Conservation agriculture

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Best Practices for Smallholder Farmers with Limited Resources

What is conservation agriculture ?

Conservation agriculture (CA) is a land management approach that saves resources and optimizes and maintains the ability of soils to produce food. In CA, sustainability is linked to the ecological preservation of agricultural landscapes. This is achieved through:

  • a reduction in ground disturbance,
  • keeps the floors covered and
  • crop diversification.

Achieving these three elements requires a combination of practices, for which there are many options. By thinking of KT as a comprehensive system, rather than a fixed set of techniques, this gives farmers and practitioners the freedom to evaluate and adopt a set of KT-related practices that are adapted to local needs.

What are the issues addressed by conservation agriculture ?

Farmers in many parts of the world, due to human population growth, have little choice but to cultivate their land permanently, not having sufficient resources to replace the nutrients removed by each successive crop. Crop residues are often lost as a source of organic matter and mulch, usually by combustion or sampling as animal feed or cooking fuel. Especially in areas where nutrient reserves are already low and topsoil is exposed to erosion (Figure 1), soils lose their ability to maintain adequate crop yields. In addition, extreme weather events, adverse climate change, human conflict and disease can all affect the ability of smallholders to maintain the productive capacity of their land.

Conservation agriculture is a means by which the land degradation cycle can be reversed. Reducing tillage, keeping soils covered and diversifying crops help to preserve soil structure and biology, conserve moisture and nutrients, prevent soil erosion and maintain organic reserves of soil fertility. Farmers are most likely to benefit from CA when the three components of CA are implemented together (Corbeels et al. 2014). Since increasing constraints vary by location, understanding the underlying principles is essential in selecting best KT practices for each local context.

Tillage: Principles

Agricultural uses

On previously uncultivated land, tillage creates the conditions under which the seed planted has good contact with the soil and can grow without competition from weeds. Using a technique ranging from a simple hoe to tractor-drawn machinery, tillage has also been used to break up compacted soil layers, control soil-bound pests by exposing them to sunlight, incorporate cover crops and fertilizers into the soil, and uproot and bury weeds in the soil.

Traction and pushing hoes used to cut weeds just below the soil surface. Source: Tim Motis

Soil structure

However, frequent soil disturbance can have a negative impact on the soil structure. Excessive tillage – especially when the soil is repeatedly turned over and left bare – also negatively affects microbial life and soil organic matter (Kushwaha et al. 2001), which help soil particles “aggregate” (stick together). Soil aggregates become smaller each time they are broken. As soil porosity (open space between soil particles) decreases, more rainwater flows on the soil surface instead of being retained in the field. The loss of topsoil due to erosion, in turn accelerates nutrient depletion and soil degradation.

Weed Control

While tillage kills many weeds, it can increase the proliferation of others. Ploughing cuts weeds into pieces, spreading those that spread through underground stems. In addition, ploughed seedbeds promote weed seed germination in addition to crop seeds. Weed seeds buried in the soil by the plough can be dormant in the soil, then germinate when they are brought closer to the soil surface by subsequent tillage.

Weed seeds on the soil surface lose their viability more quickly than seeds buried in the soil (Anderson, 2005). Therefore, if no additional weed seeds are added to the soil, weed emergence decreases more rapidly in uncultivated soils than in cultivated soils (Mohler, 1993). To avoid adding new weed seeds to the soil, reduce tillage with a weed control strategy.

Tillage: Practices

Tillage the soil as little as possible

Some form of tillage, or herbicides, may be required the first time a field is cultivated. Subsequently, strategies can be put in place to avoid further soil disturbance.

The “no tillage” or “zero tillage” methods do not include mechanical tillage. Seeding is done by pushing the seeds into the soil, as it is done with sowing sticks or with an instrument that creates a narrow slit in which the seeds are placed.

Otherwise, various forms of “plough reduction” can be used. When the hoe is already used for weeding, reducing tillage would require less behavioural change than opting totally for no-till. In the systems of the Foundations for Agriculture and God’s Way of Agriculture, cultivation basins are dug with hoes. Hoes are also used to remove weeds when they are small, avoiding deep soil disturbance. The zaï hole system, developed in West Africa, also uses cultivation basins; although they require a lot of labour to dig them, they produce a crop on soil that would otherwise be too hard/crusted to allow the plants to grow.

In the event of flooding, CA can be practiced using raised permanent beds or ridges. Such an approach maintains the roots of the crops above the water table while avoiding the digging of new ridges each year. On steep slopes, these ridges are established along the contour line to reduce erosion. The seeds are sown on the upper part or on the sides of the ridges.

