Saturday, April 5, 2008

Throwing light on African soils

The future is infrared (From New Agriculturalist)

In the last four decades, more than half of the land on the plains of Lake Victoria in western Kenya has been abandoned as a result of soil degradation. Millions of tonnes of fertile soil are lost by soil erosion at an astonishingly fast rate. According to Dr Keith Shepherd at the World Agroforestry Center (ICRAF) "Over twenty years, land degradation can mean the difference between productive land, and land that becomes like stone." If vegetation is removed, the process speeds up, and the physical structure of the soil starts to degrade and collapse, depleting vital nutrients. Reversing the process of desertification is difficult and expensive. The key, says Shepherd, "is to identify the problem early enough to prevent switches that occur from a very healthy ecosystem to a degraded state."

Throwing light on the problem

In desertified regions, one of the main priorities is to identify areas that are not yet extensively degraded but the first step is to determine the exact level of degradation. Until recently, analysing soil samples in the laboratory cost more than US$ 50 per sample - rendering it unavailable to farmers. Representing a major advance in field diagnostics however, new infra-red technology (IR) offers a low cost and accurate method of quality diagnosis. The process, known as infra-red spectroscopy, works by shining light on a soil sample. The reflections from a range of wavelengths are collected to create a 'digital scan', which is used to determine soil composition and type.

The technology, extensively used in the pharmaceutical industry to ensure quality control in medicines, offers rapid and non-destructive soil analysis. It is able to predict potential shifting points in the soil, allowing restoration before the soil structure is entirely degraded. For example, soils susceptible to physical degradation start to degrade once a critical amount of vegetation cover has been removed. Soil erosion then sets in and finer particles such as silt, clay and organic matter are washed away and the physical structure of the soil starts to breakdown. The system therefore reaches a point where the soil structure suddenly collapses and soil nutrients are depleted.

With IR technolody, ecologist Alex Awiti states that the method can be used as part of overall soil diagnosis. For instance, low nutrients levels can be redressed by using specific fertilisers. The analysis can also predict soil and plant performance, enabling advice and recommendations about how to boost crop productivity. The Western Kenya Integrated Ecosystem Management Project combined the IR technology with Geographical Information Systems (GIS), providing the potential to develop detailed soil health maps throughout Africa.

Other technologies

The scanner powered by electricity costs US$75,000 but can cost-effectively analyse samples at less than a dollar per sample. Awiti reports that farmers have responded to the technology trials with "enormous excitement". However, infra-red spectroscopy is only one of many interventions introduced by ICRAF to prevent land degradation. Tree planting and sustainable agriculture are also promoted. One tree in particular - 'the tree of life' has proved considerable success. A popular leguminous tree, Gliricidia sepium, produces leaves containing 3-4 per cent nitrogen. By capturing nitrogen from the air, the plant is capable of producing nitrogen-rich bio-mass that can double or even triple maize yields. (See Agroforestry comes of age in Orlando -

ICRAF also advises on reintroducing high value trees such as baobab or sheanut, and other fruit trees which provide an important source of nutrition. These trees are particularly valued in areas such as the parkland systems of West Africa, where trees have been cut down, or died as a result of drought, and land is left open to erosion. However, planting trees is not always the way forward. For Shepherd, there are no single answers: "I would like to see a broad approach to recommending land use. There is no silver bullet. It is a question of recognising the types of problems and then targeting them with the right kind of interventions."

Prevention is better than cure

The IR technology is currently being used as part of the UN Millennium Villages Project, and a World Bank initiative to halt desertification. But as Awiti explains, a major consideration will be capacity building, and how to "help extension workers come on board with these technologies." The best way, he says, is to plan ahead and involve communities. Reversing desertification is a difficult and expensive process, and prevention will be much more effective, not to mention cheaper, than cure. The process will need to be sustainable in the long term, as well as effective in the short term. Soil mapping for Africa is an exciting prospect and IR technology promises to revolutionise soil diagnosis and analysis. In summary Shepherd says, "if you can't measure it, you can't manage it."

For further information see World Agroforestry Centre -

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