A review of soil nutrient detection by laser induced breakdown spectroscopy

Abstract

Nitrogen (N), phosphorus (P) and potassium (K) are not only plant macronutrients but also soil fertilizer indicators, soil N, P and K rapid analysis are of significant importance to agricultural industry. This paper reviewed the recent progress and applications of laser induced breakdown spectroscopy (LIBS) on soil N, P and K detection. To reduce sample diverseness and spectral interferences, samples were mixed, milled, pressed to pellets, and even prepared with a simulated soil matrix of sand. The LIBS instrument was commonly set up with a fundamental frequency laser of 1064 nm and 10-102 mJ, detector usually set to the wavelength of ultraviolet, visible and short-wave near-infrared, collecting spectral at time window of a microsecond scale. Several means were employed to remove air interference and enhance signal quality, including low-energy laser, femtosecond laser, sequenced laser pulse, pre-ablate, optical path optimization, spatial confinement, buffer gases purge and reduced pressure. 746.83 nm was the commonly used N emission line, under the condition of interference avoidance from air N, correlated curves have been used to predict N concentration. Usually used P characteristic line, such as 255 nm, was located at ultraviolet range, methods of linear regression, intensity correction and calibration-free LIBS have been used for soil P analysis. 404.72 and 766.49 nm were the widely used K lines. Beside linear regression, internal standard and calibration-free LIBS, multivariable data mining methods, including partial least squares and support vector machines, were used to analyze soil K. Furthermore, variable selection methods of genetic algorithm, variable importance in projection and the coefficients graph were employed to improve model precision. These researches demonstrates that although challenges remain in terms of matrix effects, spectra and model processing, and instrument development, LIBS is a potential technique for rapid, in suit and multi-elements analysis on soil nutrient.