Fig. 4 Two cross-polarization spin-lattice relaxation time (CP/T₁)–total sideband suppression (TOSS) spectra of humic acids (HAs) with different relaxation rates: A = IHSS-LEON, fully relaxed at filter time t_±z = 5 s; B = Florida HA, nearly fully relaxed at filter time t_±z = 5 s; C = Amherst HA, relaxed nearly half at filter time t_±z = 5 s; D = German HA, relaxed more than half at filter time t_±z = 25 s