farmer with firewood and posts, which give an extra value to the production. Because of the increased production of maize, the quantity of stover increased as well; this can be sold as livestock fodder. Additionally, from the first year onwards the farmer can rent out the land for livestock grazing, because of the increased biomass production. Usually this is done for two months. The application of the Quesungual system not only meets the household subsistence needs for fruit, timber, firewood and grains, but generates a surplus, which generates an extra income when sold in the market.

Other benefits quickly appreciated by farmers include the reduction of the amount and costs of labour and energy required for land preparation and sowing, due to the fact that the soil becomes soft and easy to work. Ploughing the soil is by far the most energy- and time-consuming operation for the farmer. In many farming systems, it constitutes an important bottleneck, often due to the necessity to hire equipment which does not arrive in time and therefore delays planting (Plate 5 and Box 7).

Plate 5
Land preparation is by far the most time-consuming activity for the farmer and family

[J.R. Benites]

BOX 7: Conservation of time and energy Weeding accounts for more than 60 percent of the time a peasant farmer spends on the land. Conservation tillage reduces the energy (for example fuel for machines and calories for humans and animals) and time required. Thus a large-scale trial at the IITA in Nigeria found zero tillage required 52 MJ energy and 2.3 hours labour per hectare compared to 235 MJ and 5.4 hours on conventional tillage (Wijewardene, 1979). Use of pre- and post-plant herbicides in no till in Ghana required only 15 percent of the time required for seedbed preparation and weed control with a handhoe, while the reduction in labour days required in rice in Senegal was 53-60 percent (Findlay and Hutchinson, 1999).

Of the total energy used in crop production in North Africa in 1987, 69 percent was derived from people, 17 percent from animals, and 14 percent from tractors (Twomlow et al., 1999). In sub-Saharan Africa this ratio was 89:10:1. Findlay and Hutchinson (1999) estimated that 80-100 person-days/ha would be needed to prepare a land for planting with hand hoes. Animal-drawn mouldboard ploughing may take two or three days, whereas tractor ploughing may require only two or three hours.

Although it is often recommended that farmers should plough immediately after harvest, most farmers wait until the first rains before commencing seedbed preparation. Because the majority of African farmers have no direct access to animal or motorised traction, seedbeds are often prepared too late, the cropping season shortened, and crop yields reduced (Ellis-Jones and Mudhara, 1997).

Under conservation agriculture, in most systems only a small proportion of the land is worked instead of ploughing or hoeing the whole area to be planted (Plate 6). Cultivation is also usually shallower than conventional tillage. Herbicides may be used in some systems (Findlay and Hutchinson, 1999), hand hoes in others, and farmers who have animal-drawn ploughs can fit simple and inexpensive tines or subsoilers (Bwalya, 1999). Farmers using conservation tillage reduced the production costs of soybeans per hectare by US$67 in Argentina, by US$35 in the USA and by US$27 in Brazil (FAO, 1998a).

The farmers' point of view is a central consideration in an adoption process (Box 8); they will not change their practices if they do not see any benefit. In fact, the reductions in costs and time required are usually the most compelling reasons for farmers to adopt conservation tillage.

BOX 8. The farmers' point of view - Lempira (Honduras) Among the benefits farmers found in applying conservation agriculture practices within the Quesungual system, were: improved soil moisture conservation, which permits a good development of the crop, even in very bad conditions (such as El Niño in 1997); less soil erosion (even during the heavy rains of hurricane Mitch in 1998); reduced disease incidence in the bean crop due to the mulch; production of firewood and fruits from the trees and shrubs; timber trees can also be cut after about 7 years and used for construction or sold; the soil becomes more fertile and the effect of fertilizers on the production is higher; agricultural production is higher than in traditionally managed plots; plots can be cultivated for longer periods than in the slash and burn system; less labour involved in the establishment and maintenance of the system; improved soil workability, which implies less labour need during land preparation and sowing; harvested products, such as beans and maize, can be dried by hanging them over the tree trunks; cattle can feed on the residues after the harvests of maize and sorghum; trees and shrubs in the plot provide shade for the farmer and attract animals and insects, birds and butterflies. The disadvantages mentioned by the same farmers were: equal or slightly lower grain production during the first year compared to the traditional system; higher incidence of slugs in the bean crop during the first years; sometimes too much soil cover impedes the germination of the seeds; the shade can result in higher disease incidence during periods of high rainfall because of higher humidity.

