Science in Society Archive

"The Answer Lies in the Soil"

Traditional farming practices in the South are playing a major role in preserving tropical soils and enhancing food security. Lim Li Ching reports on some ingenious strategies used by the farmers.

Soils are fundamental to agriculture, but tropical soils can be rapidly depleted or eroded, depending on soil type, local conditions and farming techniques. Although crop yields decline for many reasons and numerous complex interactions are involved, research has shown that declining crop yield is in most cases exponentially linked to loss of soil quality, in both the tropics and the temperate regions.

Soil quality is a holistic concept, as soils are part of a dynamic and diverse production system with biological, chemical and physical attributes that are intimately linked to the needs of human societies. Society, in turn, actively adapts the soil to its needs, mining its nutrients and replenishing them in times of plenty.

Soil quality varies spatially and temporally, and is affected by management and use of the soil resource. How actual production is affected by soil quality depends on two factors: the intrinsic susceptibility of the soil to erosion (resilience) and the variable impact of that erosion on yield (sensitivity).

Resilience includes soil 'strength', or its resistance to shocks such as severe rainstorms. It is manifested through specific degradation or erosion rates on different soils subject to the same erosive conditions. Sensitivity, on the other hand, denotes fragility or susceptibility to decline in production per unit degradation. It is a measure of how far the change induced in soil quality affects the soil's productive capability.

The different combinations of resilience and sensitivity of tropical soils, in turn, require different management strategies. The strategies are themselves related to the capacity of farmers to provide remedial action. For example, strategies that require mechanical structures will need financial and human resources, which may be beyond the reach of many smallholder farmers.

Other strategies, such as using crop residues, green manures and alley cropping, are effective ways to address both erosion and fertility decline. Even at moderate levels of management such as that which most smallholder farming households can afford, soils that have moderate resilience and low sensitivity can effectively continue to withstand degradation and produce indefinitely, at least for 50 years.

Professor Michael Stocking, from the School of Development Studies, University of East Anglia, UK, has highlighted these dynamic links between soils and farmers in a recent paper in Science.

Soil is thus not a static, homogenous medium; instead, it is a dynamic entity, responding to how humans use it. The dynamism is reflected in traditional farming practices that do more than just extracting nutrients; they evolve in response to changing conditions over many years, through informal experimentation and experience.

This capacity to respond draws on the farmers' own local knowledge and resources. According to Stocking, "smallholder farmers in the tropics have skills and social networks that give us cause for optimism for future soil quality and food security". Many, he stresses, are managing their soils sustainably and productively; they "adapt technologies to their local needs (using indigenous knowledge and innovation) and avoid labor-demanding and expensive practices".

Traditional farming practices such as multiple cropping - planting several different food crops in the same field - are excellent ways to preserve soil quality and ensure food security, Stocking told Inter Press Service (IPS). Multiple cropping, as opposed to monoculture, minimises risks. When farmers plant a variety of crops, if the weather is bad for one it will likely be good for another, or if disease affects one, it is likely that others will survive. Not only does multiple cropping spread the risks, but it also provides better nutrition, and the surpluses can be sold.

"The better strategy in subsistence agriculture is not to maximise production but minimize risk," said Stocking. Local knowledge enables smallholder farmers to develop various techniques to minimise risk, including maintaining soil quality for future years.

Success stories are increasingly common, such as that of Carlos Crovetto Lamarca, who transformed an eroded farm along the steep coastal slopes in central Chile. The farm started as little more than rocks and a series of 20m deep gullies. But Crovetto developed a system for building soil based on adopting a gentler tillage of plant roots, rather than ploughing. With crop residues protecting the soil surface from the impact of rain, the soil structure reformed. Rather than burning or ploughing, he spread straw or other crop stubble over the soil. "I sell the grain today, and I preserve the stubble so I can harvest tomorrow," he says.

It took a long time, but now, after nearly 30 years, Crovetto has created over 30mm of organic topsoil. This happened at least 20 times faster than what would have been made by natural rock-weathering processes. Crovetto's soil is so rich that his crop yields are as good as any in the North. And, his soils are getting better each year. For farmers like Crovetto, the importance of protecting and improving soil is clear - "we depend daily on the most valuable resource, the soil".

