New X-ray technology to reveal the makeup of Africa soils
This work paves the way for the development of environmental quality guidelines for the continent’s soils.
Scientists at the World Agroforestry Centre (ICRAF) and partners have adapted a technique commonly used in clinical, archeological, and other applications so it can be used to determine the total chemical composition of the soils of sub-Saharan Africa.
Using the technique, total X-ray fluorescence spectroscopy (TXRF), it is now possible to quantify with good accuracy—and with time and cost savings—the concentration of most of the major and trace elements in soils. This work paves the way for the development of environmental quality guidelines (EQGs) for tropical Africa soils, which would allow better-informed land use planning, as reports a new article in the journal Science of the Total Environment.
Erick Towett, the article’s lead author and a research analyst with ICRAF’s Land Health research programme, worked with ICRAF colleagues and partners to develop the new procedures for TXRF for soil analysis. A portable benchtop instrument known as the S2 PICOFOX™ TXRF spectrometer and soil samples from the ICRAF soil archive (Africa Soil Library) and the Africa Soil information Service (AfSIS) project were used in the development.
- Developing and testing an improved analytical method using the TXRF spectrometer;
- Calibrating the results of element concentrations measured using the TXRF instrument against those obtained using an international standard procedure (called ‘total acid dissolution inductively coupled plasma–mass spectroscopy’, or ICP-MS);
- Refining the TXRF analytical method in an iterative way to reduce measurement error.
Thanks to this work, TXRF can now be used as a rapid screening tool for total element concentrations in soils. “It is simpler to use than conventional chemistry analyses like ICP-MS, and it saves time and money,” said Towett. With TXRF, it takes just 13 minutes to do total soil chemical analysis on a sample, while ICP-MS analysis takes approximately 1 day per sample. The new TXRF protocol costs around US$ 8-10 per sample, which is around ten times cheaper than ICP-MS. Moreover, soil samples from tropical African countries have hitherto needed to be shipped abroad for total element analysis using ICP-MS, greatly raising expenses.
Towett is quick to add that calibration measures have to be followed closely for soil analyses using TXRF, and that for now, the technique cannot be used for all elements.
“TXRF is as accurate as ICP-MS for 11 key elements, including many that relate directly to soil fertility, such as aluminium, potassium, iron, zinc and copper,” he said. “The concentration of nine other elements, including phosphorus and calcium, can also be determined with 60% or higher accuracy relative to ICP-MS; these may be under- or overestimated by TXRF analyses as calibrated.”
Towett also points out that some elements, owing to interactions with the TXRF instrument or with other elements found in soil samples, could not be measured with the new protocol. For instance, the element silver (Ag) could not be detected; mercury escaped from the surface of the reflector before it could be measured; silicon returned a blank result against the quartz glass sample carriers; and cadmium and thallium could not be accurately analysed using TXRF because of properties inherent to soils.
Despite these limitations, the new procedure opens up many new possibilities for land health monitoring and land-use planning in Africa.
“This research marks a huge step in our progress in developing rapid tests to analyse soils. Soil health is the basis of agricultural productivity and it has implications for both human and animal health,” said Keith Shepherd, leader of Land Health research at ICRAF.
“With good quality data, environmental quality guidelines (EQGs) for soils in Africa can start to be developed,” he added. In agriculture, EQGs for soils can inform land-use planning by predicting the types of crops, trees and livestock suited to particular areas, and also help with environmental management by monitoring soil pollution. At present, no such guidelines exist for the soils of tropical Africa.
Towett points out that the huge variability of Africa soils means that many different soil EQGs would need to be developed, to take into account variations in soil element concentrations, which are influenced by geological and biological characteristics, as well as recent management of the natural environment.
Shepherd says routine soil health monitoring is improving in Africa. Already, there is a strong demand on the continent for technologies that use electromagnetic radiation for soil analysis (such as infrared, X-rays, and laser light). ICRAF has assisted with the establishment of infrared spectral diagnostic labs in 10 African countries, under the Africa Soil information Service project (AfSIS), funded by the Bill & Melinda Gates Foundation and the Alliance for a Green Revolution in Africa.
“Reliable knowledge on soils can support agriculture in Africa, as well as human and animal health, particularly if the findings are used to guide action and policies, ” states Shepherd.
The new TXRF technique, used as a rapid screening tool for Africa soils, has much to bring to this endeavour.
Download full open access article: Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF) by Erick K. Towett, Keith D. Shepherd, and Georg Cadisch
Africa Soil information Service (AfSIS) project (www.africasoils.net)