Humic Substances in Biological Agriculture

Humic substances are the most widely distributed organic products of biosynthesis on the face of the earth, exceeding the amount of carbon contained in all living organisms by approximately one order of magnitude.

Soil organic matter is defined as the total of all naturally occurring organic (carbon-based) substances found in soils that originate from living things. The process of changing from recognizable bits and pieces of plants (or animals) to an amorphous, “rotted” dark mass is called humification. Humus is defined as the organic matter in soil, a mixture of partially and totally humified substances. Most humic substances result from plant matter subjected to the natural process of decay. Humic substances make up about 80 percent of the soil organic matter in dark soils.

Humic substances in soils are the dark brown, fully decomposed (humified) remains of plant or animal organic matter. They are the most chemically active compounds in soils, with cation and anion exchange capacities far exceeding those of clays. They are long-lasting critical components of natural soil systems, persisting for hundreds or even thousands of years — although they can be destroyed in less than 50 years by some agricultural practices.

Humic substances are the most widely distributed products of biosynthesis on the face of the earth. Besides soils, they can be found in varying concentrations in a number of different sources: rivers, lakes, oceans, compost, sediments, soils, peat bogs and soft coal.

The worldwide usage of humic substances is extensive. Their benefits in agricultural soils is well established, especially in soils with low organic matter. They are an integral part of all ecosystems and play an important role in global cycling of nutrients and carbon.

Humic substances are extremely versatile. They provide a concentrated and economical form of organic matter that can replace humus depletion caused by conventional fertilization methods as well as being used in biological programs. The addition of humic substances to soils, including calcareous soils, can stimulate growth beyond the effects of mineral nutrients alone.

HUMIFICATION

Humification is the natural process of changing organic matter such as leaves into humic substances by geo-microbio-logical mechanisms. Compost is an intermediate product consisting of humic substances and partially decomposed organic matter. As the conversion process continues, different chemicals dominate at different times. Complete conversion to humic substances will eventually occur.

Unlike most other natural biosynthetic processes, humification occurs in a complex, chaotic “open” system where there is no “closed” control of the process by enzymes, cell structures, membranes or cellular transport systems. With the infinite variety of plant materials that exists in nature and with the infinite access to chemical radicals, humification should produce infinitely variable humic substances. It would seem impossible to find two humic molecules with the same structure.

CONFUSION& NON-STANDARDIZATION

Humic substances have been a matter of scientific controversy for over 200 years. They are incredibly complex colloidal supermixtures that have never been separated into pure components. Inconsistent use of terminology and the previous lack of standard materials for comparison purposes have compromised the ability to translate the sparse amount of scientific knowledge to practical applications in soil environments. Traditionally, humic substances have been defined by their solubility in aqueous (water) solution at arbitrary pH levels and molecular weights.

The use of numerous names to describe commercially available humic materials has contributed to the confusion. Humates, humic acid, leonardite, brown coal, lignite, slack lignite, oxidized lignite, weathered lignite, humalite, fulvic acid, fulvates, ulmic acid, humic shale, carbonaceous shale, colloidal minerals, that are used to describe and/or market humic substances.

Non-standardization and confusion is not limited to humic substances. For example, many labs are using soil tests that may not accurately determine soil organic matter content due to oversimplification. There are numerous tests for soil organic matter, but there is no standardized test protocol for all soils. Some of the tests for soil organic matter have to be interpreted with much caution. Additionally, conventional analyses do not predict possible adverse interactions of trace elements.

BENEFITS OF HUMIC SUBSTANCES

While the complete structure of humic substances has eluded scientists, their effects on everything from apples to zucchini have been extensively studied.

Humic substances are renowned for their ability to:

• Chelate soil nutrients
• Improve nutrient uptake, especially phosphorous, sulfur and nitrogen
• Reduce the need for nitrogen fertilization
• Remove toxins from both soils and animals
• Stimulate soil biological activity
• Solubilize minerals
• Improve soil structure
• Act as a storehouse of N, P, S and Zn
• Improve water-holding capacity for better drought resistance and reduction in water usage

Extensive research on the stimulatory effects of humic substances has been conducted by the USDA-ARS soil lab in Minneapolis, Minnesota, and worldwide. Most of the research conducted in Eastern Europe on improving nitrogen utilization has not been translated into English.