Combine reduced tillage with weed control practices

As mentioned above, weed control is important in no-till or reduced tillage systems. One way to monitor weeds is to use herbicides wisely. Although controversial, their use makes it possible to control weeds without disturbing the soil. Herbicides also reduce the work associated with weeding, which facilitates scaling up CA (Nyamangara et al., 2014). Challenges and concerns to be addressed include cost, availability, negative effects on soil microorganisms and weed resistance to herbicides. When herbicides are available, training will likely be required for safe and appropriate use. To minimize the environmental impact of herbicides, Bajwa (2014) suggests combining chemical weed control with approaches such as hand weeding, mulching, optimal crop spacing (to shade weeds) and cover crops.

Some hand weeding methods require less work than deep weeding. In ECHO, we found that “Dutch hoes” were less laborious than conventional hoes to pull weeds. Dutch hoes cut weeds just below the soil surface, disturbing only a very shallow layer of soil. They also provide an easy way to weed the underside of a layer of plant mulch. Such tools could be manufactured by local metallurgists. Larger weeds can be cut close to the ground with a more traditional hoe or chopped with a machete.

Land Cover: Principles

Mulch can be any material that covers and protects the soil. The most applicable type of mulch for smallholder agriculture is plant matter, which can be either living (as in the case of cover crops) or dead (stem and leaf residues that remain after harvest). Due to competing uses for crop residues, mulch is the most difficult component of CA to apply. Without this, yield gains are still possible if the other two elements of CA are combined with fertilizer and effective pest control. Yet mulch is essential to the success of CA, benefiting farmers and their soils in important ways.

Weed control

Crop residues, if left on the soil surface, act as a barrier that blocks light and the emergence of weeds. The leaves of the ground cover also deprive weeds of light. Weeding is laborious and costly, so eliminating one or more weeding operations is significant. This is all the more true in the context of small-scale agriculture, where 50 to 70 percent of farmers’ working time is spent on manual weeding, and most weeding is done by women and children (Gianessi and Williams 2011).

Erosion of the soil

Mulch protects the soil from wind and water erosion, preserving the precious topsoil.

Soil temperature and humidity

Mulch also protects the soil from the drying and heating effects of the sun. Unlike residues from non-living crops, an actively growing cover crop removes soil moisture (Frye and Blevins 1989) when water is absorbed by the roots and evaporated into the air through the leaves. At the same time, the leaves of the ground cover retain soil moisture by reducing evaporation of water from the soil surface. In a field test carried out by ECHO in South Africa, the dense cover of cowpea (Vigna unguiculata) and lablab (Lablab purpureus) preserved soil moisture and cooled the soil.

Nutrient release

Plants absorb minerals from the soil during their life cycle. These minerals can be returned to the soil. Since CA involves no tillage or reduced tillage, the above-ground biomass retained in farmers’ fields remains on the soil surface. The above-ground and underground biomass (leaves / stems and roots, respectively) eventually decomposes, enriching the soil. The rate of nutrient release depends on a number of factors, including the degree to which the tissue is woody (this concerns the carbon:nitrogen ratios, woody tissue contains more cellulose / lignin and, therefore, more carbon than softer leafy tissue). Thin-leaved legumes such as velvet bean (Mucuna pruriens) generally degrade more quickly than corn stalks (Zea mays). Decomposition and nutrient release rates also increase with increasing temperature, humidity and microbial activity.

Soil ecology and organic matter

By protecting soils from erosion, moderating extreme temperatures, and providing a food source for macro- and microorganisms (e. g. earthworms and bacteria), soil cover contributes to a healthy soil ecology. When organic matter is broken down by soil life, nutrients are released into plants. Because organic matter decomposes rapidly under heat and precipitation, permanent soil cover with plant mulch may be required to maintain adequate soil organic matter in many tropical and subtropical areas.

Land Cover: Practices

Cultivate a healthy culture

More biomass will be created if a farmer can use quality seeds, control pests and provide the nutrients necessary for a healthy crop. Inputs are often scarce or expensive, making it necessary to optimize efficiency. Instead of completely modifying a field, fertilizers can be used more efficiently by concentrating them in smaller amounts near the roots of the crop. This is done by microdosing and placing inputs in basins or furrows/cropping strips.

Leave the residues in the field

Some crop residues may be required for livestock feed, but as much as possible should be left in the field.

