Near infrared spectroscopy (NIR) can provide rapid and accurate prediction of organic resource quality attributes across a wide range of resource types. Methods developed at the World Agroforestry Centre (ICRAF) can be used for calibrating models for prediction of organic resource quality. The calibrations provide predictions of the concentrations of nitrogen, phosphorus, potassium, calcium, magnesium, lignin, and total soluble polyphenol across a wide range of tropical organic resource types, including sawdust, cattle manure, crop residues and green leaves of leguminous trees of diverse quality. Predictions for nitrogen, lignin and total soluble polyphenol (TSPP) are expected to be quite reliable as these organic compounds directly produce absorption features, whereas mineral concentrations are predicted through indirect associations and are therefore more prone to prediction failure. ICRAF does not bear any responsibility for use of predictions produced with this protocol and validation of results using conventional reference methods on a subset of samples is always encouraged. 

Figure 1: Scatter plot of nitrogen content (%) measured be means of chemical analysis and predicted by NIRS using PLS regression for the Improved N model after outliers were removed (n = 1027).

The calibrations are based on the organic resource database (ORD) described by Palm et al. (2001), Shepherd et al. (2003) and Vanlauwe et al. (2005), and represent sources of organic inputs typically used for soil improvement in tropical farming systems.

Although not all analyses were available for all the samples used, there was a wide variation in individual quality attributes. The improved N calibration contains an additional sample set consisting of 200 cowpea leaves samples collected from Kenya and Tanzania (Towett, 2008), which ranged in N concentration from 3.5% to 7.0%.

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References

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Palm, C.A., Gachengo C.N., Delve R.J., Cadisch G., and Giller K.E. 2001. Organic inputs for soil fertility management in tropical agroecosystems: application of an organic resource database. Agriculture, Ecosystems and Environment, 83, 27–42.

Parkinson, A., and Allen, S.E. 1975. A wet oxidation procedure suitable for determination of nitrogen and mineral nutrients in biological materials. Commun. Soil Sci. Plant Anal. 6:1-11.

Shepherd, K.D., Palm C.A., Gachengo C.N., and Vanlauwe B. 2003. Rapid characterization of organic resource quality for soil and livestock management in tropical agroecosystems using near-infrared spectroscopy Agron. J., 95, 1314–1322.

Towett, E.K. 2008. Optimizing the use of Near Infrared Reflectance Spectroscopy (NIRS) to predict nutritional quality in cowpea (Vigna unguiculata) leaves for human consumption. MSc Thesis at Georg-August-University Göttingen, Germany. 109 pp. PDF-file - 4.5 MB – Available online: https://www.uni-goettingen.de/de/document/download/810d13d380e2d08b6709a...

Van Soest, P.J., and Wine, R.H. 1968. Determination of lignin and cellulose in acid-detergent fiber with permanganate. J. Assoc. Off. Anal. Chem. 51:780-785.

Vanlauwe, B., Gachengo, C., Shepherd, K.D., Barrios, E., Cadisch, G. and Palm, C.A. 2005. Laboratory validation of a resource quality-based conceptual framework for organic matter management. Soil Sci. Soc. Am. J., 69, 1135–1145.