Predicting the contents of soil salt and major water-soluble ions with fractional-order derivative spectral indices and variable selection. - Congcong Lao; Junying Chen; Zhitao Zhang; Yinwen Chen; Yu Ma; Haorui Chen; Xiaobo Gu; Jifeng Ning; Jiming Jin; Xianwen Li (Comput. Electron. Agric., 2021)