Reflectance spectroscopy: a tool for predicting soil properties related to the incidence of Fe chlorosis

Chlorosis due to iron (Fe) deficiency (internervial yellowing) is the most important nutritional problem a susceptible plant can have in calcareous soils. Fe chlorosis is related with calcium carbonate equivalent (CCE), clay content and Fe extracted with oxalate (Fe-o). Reflectance spectroscopy (RS) is a rapid, non-destructive, less expensive alternative tool that can be used to enhance or replace conventional methods of soil analysis. The aim of this work was to assess the usefulness of RS for the determination of some properties of Mediterranean soils including clay content, CCE, Fe-o, cation exchange capacity (CEC), organic matter (OM) and pH in water (pH(w)), with emphasis on those with a specially marked influence on the risk of Fe chlorosis. To this end, we used partial least-squares regression (PLS) to construct calibration models, leave-one-out cross-validation and an independent validation set. Our results testify to the usefulness of qualitative soil interpretations based on the variable importance for projection (VIP) as derived by PLS decomposition. The accuracy of predictions in each of the Vis-NIR, MIR and combined spectral regions differed considerably between properties. The R-adj(2) and root mean square error (RMSE) for the external validation predictions were as follows: 0.83 and 37 mg kg(-1) for clay content in the Vis-NIR-MIR range; 0.99 and 25 mg kg(-1) for CCE, 0.80 and 0.1 mg kg(-1) for Fe-o in the MIR range; 0.93 and 3 cmol(c) kg(-1) for CEC in the Vis-NIR range; 0.87 and 2 mg kg(-1) for OM in the Vis-NIR-MIR range, 0.61 and 0.2 for pH(w) in the MIR range. These results testify to the potential of RS in the Vis, NIR and MIR ranges for efficient soil analysis, the acquisition of soil information and the assessment of the risk of Fe chlorosis in soils.