The total P estimation with hyper-spectrum – A novel insight into different P fractions

Abstract

Phosphorus (P) is one of the main characterization parameters of soil fertility and an important element affecting vegetation growth. This paper used the multivariate stepwise regression to estimate the soil P concentration through multiple mathematical transformation of the original spectrum, evaluate the accuracy of different models, and determine the optimal modeling method. The total P (TP) is classified as loosely adsorbed P (Ad-P), P bound by iron oxides or hydroxides (Fe-P), aluminum-bound P (Al-P), calcium-bound P (Ca-P) and residual P (Re-P). Then the relationship between various P fractions and soil properties and their spectral characteristics were analyzed. The effects of different P fractions on total P (TP) estimation were discussed from the aspects of selected bands, corresponding forms and spectral transformation forms, the direct and indirect estimation models are constructed respectively, which provided a new idea for soil spectrum estimation of soil properties. The results show that (1) Al-P is the main component of TP, and its spectral characteristics are not obvious, and the correlation coefficient between Al-P and TP is 0.437, which is the main reason for the stability of TP estimation model. (2) Soil organic matter and Ca-P have the greatest influence on TP estimation. When the band is less than 1200 nm, soil organic matter concentration is the main factor affecting TP estimation. When the wavelength is greater than 1200 nm, Ca-P is the main factor affecting TP estimation. (3) An indirect estimation model of TP was developed by using Ca-P, the R2 of the calibration reached 0.78, and the R2 of the validation reached 0.66, the model is stable and can be used to estimate TP content. It is proved that it is a feasible method to estimate the TP content through the spectral characteristics of P fractions, the relationship between organic matter and TP is not the only key reason for the successful estimation of TP.