Soil spectroscopy: an opportunity to be seized

The trade-off between the growing need for large scale soil information and its high cost could be resolved by a widespread use of visible and infrared spectroscopy. A recent workshop by the European Commission – Joint Research Centre (EC-JRC) and the Food and Agriculture Organization (FAO), focused on the measures to foster the global monitoring of soils based on spectroscopy. In 1921 Aldo Leopold wrote: ‘All natural resources…are soil or derivatives of soil. Farms, ranges, crops, livestock, forests, irrigation water and even water power resolve themselves into questions of soil. Soil is therefore the basic natural resource’ (Meine & Knight, 1999). Scientists have struggled to provide a quantitative assessment of soil due to the heterogeneity of its constituents. In the époque of ‘climate change caused by CO2 emissions’, soil, being the largest terrestrial carbon pool, suddenly became important for stakeholders and policy makers and the demand for high-resolution soil data for large areas grew accordingly. Unfortunately, the availability of such data is scarce. Sanchez et al. (2009) stated that ‘communicating soil information among diverse audiences remains challenging because of inconsistent use of technical jargon, and outdated, imprecise methods’. Inconsistent, outdated, and imprecise methods also hamper attempts to compare and synthesize soil measurements from different countries, regions, times, and studies. At present, these inconsistencies are the main constraint to merging existent soil data and evaluating the state of soil resources at regional-to-continental scales. The development of cost-effective soil analysis methods is becoming a priority in contemporary soil science (Grunwald et al., 2011), and this is particularly important when considering the creation of new, harmonized, continental-scale soil datasets. A workshop recently hosted by the European Commission – Joint Research Centre (EC-JRC) and the Food and Agriculture Organization (FAO) focused on the potential role of soil spectroscopy in addressing the growing need for low-cost and standardized soil data. Over the past 30 years, diffuse reflectance spectroscopy has proved to be a high-throughput, inexpensive, reproducible and repeatable soil analytical technique (Soriano-Disla et al., 2014). Briefly, spectroscopy measures the amount of light reflected (i.e. reflectance) by a soil sample in the visible (Vis; 0.4 – 0.78 μm) and infrared spectral regions (IR; 0.78 – 25 μm), reacting to its organic and inorganic composition. Soil moisture, organic matter, carbonates, mineralogy and texture are the main constituents influencing the shape of soil reflectance in the Vis-IR region. While the first studies, in the 70–80's, focused on the interpretation and classification of soil spectra (e.g., Stoner & Baumgardner, 1981), soil spectroscopy rapidly adopted a quantitative approach to predict many soil properties such as organic carbon (Ben-Dor & Banin, 1995), texture (Sørensen & Dalsgaard, 2005), cation exchange capacity (Janik et al., 1998) and moisture (Dalal & Henry, 1986). Soil spectroscopy, being an indirect method, generates predictions by multivariate calibration models developed on spectral libraries (Fig. 1). These contain both spectral measurements and corresponding soil properties measured with reference methods of soil analyses. Calibrations are not reliable for soils not represented in the spectral library, hence the need for building libraries representative of the soil diversity for a region of interest. While soil spectroscopy estimates of soil properties are not as accurate as reference soil analyses, they can improve regional-to-continental soil resource assessments, because more samples can be analyzed for a given budget. Light reflectance, being a physical measurement, can provide greater consistency across laboratories compared with chemical reference methods. This is the strategy followed, for instance, by the Africa Soil Information Service (AfSIS; Box 1). Once a library is constructed, only a fraction of new samples (10% for the AfSIS project highlighted in Box 1) needs to be submitted for reference laboratory analysis to make reliable predictions. This causes a dramatic drop of costs. The scientists assembled at the FAO workshop identified the use of a common methodology for the collection of laboratory soil spectra as the most critical bottleneck hindering the implementation of spectroscopy in a regional/continental soil monitoring system. Using a common protocol, including wet-lab reference analyses, spectra and calibrations could be shared. This is required to enable the merging and interoperability of soil spectral libraries, and hence provide harmonized and more accurate soil data. A concerted adoption of spectroscopy for soil analysis will facilitate the collection and exchange of harmonized soil data, and thus will allow a better integration of the soil compartment in the research on global biogeochemical cycles. This will address the bottleneck identified by Sanchez et al. (2009). The project should be based on local and regional partnerships among existent laboratories, already operating Vis-IR spectrometers. The development of these collaborations would be supported by EC-JRC and FAO, consistent with their effort to contribute to food security through more continuous and sustainable soil monitoring activities. Soil IR spectroscopy has been widely researched globally but there are still relatively few examples of its routine use, despite its potential of rapid low-cost soil analysis. The principles for use of IR spectroscopy in enabling soil quality surveillance systems, mirroring the type of diagnostic surveillance systems used in public health, have been established and illustrated in the West African Sahel (UNEP, 2012). Spectroscopy provides a powerful tool for digital mapping of soil functional properties when coupled with systematic georeferenced ground sampling and remote sensing. The AfSIS has deployed IR spectroscopy as its frontline screening tool in characterizing 20 000 soil samples taken from a stratified random sampling frame across Sub-Saharan Africa. The Ethiopian government has taken up the methods at national level to provide soil fertility mapping for targeting of fertilizer and lime recommendations. There is a growing network of IR laboratories in Africa, with more than ten operating laboratories, including two private sector soil testing companies.