The use of Vis-NIR-SWIR spectroscopy in the prediction of soil available ions after application of rock powder

Some of the problems attributed to traditional laboratory analyses that limit the correct assessment of the nutrient contents in the soil are time requirements and high cost of the soil nutrient determinations. To solve these problems, a study was carried out to evaluate the use of visible, near-infrared, and short-wave infrared (Vis-NIR-SWIR) spectroscopy in the prediction of soil available ions submitted to the application of rock powders. The study was carried out on an Arenosol in Paranavaí City/Brazil. Treatments (rock powders) were arranged within a split-plot system designed in randomized blocks with four repetitions. Sugarcane was cultivated for 14 months after the application of rock powders. Later, 96 soil samples were collected for measuring the pH and available ions P, K⁺, Ca²⁺, Mg²⁺, S-SO₄^2-, Si, Cu²⁺, Fe²⁺, Mn²⁺, and Zn²⁺ as well as spectral reading through a Vis-NIR-SWIR spectroradiometer to predict the soil chemical attributes through the partial least square regression (PLS) technique. The results showed that the elements K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Fe²⁺ could be predicted with a reasonable rightness degree (R²_p > 0.50, RPD_p > 1.40) from spectral models. However, for the attributes pH, P, S-SO₄^2-, Si, Mn²⁺, and Zn²⁺, there were no satisfactory models (R²_p < 0.50, RPD_p < 1.40). Thus, the application of rock powder changed the spectral curves and, because of that, allows the building of PLS models to predict the elements K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Fe²⁺. Therefore, Vis-NIR-SWIR spectroscopy is a promising alternative to the routine analyses of soil fertility since it has advantages such as fast analytical speed, low cost, easy to operate, non-destructive, and environmentally friendly, because it does not use harmful chemicals.