Towards a Direct, Non‐destructive Method of Characterising Soil Humic Substances Using 13 C N.M.R.

Summary The structure of a humic acid from a New Zealand pasture topsoil has been investigated by conventional solution 13 C n.m.r. spectroscopy and by the cross polarisation technique. Relaxation results showed that a pulse spacing of about 0.2 s was optimum for n.m.r. of the humic acid dissolved in 0.5 M aqueous NaOH. Aromatic rings accounted for 26 per cent of the total carbon. Cross polarisation n.m.r. is a viable method for obtaining 13 C spectra of solid humic acids, although aromaticity cannot be estimated as accurately as for coals.