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Use of near infrared spectroscopy to predict chemical parameters and phytotoxicity of peats and growing media

Ludwig, B., Schmilewski, G. and T. Terhoeven-Urselmans.

Sci. Horticult-Amsterdam, 109:86-91


Fast and cost efficient tests are required for a screening of chemical parameters of peats and an estimation of a potential phytotoxicity. The objective of this study was to test the usefulness of near infrared spectroscopy (NIRS) for the quality control of peats and growing media. A population of 73 phytotoxic and non-phytotoxic peats from various stockpiles and growing media of different origins was collected and, after storage, characterized for their pH, contents of salt (estimated using the electrical conductivity), P, K, NO3- and NH4+ and their phytotoxicity using Chinese white cabbage (Brassica napus var. chinensis) as indicator. For unknown reasons, storage reduced the percentage of phytotoxic peats and growing media from 65 to 16 %. Spectra of the visible and near infrared region (400 - 2500 nm) were obtained for all samples after drying and grinding (variant A) or in a moist state (variant B). A cross validation was carried out using a modified partial least square method which was based on the entire spectra and included the first to third derivate after base line correction. The chemical parameters pH, and contents of salt, P and K were predicted well by NIRS for both variants: the ratios of standard deviation of the laboratory results to standard error of cross validation (RSC) were greater than 2, the regression coefficients (a) of a linear regression (measured against predicted values) ranged from 0.9 to 1.1 and the correlation coefficients (r) were greater or equal to 0.9. Satisfactorily (0.8 £ a £ 1.2, r ≥ 0.8 and 1.4 £ RSC £ 2.0) assessed was the NH4+ content in variant A. Nitrate contents and phytotoxicity were predicted unsatisfactorily. However, investigating a less diverse subpopulation of 38 samples which were all growing media for plant propagation from two different origins showed a good prediction accuracy for nitrate contents (variant A) and a satisfactory one for phytotoxicity (variant B). The good and satisfactory predictions reported above indicate a marked usefulness of NIRS in the quality assessment of peats and growing media.