Analyzing Africa’s Soils Using Spectroscopy

by | September 20, 2012
Category: News

Using the latest technology to screen soil health

Significant progress is being made in analyzing soil and plant samples collected from the AfSIS sentinel sites, with over 17,000 samples being received since the project began. Of this, 70% of the samples have been characterized with infrared spectroscopy and reference methods. After harmonization and quality checks, the data is stored in a meta-file database. This database also includes data from non-AfSIS sentinel sites that have been analyzed using the same methodology, as they provide additional information — such as high values of soil carbon, strongly acid or alkaline soils — to expand the range in the spectral calibrations.

Scanning a soil sample with a portable mid-infrared spectrometer

Scanning a soil sample with a portable mid-infrared spectrometer

AfSIS utilizes the innovative practice of infrared spectroscopy (IR) to analyze soil samples gathered at the sentinel sites. IR is a proven technology for rapid, non-destructive characterization of the composition of materials based on the interaction of electromagnetic energy with matter. It is now routinely used for analyses of a wide range of materials in laboratory and process control applications in agriculture, food and feed technology, geology, biomedicine, and space exploration, and has been a key technology in enabling the development of soil health surveillance systems by providing a rapid and reliable tool for soil health screening (Shepherd & Walsh, 2007).

The graphs on the next page show the actual values determined by conventional laboratory methods plotted against values predicted from mid infrared spectral measurements samples for soil organic carbon and soil pH. The graphs on the left show the calibration values and those on the right show validation values. The validation values are ‘out-of-bag’ validation samples, which provide an indication of the prediction performance one can expect if a new sample belonging to the same population of soils was scanned.

Figure 1 part 1: results obtained from diagnostic screening tests of sentinel site data

Figure 1 part 1: results obtained from diagnostic screening tests of sentinel site data

The infrared spectral measurements provide capability for rapid and low cost prediction of a suite of soil properties in one pass. Only 5% of the AfSIS soil samples need to be analyzed using conventional laboratory measurements and the remaining 95% are predicted using the spectral method. The technology enables characterization of important soil properties in landscapes and at the scale at which AfSIS operates. This not only allows population-based sampling of Africa’s soils, providing unbiased estimates of different soil health constraints, but also provides a basis for wall-to-wall mapping of soil properties at Africa scale through calibration of the georeferenced soil observations to remote sensing imagery.

Figure 1 part 2: results obtained from diagnostic screening tests of sentinel site data

Figure 1 part 2: results obtained from diagnostic screening tests of sentinel site data

AfSIS is also building the capacity of laboratories in Africa to conduct their own spectral analysis. The central ICRAF facility located in Nairobi, supports Near Infrared Spectroscopy (NIR) labs: in Mali at the Institut d’Economie Rural (IER) in Bamako, in Tanzania at the Agricultural Research Institute (ARI) in Mlingano, near Arusha, in Malawi at theDepartment of Agricultural Research and Services in Chitedze, near Lilongwe, and atthe Agricultural Research Institute of Mozambique (IIAM) in Maputo, Mozambique. Mid infrared spectroscopy labs have been established with national institutions in Cote D’Ivoire, Cameroon, Kenya, Nigeria, and Ethiopia (in progress). With this capacity, these labs can support national soil information systems and soil health baselines and impact assessment in land management projects. These laboratories will be able to benefit from the Africa-wide infrared spectral calibrations that AfSIS is building and also contribute to the continuous improvement of the calibration libraries by inclusion of new samples.

AfSIS is also working with private sector soil testing companies to develop spectral-based soil testing services that are accessible and affordable by smallholder farmers. For example AfSIS is working with BLGG Netherlands to pilot a soil testing service for smallholders in Kenya. In addition, assistance has been provided to the US National Soil Survey Center Laboratory in Lincoln, Nebraska to set up a high throughput mid-infrared scanning system, similar to the robotic system already operational at ICRAF.

It is anticipated that analysis of 40 sentinel sites will have been completed by the end of 2012.

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