Humic Substances and their Agronomic Importance

Research over the last half century has shown that humic substances are major components of natural organic matter in soils and, after years of humification, have formed molecules which contain complex functional groupings.

These functional groups can promote improved soil structure and facilitate nutrient and water interactions with plants. Furthermore, university research on turfgrass has shown that humic acids can have certain plant-beneficial effects beyond that of normal mineral nutrition. However, there remains some confusion on what humic acids are, where they come from and how that translates into improving soils and ultimately, the performance of turf and ornamentals.

Where to Start:

The Right Source

According to the International Humic Substance Society (IHSS), high quality humic substances can be derived from certain soils, peats, leonardite (highly oxidized lignite), or river water. In agriculture, leonardite humic acid has become the bench mark standard in the industry for high quality humic and has been the source used in most university-level turfgrass research. Sub-bituminous coal is similar to leonardite in that it is usually found in association with highly oxidized lignite but usually at deeper layers. The deeper layers typically contain higher levels of oxidation which yields higher concentrations of humic substances and less materials that have no agronomic potential (ash, toxic metals, etc.) and will eventually need to be removed with further processing.

The Next Step: Humic Fractions & Wet Chemistry

After identifying a high quality humic source, the next step is to make it useful in an agronomic setting. Some products will take the raw ore and will grind it up and blend it with a fertilizer or will granulate it and apply it to the soil. The problem with this is, leonardite ore is not very soluble or ionic in its raw state and, consequently, is not very useful in the soil unless some steps are taken to activate it. Dr. Muir out of the University of Idaho, has invested many years researching humic acids and is firmly convinced that humic substances which have been activated using a process called ‘Wet Chemistry’ yields humic products that are superior in their agronomic performance. Once activated, the humic molecules are relaxed so that the functional groups are more accessible to assist in soil-nutrient and soil-water interactions.

The key step in Wet Chemistry Activation is to react the raw ore with an alkali reagent (typically potassium hydroxide) so that the humic substances are hydrolyzed. Once in solution the humin fraction, which is not soluble at any pH, will precipitate out of solution and when removed, only the humic fraction and the fulvic fractions remain. If the source quality is high and the raw ore has been activated, this humic/fulvic solution is often marketed as a concentrated liquid humate product.

A possible final step in processing is to separate out the humic from the fulvic fractions to get access to straight fulvic acid. Humic acids are not soluble under acid conditions so to do this, an acidifying agent (typically HCl) is added to the humic/fulvic solution to lower the pH (pH 1-2). Once the pH reaches that level of acidity, the humic fraction will fall out of solution, yielding only the fulvic fraction which is soluble at any pH.

A Consideration: Measuring Humic & Fulvic Acid Content

Currently there is no standardized method in the U.S. for measuring humic and fulvic acid content in products containing these substances. Unlike the colorimetric method which determines the amount of humic acid by measuring how much light the humic substances absorb, the CDFA method, which is used by California and Oregon, only measures the humic acid fraction and does not include the fulvic acid fraction. A high quality, concentrated liquid humic/fulvic product that reports 12% humic acid (colorimetric) will typically be around 6-8% humic acid using the CDFA method, depending on the source material used. Humic products which have not been activated will exhibit much lower concentrations when using the CDFA method. While the reported humic acid content is not the only characteristic that should be evaluated when selecting a humic product, it is important to understand which method was used to determine the amount of humic acid in the product.

Performance: What to Use & When?

With a high quality humic source which has been fractionated and activated, the remaining question is when to use which products? The section below addresses each humic fraction in kind and aims to assist in the decision-making process when answering these questions.

Humic Acid
Humic acid is the higher molecular weight fraction of humic substance which is insoluble under acidic conditions (pH < 2) and soluble under alkaline conditions. Humic acids are typically a dark brown to black in color and have a higher carbon and lower oxygen content. The higher molecular weight, high cation exchange capacity and higher carbon content makes humic acids ideal as a soil amendment and, when incorporated into the soil, will help promote plant and soil health by mediating water and nutrient interactions in the rootzone. Since humic acids are pH sensitive, mixing humic acid products with low pH products or tank solutions should be avoided.

Fulvic Acid
Fulvic acid is the lower molecular weight portion of humic substance which is soluble under all pH conditions. Fulvic acid is typically light yellow to yellow-brown in color and typically contains higher oxygen than carbon content. Research has shown that humic substances with the lowest molecular weight and highest acidity (fulvic acids) have the ‘most hormone-like’ activity on plants (Piccolo, 1992). Fulvic acids contain a high cation exchange capacity and can be mixed with low pH tank solutions, making them ideal for foliar spray applications.

The Right Source

High quality humic substances can be derived from certain soils, peats, leonardite (highly oxidized lignite), or river water.

  1. Humic substances will:

    Continue to gain importance as turfgrass managers strive to balance higher level performance demands while promoting improved soil health for the future.

  2. Moving forward:

    It has been projected that humic substances will continue to play a larger role in the process of improved agronomics and soil health.

  3. Conclusion:

    This article has attempted to show not only how humic and fulvic acids can promote soil and plant health, but also provide some key points for consideration when making the decision to purchase and use a humic-based product.

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