Chemometric soil analysis on the determination of specific bands for the detection of magnesium and potassium by spectroscopy

- Spectroscopy was an important tool for determining soil attributes.
- We determined the spectral responses of potassium and magnesium on soil.
- Spectral sensing was carried out in the VIS-NIR-SWIR regions (350-2500 nm).
- Responses occurred mainly in the regions 2186, 2189, 2200 nm.

The laboratory soil analysis is traditionally used to establish elements, such as those related with fertility. It is costly and time consuming, which insures issues for future of precision agriculture, which is stagnated in some countries. Reflectance spectroscopy has recently emerged as a potential tool to reduce these issues. However, chemical elements are usually only underlined with spectra by a “coincidently” statistics and doesn't provide real detection. This study aims to investigate to interaction of K+ and Mg2 + with electromagnetic energy for decrease the demand of soil analysis and rationalize the use of fertilizers. The experiment was carried out using three major soil classes with different textures from tropical environment in Brazil. To reach K+ and Mg2 + saturation in different levels, the soils were saturated with concentrated solutions of KCl and MgCl2 in vertical columns and then washed with distilled and deionized water to extract the residual elements. A second experiment was made by incubation of these elements in soils during four days in room temperature around 30 °C. Laboratory spectral sensing was carried out in the VIS-NIR-SWIR regions (350-2500 nm). Spectra were processed by the Continuum Removal, the Principal Components Analysis (PCA) and Partial Least Squares regression. The PCA showed a high degree of association between spectral and chemical variations. There was no alteration on mineralogy, texture, organic matter, moisture and effective cation exchange capacity (CEC) after the experiment occurs. On the other hand, differences on spectral, mainly where occurs CEC around 2200 nm, did change. Thus, the incident energy interacts with K+ and Mg2 + which promoted these alterations, mostly for Arenosol and Ferrasol. In Cambisol (2:1 mineralogy) we had a double effect due to effective CEC and cations alteration. Were encountered specific bands which altered features due to K+ and Mg2 + content mainly in 2186, 2189 and 2200 nm. Based on the results, the identified bands (related to K+ and Mg2 + contents in the soil) were extracted from the spectral data of soil samples obtained from a Brazilian soil spectral library. Indeed calibrations of K+ and Mg2 + models allowed to quantify these elements with 0.66 R2 for the selected bands and 0.64 for the entire spectrum. Thus, the results indicate that it is possible to detect chemical elements, such K+ and Mg2 + in VIS-NIR-SWIR, looking forward on to assist soil analysis and all inherent approaches.