Fig. 10 Bar-graph plot of the composition of two plant-extracted materials (PEMs), International Humic Substances Society (IHSS) Florida peat, and three commercial humic acids (HAs), as obtained from direct-polarization magic-angle spinning (DPMAS) corrected by cross-polarization spin- lattice relaxation time-total sideband suppression (CP/T₁–TOSS). On the right, bar graphs of compositions calculated from two models of humic acid (HA) structures are also shown. The chemical-shift ranges are the same as in Fig. 9