Fig. 2. Cross polarization magic angle spinning ¹³C-nuclear magnetic resonance spectra for humic acids (HAs) isolated from a silty clay grassland soil by the International Humic Substances Society procedure at pH 7 (1), 10.6 (2), or 12.6 (3), and for the HA precipitated from dilute solution at pH 2 from the extract at pH 12.6 (spectrum 4) and subjected to the resin in tandem separation procedure. Spectra 5, 6, and 7 are for fulvic acids, and 8, 9, and 10 are for the XAD-4 (styrenedivinylbenzene) acids from the pH 7, 10.6, and 12.6 extracts, respectively, and fractionated by the resin in tandem process. Spectrum 11 is for the XAD-4 neutrals.