Depending on the form of fertilizer applied, nitrogen may become a structural component of humic substances as a stable organic material, preventing it from leaching through the soil. In their natural state, humic substances contain anywhere from 1 to 5 percent nitrogen.

NITROGEN MANAGEMENT

Other effects of humic substances include increased CEC (cation exchange capacity), stabilization of soil structure, and the reduction of nitrogen and phosphorus fertilizers. The importance of humic substances to the fertility of soils and the stabilization of nitrogen has been well documented. One study done by a science team from West Texas A&M University and the USDA-ARS demonstrated the potential of humic substances in reducing ammonia emissions from feed lots.

If there are sufficient humic substances present, up to 35 percent of the soluble nitrogen applied to soils as fertilizer can be retained in the soil in organic forms at the end of the first growing season, thus converting the nitrogen to a stable, bioavailable form.

PELLETIZED LEONARDITE

One of the biggest obstacles to using humic products is the dustiness of the dry materials, making them almost impossible to handle. Liquid humates are easier to handle, but their use is restricted to foliar application at very low concentrations. Because of extremely low application rates, they have no effect on soils. The water-soluble derivatives from alkali extractions are only compatible with high- pH liquids, and they are expensive. Pelletizing humic substances can improve their handling and allow them to be blended with fertilizer “in the row,” where they can do the most good.

A team of scientists with the U.S. Bureau of Mines, University of North Dakota, combined standard NPK fertilizer with leonardite into a pelleted form. Although the addition of leonardite lowered the soluble analysis for NPK to 10-10-5, thus lowering the relative amount of applied fertilizer, the pelleted leonardite combination (10-10-5 L) was effective on barley, potatoes and sugar beets.

Despite the fact that the results concentrated primarily on yield — typical of conventional NPK fertilization programs — the report underscores how humic substances can improve nitrogen utilization and impact overall crop quality by increasing the efficiency of fertilizers. Additionally, the reduction in nitrogen usage demonstrates the environmental significance of using humic substances blended directly with fertilizers.

CARBON CYCLING

The carbon held in soil humic substances is so stable that it can be retained in soils for thousands of years, depending on conditions. The sheer complexity of these materials may explain why they are not broken down more quickly by microbial action. It is possible that the surfaces of humic substances are unrecognizable to microbes.

Conventional fertilizers rapidly age soil components, resulting in acidification of soils and dissolving the humic materials with soluble nitrogen. Urea is so effective at dissolving humic substances that it is used in some laboratory extraction procedures. A typical Iowa soil under conventional agricultural management retains its carbon for as little as 90 years.

The negative effects of high soil acidity have been extensively researched. Liming, the use of dolomitic limestone (calcium magnesium carbonate), improves soil productivity by providing cations of calcium (Ca₂₊) and magnesium (Mg₊) The carbonate ions raise the pH by combining with the excessive hydrogen protons.

The ability of humic substances to complex with cations such as calcium is decreased as the bulk pH of soils goes down (that is, as they become more acid) due to aggregation of the humic molecules. The aggregation reduces the exposure of functional groups, cutting off the access of nutrients to the molecules. Functional groups attached to carbon chains are primarily responsible for the biochemical characteristics of organic compounds.

HUMIC ACID, FULVIC ACID & HUMIN

About 200 years ago, the names humic acid, fulvic acid and humin were used to describe what workers believed to be three distinct fractions of humic substances. The three fractions were separated from various materials by using “classical” extraction techniques with aqueous (water) solutions. First, the humic material was treated with a strong alkali (base), then an acid was added. The acid caused a coagulated, black, sludge-like material to precipitate out of solution. They named the precipitate “humic acid.”

The remaining mixture that survived the base acid treatment consisted of an acidic liquid and a solid. The liquid was named “fulvic acid,” and the solid that was unaffected by the treatments was named “humin. Despite the fact that manufacturers use variations of these operations, which don’t necessarily duplicate the process described above, the names “humic acid,” “fulvic acid” and “humin” persist.