Farmers are likely to invest in soil conservation as they are unlikely to do anything that undermines their future or puts household livelihood and food security at risk, unless immediate survival were in question. They also adapt and experiment with methods that can increase benefits.

For example, Stocking cites the use of Mucuna pruriens (velvetbean) as green manure mulch in sub-humid Benin, as evidence of smallholder farmers' adaptability, flexibility and responsiveness to techniques that bring private benefits. From 15 original farmers involved in experimentation in 1987, 100 000 were reported to have embraced this practice by 1996 to counter soil fertility decline. Planting Mucuna beans has also restored soil fertility on depleted soils in Latin America.

More often than not, farmers make better decisions that the 'experts', because of the experience gained in integrating the myriad local factors responsible for controlling production. They are also often the best arbiters when it comes to technologies. "Science does not always get it right and does not necessarily provide workable or acceptable solutions".

An example is in semi-arid Kenya, where farmers choose 'trashlines' (bands of uprooted weeds and crop residues) to intercept sediment and runoff, a technique never promoted by the advisory services. Yet, when the marginal rates of return and net present values over 10 years are calculated, trashlines are almost always the only technique of soil quality maintenance that consistently benefits farmers' livelihoods.

In Ghana, farmers plant up to 54 different varieties of climbing yams (Dioscorea spp.) inside the forest to protect the vegetables. At the same time, the forest keeps the soils moist and rich in organic matter. This traditional means of maintaining soil-plant relationships also conserves biodiversity, as the natural forest vegetation is kept to protect the climbing yams.

A multi-faceted, flexible and diverse approach to managing soil quality is obviously needed to address the complex realities that smallholder farmers face. There are many examples from around the world, of farmers in the South adopting various practices to improve soil quality and conserve soils, leading to productivity increases (see Chapters 15 & 16 of the report of the Independent Science Panel, The Case for a GM-Free Sustainable World, ). All this is in striking contrast to the simplistic, mechanistic approach of the dominant Western model of industrial monoculture.

Agronomists and development professionals from the North often discount the practices of smallholder farmers because the yields for individual crops are low by comparison, as farmers tend to plant a diversity of crops. This one-dimensional yield is a poor yardstick for food security. Maintaining soil quality and fertility is by far the more important.

North American and European soils have been badly depleted, and remain productive only on account of the massive amounts of chemical fertilizer applied to them. "The soils here are just a passive medium keeping the plants upright," says Stocking.

Agricultural productivity in the North is also wholly dependent on the plentiful supply of cheap energy available to make fertilizers and to power machinery, which makes it totally unsustainable. In contrast, with little or no outside inputs, farming systems in the South are clearly much more sustainable and productive in multi-faceted ways.

A major threat to productivity in the South is lack of tenure or access to land. Farmers cannot maintain their soil in areas affected by civil conflict or insecure land tenure. The greatest damage to soils occurs where conditions are volatile, as with migrants and refugees, whose local knowledge is poor. In such cases, depleting the soil of its nutrients is essential for survival, at least in the short term. The security of tenure for smallholders could be made even more difficult by changing national and global conditions.

Thus, efforts to improve declining trends in food security not only ought to focus on local solutions that enhance farmers' efforts to protect and improve their soils, and allow them to produce enough of their own food, but should also encourage policies that can bring about greater stability. They must be rooted in secure rights to land, water and other resources, including necessary credit and market. Traditional and appropriate modern techniques and knowledge should be balanced.

There is much to be done in partnership with smallholder farmers, and through learning from them. Stocking says, "Interventions that use community-based approaches that empower farmers to manage their own situation therefore hold the greatest promise for maintaining soil quality and ensuring food security".

Article first published 17/02/04


  1. Stocking MA. Tropical soils and food security: The next 50 years. Science 2003, 302, 1356-9.
  2. "Development: Small farmers lead the way in preserving soil", by Stephen Leahy, IPS, 27 November 2003.

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