ICRAF-ISRIC VNIR Soil Spectral Library now available here.

Infrared spectroscopy (IR) is a proven technology for rapid, non-destructive characterization of the composition of materials based on the interaction of electromagnetic energy with matter. IR 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 (e.g. the Viking Lander Mission to Mars already used this technology in 1976-78). Both the visible–near-infrared (VNIR, 0.35-2.5 µm) and mid–infrared (MIR, 2.5-25 µm) wavelength regions have been investigated for non-destructive analyses of soils and can be applied to predict a number of important soil properties (Shepherd & Walsh, 2002; McBratney et al 2006; Brown et al. 2006). Infrared spectroscopy 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; also see www.worldagroforestry.org/sensingsoil).

In this project we are using both VNIR and MIR spectroscopy, for which these properties include: soil color, mineral composition, organic matter and water content (hydration, hygroscopic, and free pore water), iron form and amount, carbonates, soluble salts, and soil aggregate and particle size distributions. Importantly, these properties also largely determine the capacity of soils to deliver various provisioning and regulating ecosystem services specified in the Millennium Ecosystem Assessment. Rather than calibrating individual nutrients and properties to infrared spectra, indicators that summarize syndromes (or multivariate associations in soil properties) are used. These relationships are established using conventional reference laboratory analysis on a limited number of samples carefully selected to cover the range in spectral diversity. In this way spectral information can provide information on properties that may not be strongly predicted individually. For example, micronutrient deficiencies tend to occur in sandy soils with low organic matter content, and this syndrome is readily diagnosed spectrally. Interventions need to be targeted at the syndromes and not individual nutrient deficiencies (e.g. build up organic matter to supply missing nutrients, increase nutrient retention capacity and improve soil structure).

We are building spectral libraries of all soil samples taken from the sentinel sites as well as from the soil management experiments. We will also expand the African soil spectral library maintained at ICRAF with additional legacy data, where the physical samples are available to do this.

Tthree regional laboratories, in addition to the central ICRAF facility located in Nairobi, have been equipped to handle these types of analyses: in Mali at the Institut d’Economie Rural (IER) in Bamako which is partially supported by ICRAF and operates a NIR spectrometer, in Tanzania at the Agricultural Research Institute (ARI) in Mlingano, near Arusha, and in Malawi at the Department of Agricultural Research and Services in Chitedze, near Lilongwe.

The NIR instruments will also provide the regional labs with the capacity to analyze a wide range of agricultural inputs (manures, fertilizers, fodder) and products (grain and cash crops, biofuels), as described by Shepherd and Walsh (2007). The capability to analyze organic resources can be an important additional contribution to validation of integrated soil fertility management practices (e.g. monitoring improvements in manure/compost nutrient status).

Reference analyses and MIR spectral analyses are being done at ICRAF’s laboratory to ensure consistency in methods, and quality control is conducted to ensure spectral quality across the NIR laboratory network. Capacity building support to the national NIR laboratories in addition to instrumentation and equipment and their associated milestones is also underway.

The field offices are also being equipped with portable field spectrometers to ensure that soils can be rapidly analyzed in situations where exporting physical samples to a centralized laboratory would be difficult, to select optimal subsets of samples for shipping for more expensive analyses, to develop field-based, spectrometric diagnostic tests, and to radiometrically calibrate remote sensing images.

Through the 4-year project term, 32 soil samples per sentinel site (1,664 in total) are being analyzed using standard laboratory methods for soil texture, pH, exchangeable acidity, Na, Ca, Mg, and extractable P and K (at the ICRAF soil lab in Nairobi), and for C, N, S, d13C an d15N (at the Lamont-Doherty Geochemistry labs at the Earth Institute in New York). Micronutrients and P-sorption characteristics are also being run on further sub-sets of samples to identify deficiency syndromes. Portions of all recovered soil samples will also be archived at ISRIC to facilitate additional and/or new analyses, if and when these become available.

An X-ray diffraction instrument with capability for high throughput has also been installed at ICRAF/TSBF, allowing fully quantitative, non-destructive soil mineralogy profiling. This new technology provides the missing link for evaluation of soil fertility and functional capacity and complement infrared spectral analysis as a diagnostic screening tool. There is little information on the mineralogy of African soils and there have been very few attempts to link mineralogy to soil function despite the recognition of parent material as a key soil forming factor.