Impacts on the environment

Experience has shown that conservation agriculture systems achieve yield levels as high as comparable conventional agricultural systems but with less fluctuations due, for example, to natural disasters such as drought, storms, floods and landslides. Conservation agriculture therefore contributes to food security and reduces risks for the communities (health, conditions of living, water supply), and also reduces costs for the State (less road and waterway maintenance, less emergency assistance).

Conservation agriculture also contributes to wider environmental benefits such as:

• improved management of soil and water resources from farm to watershed levels: less flooding, less erosion, less desertification, more constant flow in the rivers, better recharge of groundwater resources, improved water quality (less pollution) and reduced siltation effects downstream;
• increased carbon sequestration and less carbon release (less fuel used, less organic matter degradation);
• increased biodiversity through diversification.

Plate 6
Only a small percentage of the total area is worked in reduced tillage systems

[J. Kienzle]

Impact of management practices on soil fauna and soil fertility

The greater production of biomass in a system with cover crops and zero or reduced tillage compared to monocrop cultures with conventional tillage, leaves a protective blanket of leaves, stems and stalks from the previous crops on the surface. In this way organic matter can be built up in the soil, which has great influence on the activity and the population of the micro-organisms. This results in a greater biological activity (Box 9) , more humus formation, and hence a darker coloured topsoil. With time, in reduced or zero tillage systems, soil fauna take over the functions of traditional soil tillage, which is loosening the soil and mixing the soil components. In addition, the increased biological activity creates a stable soil structure through accumulation of organic matter.

BOX 9: Soil microbial communities and zero tillage A component of soil quality maintenance is favouring the activity of beneficial soil organisms. Among the most important species are the root nodule bacteria involved in biological nitrogen fixation. Several studies have indicated that zero tillage systems increase soil microbial biomass and the size of the microbial population (Ferreira et al. 2000). For zero tillage systems in southern Brazil, differences of about 50 percent in soil biomass and rhizobial populations compared to conventional tillage were reported (Hungria, et al. 1997). Evaluations have demonstrated that some crop rotations and zero tillage favour bradyrhizobia populations (Figure 2), nodulation and thus nitrogen fixation and yield (Voss and Sidirias, 1985, Hungria, et al. 1997, Ferreira, et al 2000).

Figure 2

Population size of root nodule bacteria under zero tillage (left) and conventional tillage (right) with different crop rotations [S=soya; W=wheat; M=maize] (Voss and Sidirias, 1985)

The vegetal cover on the soil surface creates a more humid environment, which is conducive to the activity of soil organisms. The greater numbers of worms, termites, ants and millipedes combined with a higher density of plant roots result in more large pores, which in turn increase water infiltration (Roth, 1985). Thus in an experiment in southern Brazil rainwater infiltration increased from 20 mm/h under conventional tillage to 45 mm/h under no-tillage (Calegari et al., 1998). As a result soil erosion may be reduced to a level below the regeneration rate of the soil and the groundwater resources may be maintained or even enhanced (Derpsch 1997). Leaching of plant nutrients or other substances into the aquifer is also reduced (Becker 1997) compared to conventional arable agriculture. Soil moisture storage and availability is also improved both by soil cover (less evaporation, more infiltration) and soil organic matter. All these phenomena are improving plant nutrition (Box 10).

BOX 10: Nutrient availability under various cover crops (southern Brazil) Different cover crops and tillage systems may affect the availability of plant nutrients, especially nitrogen. These effects are being evaluated in long-term soil management experiments in southern Brazil: The reduction of tillage and the addition of nitrogen by legumes in the cropping system increased the total nitrogen in the soil. The intensive system consisted in oats and clover as the cover crops and maize intercropped afterwards with cowpea (Vigna unguiculata), under zero tillage. After five years, the 0-17.5 cm soil layer contained 490 kg/ha more total soil nitrogen than the traditional system oats-maize under conventional tillage. After nine years, the system even resulted in a 24 percent increase in soil N compared to conventional tillage (Amado et al., 1998). Calegari and Alexander (1998) found that after nine years, the phosphorus content (both inorganic and total) of the surface layer (0-5 cm) was higher in the plots with cover crops. Depending on the cover crop the increase was between two and almost 30 percent. This indicates that the different cover crops have an important P-recycling capacity and this was even improved when the residues were retained on the surface. This was especially clear in the fallow plots, where the conventional tillage plots had a P-content 25 percent less than the zero tillage plots. According to Burle et al. (1997), many studies report soil acidification in legume-based systems caused by intense nitrification followed by NO3- leaching, H3O+ excretion by legume roots, and the export of animal and plant products. In general, legume-based systems did not increase soil acidification in the surface layer, where the greatest soil organic matter accumulation occurred. The highest soil cation exchange capacity (CEC) is found in legume-based cropping systems with the highest organic matter content. Especially systems with pigeon peas (Cajanus cajan) resulted in a 70 percent increase of the CEC compared to a fallow-maize system. The highest levels of exchangeable K, Ca and Mg were found in systems containing pigeon peas and lablab (Dolichos lablab) and lowest in systems containing clover. It is possible that clover systems had increased NO3- leaching, which may be accompanied by increased leaching of exchangeable Ca and Mg (Burle et al., 1997).

