Abstract

In soil science, structural features of soils are an important attribute; soils can be characterised by proportional size distribution of aggregates and the pore space between and within those aggregates. The objective of this study was to evaluate the relationship between the visible and near infrared (Vis-NIR) spectra and microaggregates from a collection of Ferralsols. Our dataset consists of 56 spectra from seven soil profiles of kaolinitic and oxidic–gibbsitic Ferralsols. Soils were characterised according to their morphological features: those with a typical porous massive structure and those with a cohesive massive structure. Size distribution of five classes of microaggregate was determined (1000–2000 μm, 500–1000 μm, 250–500 μm, 105–250 μm and <105 μm). From the soil spectra, we determined the CIE (Commission Internationale de l'Eclairage) red index (RI_CIE) and the absorption intensities of kaolinite (I_Ka) and gibbsite (I_Gb). The predominant fraction of microaggregates exhibited sizes between 250 and 1000 μm (Φ_250–1000). The proportional increase of this microaggregate class in the soils increased the scattering and variability of reflected light, particularly when soil samples showed more than 0.50 g g⁻¹ of Φ_250–1000. The microaggregate size explained over 55% of reflectance intensity variability of wavelengths in the Vis-NIR regions, principally on reflectance intensity and concavity of the spectra at 436 nm, 546 nm and 2236 nm, which decreases with increased proportion of Φ_250–1000. Thus, different structural features of Ferralsols can be directly evaluated through Vis-NIR spectroscopy, even in samples sieved through 2.00 mm. These results highlight the potential use of Vis-NIR spectroscopy in studies of soil classification, soil physical behaviour and soil physical fertility and environmental studies, principally by a new method of analysing bulk density and structure of soil directly from the soil spectra, without the need for predictive models. As well as spectra, RI_CIE, I_Ka and I_Gb can be used to characterise the various structural features of Ferralsols.

© 2016 The Author(s)

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