Potassium hydroxide is the typical alkali used by manufacturers to extract humic acid from leonardite. Since the remaining liquid solution is very alkaline, in the range of 8 to 12 pH, it is incompatible with acids. Here lies some of the confusion, the humic acid synthesized by this operation is not actually an acid. Because it can also be described as the product of adding acid to an alkaline solution, it is a salt — therefore the word “humate” may be more appropriate.

Some manufacturers follow the traditional method described above by treating the alkaline extract with acid, precipitating out the humic acid portion, leaving behind the so-called fulvic acid fraction in solution. The fulvic fraction is acidic, with a distinctive yellowish tint. Note, however, that the operation is vague. There is no definite pH at which the precipitate and acid are separated.

As various fractions of humic substances are soluble in a wide pH range, it makes sense that some fractions must be soluble at neutral pH. Some manufacturers treat humic materials with water, extracting the water-soluble fraction, calling that fraction either fulvic acid or “colloidal minerals,” which are promoted in human neutraceutical markets. Fulvic acid can be operationally defined as “the fraction of humic substances that is soluble in water under all pH conditions.”

The marketing of humic substances is interesting in that there is a lack of standardized analysis within the industry for fulvic acid and humic acid. For example, if liquidized humic materials are subjected to analysis, it is difficult to determine what the analysis reveals because of the infinite number of re-associations of free radicals that are possible during the extraction process. Some scientists argue that the reaction products are substances created by alkali treatment as complex degradation products, stripped of many of the original functional groups and recombined into an indescribable material. This may seem to be a nit pick, but some scientists like to argue about it.

Some people think that fulvic acid is more biologically active than humic acid because of its smaller molecular size. There is some truth in these representations as there is evidence that the lower molecular weight fractions have the ability to cross plant membranes and improve permeability of cell walls. It is true that fulvic acids have a higher “total acidity” than humic acids, but the chemical reactivity and chelating ability of humic acids is equal to or greater than fulvic acid, making them very bioactive substances. The humic acid fraction may be more effective than fulvic acid at solubilizing extremely stable aluminum and iron phosphates.

Thirteen carbon nuclear magnetic resonance and mass spectrometric analyses have revealed that the main structural features of humic acid, fulvic acid and humin are nearly identical. To scientists who study humic substances, the names have no meaning chemically. Some scientists say that humic substances from different sources are essentially the same.

COMPLEX GEOBIOLOGICAL SYSTEMS

The lower molecular weight (the mass of a substance expressed in gram equivalents of its atomic mass) of fulvic acid is sometimes said to account for its greater biological availability. That is somewhat correct, but vague — the industry has not agreed on standardized molecular weights for fulvic acid. Defining humic acid, fulvic acid and humin by their molecular weights is a controversial concept.

Some wet chemistry techniques can be used to characterize different humic materials. For example, the carbon-oxygen ratio is used by some to determine the presence of functional groups. There may be some merit to this, as functional groups are high in oxygen content. The difficulty with wet chemistry techniques is that they rarely mimic the real environments in which these materials are expected to perform.

Humic substances change their structure depending on pH and the type of metals present. High pH (or the presence of multivalent ions, such as calcium Ca²⁺) makes humic substances open up their long-chain polymers, whereas low pH makes them close. In the presence of toxic metals, humic substances remove the metals from the surrounding environment by forming insoluble aggregated spheres around them.

Humic substances are polymer-like molecules that demonstrate self-organization. The bi-layers formed by humic substances to surround otherwise insoluble minerals are reminiscent of the way all living things utilize biochemical reactions. The self-organized (micellular) colloidal phases act like biological molecules in cellular systems, showing a strong resemblance to the biological mechanisms of living membranes, as described in college textbooks. Humic substances are more like living creatures than chemical entities, but they don’t reproduce.

Slight changes in pH will actually cause the humic polymers to fracture, breaking up the original molecules. The fractured molecules are then free to associate with numerous other free radicals, metals or impurities. Humic substances are made up of hundreds of different molecules of many different sizes (polydispersity) with many ways to orient themselves by twisting, bending, compressing, and expanding (conformational changes). They are held together loosely by weak forces in a colloidal state.

