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Characterization of Humic Acid from No-Tilled and Tilled Soils Using Carbon-13 Nuclear Magnetic Resonance

Plant and Soil Science

L. J. Tortorelli

Chemistry Dep., Univ. of Tennessee, Knoxville, TN 37901

* Corresponding author.

ABSTRACT

Chemical composition of humic acids is important due to their reactivity with soil-applied pesticides and fertilizer amendments. Humic acids from continuously (7 yr) no-tilled and tilled cotton, corn, and soybean plots in West Tennessee were characterized by ¹³C-NMR from soils sampled at various depths. Humic acids were extracted with dilute NaOH solution and purified with KOH-KCL and HCL-HF solution, respectively. NMR solution spectra generated on a Jeol FX 90Q spectrometer (Jeol Co., Tokyo, Japan) were divided into six regions (0–40, 40–62, 62–105, 105–150, 150–170, and 170–190 ppm) and peak areas compared. Humic acid composition differed by soil depth and crop. Small differences were observed between tillage systems. Aliphatic and aromatic carbons ranged from 48 to 65% and 25 to 40% of total peak area, respectively. Carboxyl groups of humic acids as determined by ¹³C-NMR ranged from 9 to 13%. Humic acids extracted from those soils containing larger amounts of C (surface no-tilled treatments) had greater aliphatic to aromatic ratios, perhaps due to earlier stages of decomposition.

Received for publication June 27, 1988.

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