The view from above

VHRI gives West African farmers data on soil fertility and land size

Pierre Sibiry Traoré

The Seeing is Believing project uses very high resolution satellite imagery to give farmers in West Africa information on soil fertility and accurate land size.

Smallholder farmers in West Africa, and many other tropical regions, are experts in precision agriculture, and have been for many generations. Because they work on small areas of land in variable and unpredictable environments, they have traditionally relied on a wide range of tools, knowledge and information to be able to sustain their quality of life. But farmers often have only a limited view of their landscape, and will welcome any source offering a new perspective to help them in their work.

In June 2009, the Seeing is Believing-West Africa (SIBWA) project, started working with six communities of farmers in the region – three in Mali, and one each in Ghana, Burkina Faso and Niger. Led by scientists at International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), the SIBWA team provided the farmers with very high resolution imagery (VHRI) of their land. The images are made by sensors on satellites and show a high level of detail. The images on Google Earth, for example, are VHRI quality.

When ICRISAT acquire a VHRI, they use computer software to enhance the image, add extra layers of information and analyze the data that would be useful to the farmers – estimates variations of soil fertility, land size and shape. Working with local NGOs and extension officers, the SIBWA team then visit the project sites to verify the information with the farmers.

ICRISAT further analyze the images using the feedback from the field research to build a database of information that they can use to develop an accurate map of each farm. SIBWA partners then translate the information into local languages and take the detailed maps back to the individual farmers, who can use to plan and manage their crops for the coming growing season.

Indicators

Although a satellite cannot directly detect soil quality, it can record how the soil reflects light; its colour, in other words. But to get a more precise picture of soil fertility, the scientists can analyze the images when the crops are growing at their peak. The condition of the fully-grown plants can then give a good idea of the quality of the underlying soil.

The images, therefore, cannot give an exact figure for soil fertility, as in more traditional soil sampling and analysis techniques, but VHRI gives an accurate picture of relative fertility across the landscape, rather than just the results from a few sample points. While a single VHRI image costs between US$ 1000-1500, this method of analysis is often still cheaper than visiting every individual farmer’s field and sending a comprehensive set of soil samples to the laboratory.

The cost of satellite imagery has decreased rapidly in recent years as more sources have become available, but SIBWA used images and data from two other ICRISAT research programmes. The technology and data was already available, but they had not been brought together and applied to the issues affecting small-scale farmers before.

With this overview of the soil quality, farmers can organize the distribution of fertilizer throughout their fields and plan which crops should go in which areas. Many farmers also do not know the exact size of their land, but the SIBWA team worked with the farmers to determine the area of each field. The farmers can then use this data to calculate the precise amounts of seeds, pesticides and fertilizers they need to buy.

Knowing the size and shape of fields can also help rural communities to plan for future developments and investments and if, for example, the land is suitable for mechanization. Small and fragmented fields, and fields with an awkward shape, are difficult to work with a tractor or even animal traction. There is a minimum size above which it becomes cost-effective to use a tractor and it is a simple process to determine that from the image before the community invests in any new equipment.

Another advantage of VHRI is that it shows the direction of furrows on the field and areas where farmers use contour tillage, which is when farmers plough a ridge along the contour lines of the land. Farmers use this method to encourage water infiltration and reduce soil erosion. From the satellite imagery, farmers could monitor whether they were following the contour lines accurately and efficiently. SIBWA involved local NGOs specialized in technology and extension services in each community to help farmers make use of the available data.

After only six months since the start of the project, it is too soon to see the benefits, but the team expects that the farmers will consider the data when planning for the new growing season. SIBWA looks forward to a time when farmers routinely use information from VHRI. That may be in five or ten years time, but the images, data and analysis techniques are already well developed, and the results are clearly valuable to small-scale farmers. There is no reason why they should have to wait so long.

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Pierre Sibiry Traoré is a remote sensing scientist and GIS head at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) www.icrisat.org

23 February 2010

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