FULVIC ACID

The primary reason why there is so much confusion about humic substances is the fact that the procedures used to describe them are based on “classical” aqueous extraction. If minerals are present in the parent material, they become complexed by humic substances. This allows more humic and nonhumic mate- rial to be solubilized during extraction by breaking down ion bridges that would normally hold the molecules together in higher-purity materials. Unless the supernatant is separated by special procedures (such as passing over an XAD-8 resin) to isolate the fulvic portion, the extracted substances may contain amino acids, proteins, sugars or fatty acids in addition to the fulvic acid.

In biological molecules, it is an established fact that the presence of functional groups such as carboxyl, phenol, quinone and hydroxyl are responsible for the activity of these molecules. There is some evidence that there are more functional groups in fulvic than in humic acid. The effectiveness of fulvic extracts may be influenced by the way they are synthesized during chemical processing. The fulvic fraction of humic substances is undoubtedly a beneficial part of oxidized lignites.

GEOCHEMICAL ANALYSIS

Silicon, iron and aluminum are among the most abundant elements in the Earth’s crust and the most common minerals associated with humic substances. Finding a humic substance low in contaminants is challenging, but not impossible. Since humic substances are composed mostly of carbon, a high carbon content can be used as a crude measure. Loss on ignition (LOI) is less reliable, but may be used to confirm the presence of carbon because organic material is burned off during this high-temperature procedure.

ENHANCING NUTRIENT BIOAVAILABILITY

Studies of the direct and indirect effects of humic substances on plant growth have repeatedly shown positive effects on plant biomass as long as there is sufficient mineral nutrition. Stimulation of root growth is generally more apparent than stimulation of shoot growth.

For many years, the stimulatory effects of humic substances were attributed to hormone-like activity because the action of humic substances was similar to auxins, cytokinins and absisic acid. This is no longer the case. The stimulatory effects of humic substances have been directly correlated with enhanced uptake of macronutrients, such as nitrogen, phosphorus and sulfur, and micronutrients, such as Fe, Zn, Cu and Mn.

Humic substances enhance the uptake of minerals through the stimulation of microbiological activity. Humic substances actually coat mineral surfaces with a membrane-like bi-layer, which aids in the solubilization of otherwise insoluble compounds by dissolving, complexing, and chelating the dissolved nutrients.

The bioavailability of nutrients released from rock minerals by biological activity is enhanced in the presence of humic substances. Geomicrobiologists have reported that organic acids generated by microbial activity directly influence the rate of dissolution (release of nutrients) from rock minerals.

The implications of this research are astonishing. Whereas conventional farmers are faced with the mandated reduction of soluble fertilizers, sustainable/ biological/ organic farmers can take advantage of the microbiological release of nutrients from insoluble minerals as the humic substances stabilize and improve the bioavailability of the minerals that are in soil solution.

CALCIUM

Humic substances are becoming renowned throughout the world for their importance in agriculture, especially their ability to chelate nutrient minerals and increase root mass. The benefits to soils and plants are extensive and correlate well with the benefits of humus, organic matter and calcium.

It seems reasonable to conclude that humic substances saturated with unwanted cations and heavy-metal contaminants may lower bioavailability and the efficacy of the product. Therefore, it is important to seek out a high-grade, uncontaminated source. Since there are many sites on a humic molecule for acceptance of cations, it seems reasonable to conclude that calcium in combination with humic substances should make a powerful combination.

Many of the benefits of calcium over- lap with the benefits of humic substances. Also, the low pH of humic substances along with their biological stimulation and chelating capacity combined with the right dry-calcium source may perform as well as, if not better than, chelated liquid products and calcium chloride. Furthermore, because humic substances are known to complex both cations and anions, creating a synergistic effect, the combined benefits should be greater than the individual ingredients.

In theory, the enhanced benefits should result in increased:

• Biological release of nutrients from otherwise insoluble minerals
• Root growth
• Nutrient uptake from the larger root mass
• Respiration
• Photosynthesis
• Mineral bioavailability and stabilization
• Nitrogen stabilization and fertilizer efficiency
• Disease resistance

Indeed, recent research has demonstrated how the combination of dry calcium with leonardite performed as well as calcium chloride and EDTA, a popular synthetic chelating agent.

