Quantification of Humic Substances in Caustobiolites and Commercial Products Using a New Standardization Method

Nowadays, environmentally friendly agriculture contributed to a significant interest in the production of fertilizers based on water-soluble humic substances such as humates and lignohumate. These commercial products are dark brown powders and/or concentrated alkaline solutions, containing mixture of humic substances, lingo-humic acids, and smaller proportion of lightly hydrolyzing organic compounds. Their root and foliar application increase growth of roots and leaves, chlorophyll content, and activity of plant enzymes, etc. All of this has generated intense interest for an accurate and reliable method to quantify humic substances in caustobiolites and commercial products.

The aim of this work was to determine the content of humic substances in raw caustobiolites (i.e. lignite, leonardite, and alginite) and commercial humate products. Humic substances (HA and FA) were isolated from following samples: South Moravian lignite (the northern part of the Vienna basin, Mír mine near Mikulčice in the Czech Republic); leonardite (Afşin, Kahramanmaraş, Turkey); alginite (Pinciná in the Slovakia Republic); lignohumate MAX (Amagro, Prague in Czech Republic); and HumiKey (Xi´an, TBio Crop Science Co., Ltd., China). The humic substances were extracted using a new standardized method for quantification of humic substances (Lamar et al., 2014) recommended by the International Humic Substances Society. The content of humic substances was obtained by gravimetric analysis. The wt.% HA and FA contents were corrected for moisture and ash content. Furthermore, the humic substances were used in solid powder form and characterized by thermal techniques (i.e. elemental and thermogravimetric analysis), UV/Vis spectroscopy, and FTIR spectroscopy.

The determining factor influencing the yield of humic substances from raw caustobiolites and commercial products is their origin and method of extraction. The greatest content of HA (54.22 ± 1.76%) was obtained for sample isolated from Turkey leonardite. In contrast, the lowest contents were determined for HAs extracted from alginite and lignohumate MAX. It is obvious that these samples are characterized by significant content of FK and lightly hydrolyzing organic compounds. Extremely high ash content was determined for alginate. Caustobiolites (e.g. alginite) with high ash and low contents of humic substances appear to be less suitable as sources of HS for agricultural purposes.

All examined HAs isolated from caustobiolites were generally characterized by the complex and heterogeneous molecular structure with high average molecular weight and high degree of aromaticity. On the other hand, FAs, especially those isolated from commercial products, were predominantly aliphatic, with a smaller content of nitrogen and low degree of aromaticity and greater amount of oxygen-containing functional groups (e.g. carboxylic and phenolic).

This standardized method and studies on the physicochemical properties of HS can be helpful in predicting the behavior of such fertilizer components in the environment.

Reference

Lamar, R.T., Olk, D.C., Mayhew, L., Bloom, P.R., 2014. A New Standardized Method for Quantification of Humic and Fulvic Acids in Humic Ores and Commercial Products. J. AOAC Int. 97, 721-730. https://doi.org/10.5740/jaoacint.13-393.

Acknowledgement

This work was supported by The NATO Science for Peace and Security Programme, project Nr. G6296. https://land-security.org/.