Abstract
A portable X-ray fluorescence probe (pXRF) is a tool that is used to measure many elements quickly and efficiently in various samples, without any pretreatment. However, each type of sample generally requires different calibrations to be accurate. To overcome this, our work evaluated the efficacy of determining several elements in forage plant samples using the ‘Soil Nutrient and Metal’ calibration in a commercially available pXRF probe, envisioning that a single calibration can be used to measure samples of different matrixes. For this, the net intensity of the pXRF probe was determined in place of the concentration values that are obtained directly from measurements. Elemental concentrations (P, K, Ca, Mg, S, Cu, Fe, Zn, and Mn) from forage plant samples, collected across Oklahoma, US, were assessed in a representative number of ‘modeling’ and ‘validation’ (independent dataset) samples. Linear regression (LR) associated with the d-index, polynomial regression (PR), and power regression (PwR) were tested for predictions, producing many statistical parameters associated with the models that were used for comparison goals. The pXRF elemental data provided highly reliable predictions of K, S, Zn, and Mn regardless of the regression model. Although all models can be reliable in prediction of Ca and Fe concentrations, the PwR provided better root mean square error (RMSE) values. The predictions of Mg concentrations were less reliable, although highly significant; however, the P and Cu predictions were not acceptable. Our work successfully showed that, once established, a single calibration curve that covers a wide range of concentrations of several elements in soils and plant tissues enables both soil and plant samples to be analyzed. This suggests that manufacturers can develop a new calibration model for a commercially available pXRF probe that covers a wide variety of heterogeneous samples.
Highlights
Research on the application of portable X-ray fluorescence probes for agricultural purposes has been very limited, and research regarding their use for plant analysis was nearly nonexistent [1]
We aimed to use the same technique to determine plant nutrient concentrations from a calibration method that was originally developed for soil analysis, since the range of elemental concentrations of forage plant samples used in our study is within the range of the calibration curve acquired by the pXRF probe
The ≤2 mm particle size was chosen to simulate the same physical conditions of processed soil samples that can be measured by a pXRF probe, and because of the results of Sapkota et al [3], who indicated that the heterogeneous nature of the forage plant sample did not affect the quality of results when samples are ground to ≤2 mm sizes
Summary
Research on the application of portable X-ray fluorescence probes (pXRF) for agricultural purposes has been very limited, and research regarding their use for plant analysis was nearly nonexistent [1]. Some works [1,2,3,4] have assessed pXRF probe use in determining plant nutrient concentrations, aiming to eliminate the wet chemistry sample preparations that are usually involved during acid digestion [5]. It is important to develop a single calibration curve with multiple purposes, with potential use for heterogeneous samples such as soils and plants. This would gradatively eliminate the costly use of time and resources that is involved in the standard method (wet chemical analysis via acid digestion and quantitative determination via ICP). Our work will be a pioneer in inspiring such a development, since a single calibration curve—originally designed for elemental analyses in soil samples—will be used to determine nutrient concentrations in forage plant tissues
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