Estimation of soil phosphorus availability via visible and near-infrared spectroscopy

Abstract

Spectroscopic techniques have great potential to evaluate soil properties. However, there are still questions regarding the applicability of spectroscopy to analyze soil phosphorous (P) availability, especially in tropical soils with low nutrient contents. Therefore, this study evaluated the possibility to estimate P availability in soil and its pools (labile, moderately labile and non-labile) via Vis-NIR spectroscopy based on intra-field calibration. We used soils from two different locations, a plot experiment that received application of phosphate fertilizers (Field-A) and a cultivated field where a grid soil sampling was performed (Field-B). We used the technique of diffuse reflectance in the visible and near-infrared (Vis-NIR) to obtain the spectra of soil samples. Predictive modeling for P availability and labile, moderately labile and non-labile pools of P in soil were obtained via partial least squares (PLS) regression; classification modeling was performed via Soft Independent Modeling of Class Analogy (SIMCA) on three P availability levels in order to overcome the limitation on quantifying P via Vis-NIR spectroscopy. We found that isolating P contents as the only variable (Field-A), Vis-NIR spectroscopy does not allow estimating P pools in the soil. In addition, quantification of P available in the soil via predictive modeling has limitations in tropical soils. On the other hand, estimating P content in soil through classes of availability is a feasible and promising alternative.

Combining Laser-Induced Breakdown Spectroscopy (LIBS) and Visible Near-Infrared Spectroscopy (Vis-NIRS) for Soil Phosphorus Determination

“…However, studies that compare the use of iPLS and CARS on soil properties determined by vis-NIRS have not been found. Overall, studies where variable selection strategies have been based on automated methods [45,50,51,93], which have given better results than when using the full spectra [49,52], which is in line with our findings.…”

“…They obtained an R 2 of 0.58 and 0.61 and RMSE of 3.78 and 2.01 mg kg −1 , suggesting that model accuracy depends on the reference method used. In a recent study by De Souza et al (2020) on 90 soils from the same field, PLSR models for labile, moderately labile and non-labile pools of P with reported Rv2 values of 0.42, 0.19 and 0.48, respectively, were obtained [ 52 ]. Again, the authors concluded that model accuracy depended on the P pool or method used, as we also found in our study.…”

“…Even though P is not spectrally active in the vis-NIR region, correlations between P-forms and spectrally active soil properties have been the basis for quantification of different P-forms indirectly by means of vis-NIR [45,46]. For soil P predictions, a number of studies have yielded contradicting results [47][48][49][50][51][52]. Stenberg et al (2010) attributed the contradicting results to differences in the datasets in the P pool measured, but also to the P reference method used.…”

Estimating soil chemical properties by diffuse reflectance spectroscopy: Promise versus reality

“…Secondary soil chemical properties that have been estimated by spectral reflectance include extractable macronutrient (e.g. K, P and Ca) concentrations, mineralisable N, lime requirement, exchangeable or extractable acidity and pH (de Souza et al, 2020;Munnaf et al, 2021;Niederberger et al, 2015;Pinheiro et al, 2017;Tarnik et al, 2020;Viscarra Rossel et al, 2006). These soil properties are not spectrally active, showing no consistent detectable spectral features in the VNIR region (e.g.…”

“…In the last two decades, interest in using diffuse reflectance visible and near-infrared (VNIR) spectroscopy of soils to replace the more laborious standard laboratory methods to determine soil physical and chemical properties has grown as evidenced by the steadily increasing number of published studies on this topic (Ahmadi et al, 2021;Bellon Maurel et al, 2010;de Souza et al, 2020;Gates, 2018;Greenburg et al, 2021;Nocita et al, 2015;Pinheiro et al, 2017;Soriano-Disla et al, 2014;Stenberg et al, 2010). Numerous authors have pointed to the advantages that reflectance spectroscopy presents over standard chemical tests of soils, specifically simplicity of soil sample pretreatment, lack of chemical reagents and chemical waste, rapidity and low cost (Mohamed et al, 2018;Zornoza et al, 2008), implying that conventional soil testing procedures can be replaced by this spectroscopic method.…”