Characterization of humic acids derived from Leonardite using a solid-state NMR spectroscopy and effects of humic acids on growth and nutrient uptake of snap bean

Abstract Understanding chemistry and the effects of humic substances on plant growth is important for using organic fertilizer or soil amendment for sustainable snap bean production. The objective of this study was to characterize different fractions of humic acid (HA) derived from Leonardite and evaluate their effects on seedling growth and nutrient uptake of snap bean (Phaseolus vulgaris L.). HAs extracted from Leonardite were separated based on molecular weights into three fractions (HS1, HS2, HS3) plus sediment (SED). With direct polarization combined with spectral editing techniques, functional groups of HAs were quantified and the results indicated that HAs with low molecular weights had more O-alkyl and carboxyl C groups than those with large molecular sizes. A plant growth experiment was conducted as a randomized split-plot design with three replications and repeated for three plantings. The results show that addition of HAs was beneficial to leaf and root growth of snap bean compared with the control (no HA). Plants treated with low-molecular weight HAs had significantly greater root length, root surface area, and uptake of potassium by shoot than those treated with other HAs, while leaf growth was affected mainly by HAs with high molecular weight.