Organic matter also plays an important role in the formation and stabilisation of soil aggregates through connecting the organic polymers and the inorganic surface with polyvalent cations. The hyphae fungi and bacteria slime, even if formed and decaying again rapidly, also play an important role in connecting soil particles. A strong relationship also exists between the soil carbon content and an increase in aggregate size. Castro Filho et al. (1998) found an increase in soil carbon content under zero tillage resulting in a 134 percent increase in aggregates of more than 2 mm and a 38 percent decrease in aggregates of less than 0.25 mm, compared to conventional tillage.

Where conditions are suitable, increased residues and soil cover resulting from higher yields can generate an upward spiral in soil productivity. The inclusion of leguminous green-manure or cover crops in small-farm systems has shown such effects by providing not only dense cover and large quantities of organic matter to the soil, but also significant quantities of microbially fixed nitrogen.

Castro (1991) compared water, soil and plant nutrient loss between conventional agriculture and direct seeding in a wheat-maize rotation. The losses were less under direct seeding due to the soil cover, which reduced the rainfall impact on the soil surface (Table 3).

Table 3
Water, soil and plant nutrient losses under conventional agriculture and direct seeding in a wheat-maize rotation (Red Ferralsol)

Mitigating climate changes and greenhouse gases

Emissions of the so-called greenhouse gases resulting from human activities are substantially increasing the atmospheric concentration of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Half of the increase in global warming since the industrial revolution is considered to be the consequence of an increased level of carbon dioxide in the atmosphere (Lal, 1999). Sources of carbon dioxide emissions include burning of fossil fuels, industrial production, deforestation and agriculture. Although estimates of the total CO2 emissions vary widely, the contribution of forestry and agricultural activities to the emission of carbon dioxide is estimated at only five percent of the global total (Benites et al., 1999).

Conversely, the potential of agriculture and forestry for sequestering carbon (the absorption of carbon in biomass) is significant (Box 11). For example, systems based on high crop residue addition and no-tillage tend to accumulate more carbon in the soil than is released into the atmosphere (Greenland and Adams, 1992). Bayer (1996) found that crop rotation systems accumulated about 11 t/ha of carbon in the topsoil (0-17.5 cm) after nine years. Under conventional agriculture and with monoculture systems the carbon liberation into the atmosphere was about 1.8 t/ha per year of CO2 (Reicosky et al., 1995).

BOX 11: Carbon sequestration (southern Brazil) The emission of carbon dioxide to the atmosphere is related to the mineralisation and decomposition processes of soil organic matter by micro-organisms (Lal, 1999). The CO2 emission from the soil is increased by ploughing, mixing crop residues and other biomass into the soil surface and burning of biomass. Studies in southern Brazil show an increase in organic carbon in the soil under conservation agriculture. The different cover crops showed significant effects on the organic carbon level for two depths (0-5 cm and 5-15 cm). The means of all winter cover crops presented greater values for soil organic carbon than fallow at both depths (Calegari and Alexander, 1998). During the initial years until establishment of the cropping system the increase in total organic carbon content was restricted to only the surface layers of the soil (0-2.5 cm) (Testa et al., 1992). With time, this effect reached deeper soil layers (2.5-7.5 cm). Castro Filho et al. (1998) found a 29 percent increase of soil organic carbon in no-tillage compared to conventional tillage in the surface 0-10 cm of the soil, irrespective of the cropping system. Compared to the cropping system fallow-maize, which was taken as a reference, soil carbon content increased by 47 percent in the system maize-lablab (Dolichos lablab) and by 116 percent in the maize-castor (Ricinus communis) cropping system. In systems where nitrogen was applied as a fertilizer the carbon contents increased even more (Testa et al., 1992). Bayer and Mielniczuk (1997) found that five years after the introduction of intensive cropping systems containing leguminous crops (especially the cropping systems oats+clover-maize and oats+clover-maize+cowpea), soil organic carbon contents were restored, after the loss of 8.3 tons of organic carbon per hectare under previous cropping systems.