HUMIC SUBSTANCES & ROCK PHOSPHATES

The ability of humic substances to sol- ubilize and complex with natural minerals such as rock phosphates is well documented. The bioavailability of nutrients released from rock minerals by microbiological activity is enhanced in the pres- ence of humic substances.

Humic substances can improve the effectiveness of rock phosphates by causing the release of PO^3- anions and Ca cations from hardly soluble rock minerals because of high total acidity and its ability to complex and chelate the resulting solutions and to stimulate microbial metabolism. Natural leonardite is a complex supermixture of high and low molecularweight humic substances. The lower molecular-weight constituents (fulvic acids) are primarily responsible for the solubilization of phosphate minerals. Contrary to the typical marketing claims for “fulvic” acids, the higher molecular- weight components (humic acids) also engage in solubilizing minerals, have a higher capacity for stimulating biological activity, and exhibit greater potential for chelation. In natural soil systems, the two components may act synergistically by complementing each other.

Humic substances also chelate iron, zinc and copper and complex with many other trace elements. Elements typically found in natural phosphate minerals, such as zinc and copper, are known to suppress pathogens and encourage the growth of beneficial organisms.

These phenomena have environmental implications as well because the solubilization of rock phosphates by humic substances can reduce the need for industrial acidification of rock phosphate used for the production of phosphatic fertilizers. Industrial production of phosphate fertilizers is extremely inefficient and creates enormous waste piles that are burdened with contaminants. Additionally, 60 to 80 percent of all highly soluble phosphate fertilizer applied to soils is lost to the environment.

Colloidal humic substances are part of natural soils and help retain nutrients in the soil system through soil stabilization and the stabilization of nitrogen. Furthermore, the complexing action keeps the minerals in solution instead of precipitating (locking up) with soil iron, aluminum and rare earth elements.

SUMMARY

Humic substances are formed by a process called humification. The humification process is chaotic, with innumerable reactions occurring under countless conditions. The process occurs over geological time, therefore younger deposits of humic materials generally have lower concentrations of humic acid.

Humic substances are critical components of water and soil ecosystems, which are essential to soil genesis and the global cycling of carbon and nutrients. The interactions among microbes, clays and minerals are dependent upon humic substances. The vast agronomic and environmental importance of these materials is just beginning to be appreciated.

Distinctions based on molecular mass (weight) or the quantity of functional groups and fulvic acid content seem useless if there is no agreement regarding the methods used to evaluate the materials. The quality of natural humic materials can be assessed by pH, CEC, total carbon, total organic carbon, and association with calcium, silicon, sulfur, iron, aluminum and toxic contaminants. The concentration of humic acid and fulvic acid can be analyzed by some labs since standard reference materials and procedures for the extraction and analysis of humic substances are available from the International Humic Substances Society.

The agronomic effectiveness of humic materials may be influenced by the presence of metals associated with the natural ores. Because humic substances are powerful complexing and chelating entities, association with silicon, aluminum or iron (typically found in clays) may influence the materials in soil systems. Research based on the agronomic effectiveness of humic materials (oxidized lignites) from different sources has not been performed.

CONCLUSION

The conventional tools of chemistry have not been able to explain why these materials work in complex soil ecosystems. They have all the qualities of humus in a compact convenient package. Although examination of the microscopic detail and structure of humic substances is currently not achievable, their beneficial properties are evident. The ecological and plant-nutritional benefits provide sufficient justification for using these extraordinarily complex ecominerals.

For the consumer, there is an endless variety of applications for humic substances, both as agronomic inputs and as human health aids. Humic substances are part of an environmental engineer’s toolbox for the bioremediation of toxic contaminants. Humic substances are possibly the most versatile natural substances ever known.

Humic acid fertilizer contains a large amount of soil organic matter, which can promote plant growth and development, improve soil structure, enhance the stress resistance of crops, and improve the nutritional status of plants. The humic acid structure contains a large number of hydrophilic groups, which can improve the water retention capacity of the soil and reduce the number of irrigation in agricultural production activities.