The simplicity of light is revolutionising the measurement of soil health

Technician scanning a soil sample with an infrared spectrometer

Technician scanning a soil sample with an infrared spectrometer

By Keith Shepherd

The United Nations (UN) General Assembly has proclaimed 2015 as both the International Year of Light and Light-based Technologies (IYL 2015) and the International Year of Soils (IYS). The UN has also recognized December 5th, 2014 as World Soil Day.

In celebration of these important events, the Soil-Plant Spectral Diagnostics Laboratory of the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, would like to tell you about how the simplicity of light is being used to revolutionise the measurement of soil health.

Healthy soils are fundamental to sustainable development. A healthy soil is one that has capacity, relative to its potential, to sustain delivery of essential ecosystem services – the benefits people obtain from ecosystems. These include the capacity of soil to retain and supply water and nutrients to plants, regulate water flows, sequester carbon, resist soil erosion, filter wastes, and support biological diversity.

We think soil degradation is a widespread problem, undermining sustainable development, but it is a poorly quantified problem. The lack of hard evidence makes it difficult for decision makers to give priority to soil health or to formulate appropriate policy and action.

A key reason for poor quantification of soil health is that conventional methods of measuring soil properties are cumbersome and expensive, and often lack reproducibility across soil types and laboratories. This has prevented systematic monitoring of soil health over large areas, and the development of standards, so we do not have an accurate picture of what is happening to soil health. Soil health surveillance systems that would parallel surveillance systems in public health do not currently exist.

Enter light technology. What could be simpler than measuring the amount of light that is reflected off soil at different wavelengths when you shine light on it? After all that is what our eyes do – we see light in the red, green and blue wavelength ranges.

In physics, light refers to electromagnetic radiation of any wavelength, whether visible or not. It turns out that light reflected off soil in the visible, near infrared or mid-infrared ranges provides a fingerprint or spectrum that relates to many soil properties measured the conventional way. The spectral signatures characterise the basic mineral and organic matter composition of the soil, which in turn determine a soil’s functional properties.

With modern spectrometers we can obtain an infrared scan of a soil sample in just 30 seconds. This single measure not only is able to tell us about many soil properties in one pass, but is also high reproducible, unlike most traditional soil measurement methods. The technology does away with the use of chemicals and instead deploys fundamental physics – the interaction of light with matter. What could be simpler!

ICRAF’s Soil-Plant Spectral Diagnostics Lab has already put this technology to good use in Africa, where it is needed most, since data is sparse and conventional soil laboratory facilities are often wanting. Through the Africa Soil Information Service (AfSIS), ICRAF has used infrared spectroscopy to characterise the first ever baseline on soil health in Africa. The technology is now in use in 10 African countries, and is being used for national-level soil mapping by the Ethiopia Soil Information Service. Private companies and development NGOs are also picking up the technology with an interest in providing low cost soil testing and advisory services to smallholder farmers. India and China are also knocking on the door.

Other light-based applications are in store. The ICRAF lab is using safe x-ray technology to measure the amount of different minerals in soils and the total elemental composition of soils and crops. Red and blue lasers are being used to measure how readily soil particles break up in air and water streams, to provide measures of susceptibility of soils to wind and water erosion.

What does the future hold? Light-based technology is moving rapidly towards robust handheld and miniature instruments that could be used by field agents. We foresee new spectral metrics emerging that will replace conventional soil guidelines. Soil health surveillance systems are now feasible and would rationally guide health management of the soil resource base, just as public health surveillance has done over a number of decades for human health.

Happy Soils Day!

Happy International Year of Soils!

Happy Year of Light and Light-based Technologies!

Further reading

Shepherd KD and Walsh MG (2007) Infrared spectroscopy—enabling an evidence-based diagnostic surveillance approach to agricultural and environmental management in developing countries. Journal of Near Infrared Spectroscopy 15: 1-19.

Shepherd, K.D., Shepherd, G. Walsh, M.G. (2015). Land health surveillance and response: a framework for evidence-informed land management. Agricultural Systems  (2015), pp. 93-106. DOI information: 10.1016/j.agsy.2014.09.002

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Soil knowledge vital to boosting agricultural production

New X-ray technology to reveal the makeup of Africa soils

Web sites

Africa Soil Information Service: http://www.africasoils.net/

Acknowledgements

The Africa Soil Information Service is supported by the Bill & Melinda Gates Foundation and the Alliance for a Green Revolution in Africa (AGRA). The ICRAF Soil-Plant Spectral Diagnostics Laboratory is also supported by the World Bank, the Government of Ireland, Government of UK (DFID), Government of Canada, Government of Norway, Government of Finland, Government of Switzerland, and Government of Germany, and the CGIAR Research Program on Water, Land and Ecosystems.

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