SUMMARY Humic acids were isolated from nine topsoils in New Zealand tussock grasslands. Cross‐polarization 13 C NMR spectra of solid samples were used to estimate fractions of carbon contained in different types of chemical functional groups. The degree of oxygen substitution of aromatic rings showed a strong negative correlation with soil development. Aromaticities greater than 0.25 were found in humic acids from only the two least‐developed soils.
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Newman et al. (1991) studied this question.
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