3.2. LIBS spectra of soil samples
The average LIBS spectra of the soil samples, from 200 to 1000 nm, are displayed in Fig. 4. The main Si, Mg, Ca, Na, and K lines were marked according to the National Institute of Standards and Technology (NIST) electronic database (Kramida, 2018). The Ca II (ionic) lines were at 315.89, 317.93, 393.37, 396.85, and 422.67 nm, while the Ca I (atomic) lines were at 558.88, 559.45, and 610.28 nm. The Mg II (ionic) lines were at 279.55 and 280.27 nm, while the Mg I (atomic) lines were at 517.27 and 559.45 nm. The Na I lines were at 588.99 and 589.59 nm. The K I lines were at 766.49 and 769.90 nm. The Si line was at 288.15 nm. In general, choosing suitable lines from spectra is an important and challenging task in LIBS analysis. Researchers (Rühlmann et al., 2018) have mentioned that the Ca I line at 422.67 nm and Ca II lines at 393.37 and 396.85 nm have been employed for Ca determination detection, but did not show good performance for Ca nutrient quantification in soil because of self-absorbance. Thus, the ionic lines of Mg I at 517.27 nm, Ca I at 558.88 nm, Na I at 588.99 nm, and K I at 766.19 nm were selected for further study.