Reduction of contamination and water pollution

The change in land use and management associated with conservation-effective practices leads to a significant reduction in erosion, and thus to a reduction in water pollution and contamination (Plate 7).

Indicators that can be used to measure this reduction of water pollution include:

• Water turbidity and the concentration of sediments in suspension;
• Total sediment loss and associated loss of nutrients;
• Reduction of water treatment costs.

Bassi (2000) found significant reductions of water turbidity and concentration of sediments over a period of ten years (1988-1997) in different catchment areas in southern Brazil. The reductions varied between 50 and 80 percent, depending on soil types predominating in the areas. These reductions are due to increased perennial crops (banana and pasture) on hillsides, thus reducing the erosion process. Total sediment loss decreased by 16 percent and the cost of plant nutrients by 21 percent. Reduced sediment loss and less soil particles in suspension also reduced the cost of water treatment. Data obtained in Chapecó indicated that the quantity of aluminium sulfate used for flocculating suspended solids fell by 46 percent over a period of five years.

Plate 7
Flooding and sediment transport to the river increasing cost of water treatment

[WOCAT, FAO, 2000]

Enhancement of biodiversity

The result of increasing soil cover, through living crops and crop residues, is an increase in the variety and variability of animals, plants and micro-organisms, which are necessary to sustain key functions of the agro-ecosystem.

Conservation agriculture provides more habitats for birds, small mammals, reptiles and earthworms, amongst others, and more food, including insects and seeds, which in turn leads to an increase in species and population. The increased production from conservation-based agriculture also makes it possible to set aside areas for natural regeneration (Box 12).

BOX 12: Increase of protected areas through livestock management (Costa Rica) Traditionally, livestock was produced in hilly areas, in an extensive form, without any management of the resources, leading to erosion and environmental problems. After the hurricane Cesar in 1996, a rehabilitation programme was launched to reinitiate agricultural production in a sustainable way. The solution for livestock production was to change towards a more intensified production system, with the objective to reduce degradation risk, improve the nutrition status of the cattle and liberate areas which can be used for other activities, including the natural regeneration of the vegetation. The intensified system is based on semi-stall-fed production of livestock and was initiated through a farm planning in order to define land use capacity and to select the areas most suitable for livestock raising. Part of these areas was then sown with improved pastures and the rest with fodder crops. The improved pastures were divided into smaller parcels to permit rotational grazing. The fodder crop area is fertilized with manure produced in small stables. The intensification of the livestock system has resulted in spectacular increases in the production of meat and milk. The relocation of livestock activities has led to the natural regeneration of severely eroded land and areas unsuitable for agricultural production, which is having a positive effect on the biodiversity and allows the government to "trade" the area in international treaties dealing with forest protection or carbon sequestration.

Less vulnerability to natural disasters

The improved soil conditions make the land and the agricultural system more resilient to extreme events. This effect has been studied in the Quesungual system (Lempira, Honduras - Plate 8) during the Canicula, El Niño, and excessive rainfall during hurricanes, such as Mitch in 1998. Compared to farmers who did not change their cropping system from the traditional slash and burn, the Quesungual farmers did not experience a major loss in maize production during the dry period of El Niño in 1997, as is shown in Figure 3 Even the following year, when hurricane Mitch passed over Central America resulting in excessive rainfall, and many farmers lost their crop for the second time, the Quesungual farmers obtained similar yields as in the year before El Niño.

Plate 8
The Quesungual system is an indigenous agroforestry system which most distinct characteristic is the combination of naturally regenerated and pruned trees and shrubs with more traditional agroforestry components, such as high value timber and fruit trees. In between the trees the traditional staple crops, i.e. maize, roghum and beans are grown

[A.J. Bot]

The Quesungual system is an indigenous agroforestry system characterised by the combination of naturally regenerated and pruned trees and shrubs with more traditional agroforestry components, such as high-value timber and fruit trees. The traditional staple crops, i.e. maize, sorghum and beans, are grown between the trees.

Figure 3

Maize production under the Quesungual system (Alvarez and Flores, 1998)