“…The next step of this research would be to assess the portable mid-IR and vis-NIR soil spectra obtained under field conditions and further eliminate the environmental and systematic factors [ 44 , 63 ] affecting both soil spectra in a similar way to [ 46 , 47 , 72 ]. In addition, a variety of calibration models (ranging from simple to complex), e.g., partial least squares regression (PLSR) [ 1 , 73 , 74 ], principal component regression (PCR) [ 7 , 75 ], artificial neural networks (ANN) [ 76 ], convolution neural networks (CNN) [ 77 , 78 , 79 ], support vector machines (SVM) [ 80 ], and deep learning [ 81 , 82 , 83 , 84 ], to name a few, could be studied.…”