Do not burn the residues

Crop residues are sometimes burned to remove debris to facilitate seeding and to kill crop pests. However, combustion leaves the soil unprotected and kills beneficial organisms. Most nutrients are lost in the air. Those released into the soil are quickly lost due to leaching and erosion. In fields where trees are planted with annual crops, farmers have a strong reason not to burn crop residues.

Keep crop residues on the soil surface

In cooler climates, legumes and crop residues are often incorporated into the soil for several reasons. Less nitrogen is lost to the air (as ammonia gas[NH3]) than with exposed mulch. In addition, the burial of mulch makes its nutrients more accessible to plant roots and microbes. However, the incorporation of residues requires tillage. In addition, in the warm tropics, soil cover is necessary to protect the soil from erosion and heat from the sun. When combined with no-till or reduced tillage, surface mulch still contributes to soil fertility, as soil microbes can access surface residues through channels stored in undisturbed soils.

Grow legumes as cover crops

Base crop residues are often insufficient to provide the amount of organic matter needed to support crops and livestock over time. Manure is often insufficient or difficult to transport. Legumes are an excellent option for producing organic matter directly in farmers’ fields. Choose legumes that benefit both the farmer (through weed control and possibly feeding/forage options) and the soil (by protecting the soil from erosion and soil organic matter formation). See Box 1 for other factors to consider. A brochure entitled Restoring the Soil by Roland Bunch is an excellent resource for exploring the cereal and pulse systems that farmers have used in different parts of the world.

Selection factors for leguminous plants

Climate: leguminous plants for warm areas include cowpeas (Vigna unguiculata), sweet peas (Canavalia ensiformis), lablab (Lablab purpureus), pigeon peas (Cajanus cajan), rice beans (Vigna umbellata), tephrosia (Tephrosia vogelli or T. candida) and velvet beans (Mucuna pruriens). Of these, sweet pea, lablab, pigeon pea and tephrosia are the most drought-tolerant. For cooler areas, consider faba beans (Vicia faba), Spanish beans (Phaseolus coccineus) or hairy vetch (Vicia villosa).

Growth habit: Tufted and vertical leguminous plants (e. g. pigeon pea and cowpea, rice bean and velvet bean shrub varieties) are easier to manage and harvest than creeping varieties. They are often preferred for more mechanized agriculture. The creeping varieties cover the soil and produce a lot of biomass, but they climb the stems of cereal crops.

Maturity time: There are advantages and disadvantages for each legume. Cowpeas, for example, are able to produce edible and dry beans well before the corn harvest. Lablab is slower to develop, but produces a late harvest of edible beans on creeping stems that remain longer in the dry season than cowpeas. Long-lived pigeon pea varieties tend to produce more biomass than shorter-lived types.

Cultural diversity: Principles

Soil health

Plant species differ in the mixture of minerals they contain, and in the organic substances excreted from their roots. As a result, crop diversification is more likely to feed a wide range of soil organisms – with increased nutrient cycling – than a single crop used year after year in the same field.

Efficiency and effectiveness

Crops vary according to nutrient demand and rooting characteristics. Farmers can use these differences to their advantage to maximize the efficiency of fertility resources.

  • Crops that are more nutrient-intensive, such as corn, can benefit from legumes that enrich the soil.
  •  Deep-rooted plants absorb nutrients from the deep layers of the soil and make them available to more shallow crops.
  • Surface rooting can be beneficial in phosphorus-deficient soils. Phosphorus does not wash as easily as other nutrients and is often more concentrated near the soil surface where it is accessible through lateral roots (Lynch 2011).

Examples of deep-rooted and shallow-rooted crops.

Deep-rooted plants include high-cut cereal crops such as maize, annual taproot legumes such as alfalfa (Medicago sativa), lablab and pigeon pea, and many perennials (e.g. nitrogen fixing agroforestry trees).

Shallow-rooted plants include many grasses, most vegetables and annual legumes such as groundnuts (Groundnut hypogaea) and common beans (Phaseolus vulgaris).

In general, trees produce the deepest roots, followed by shrubs and then herbaceous (non-woody) plants (Canadell et al., 1996; Maeght et al., 2013).

Resilience

Crops can be selected on the basis of their tolerance to poor soils, drought and salinity. Crop diversification reduces the risk of crop failure caused by pests or plant diseases or adverse climate change. Resilience to economic downturns is enhanced by a range of products that can be harvested for family consumption or income generation. An ideal mix of crops offers options for economic benefit while strengthening the soil.

Adaptability

Crop diversification is often considered the most applicable to smaller, intensively managed gardens. However, there are ways to integrate multiple crops into field scale production.

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