La nouvelle norme internationale approuvée par l'ISO pour la quantification des substances humiques
Humic and fulvic acid are terms that do not refer to a discrete chemical compound or molecule. There is no defined or specific chemical formula, molecular weight or structure for fulvic or humic acid. Thus, a more accurate term to use is the plural fulvic acids and humic acids. The term fulvic acid is referring to a large class of several hundreds of various complexes that have key distinguishing physical and chemical differences from humic acid complexes. There is a wide variation within fulvic acid complexes or humic acid complexes of different mixtures of multiple molecules, molecular weights and functional groups.
The most widely accepted definition for fulvic acid is the fraction of humic substances that are soluble in both acidic and alkaline conditions. Whereas humic acids are the fractions that are soluble in alkaline conditions only. Fulvic acids have: 1) smaller size, 2) lower molecular weight, 3) higher oxygen content, 4) higher chemical reactivity, and 5) greater microbial activity compared with humic acids.
There is no direct method to characterize or quantify fulvic or humic acid. All attempts to characterize or quantify to date are estimations, and as such a standard method has been difficult to develop. Early quantification attempts used colorimetric methods (Mehlich, 1984) to quantify humic and fulvic acid products by comparing the intensity of color with standard amounts for Aldrich humic acid. Researchers have shown that different batches of humic acid samples have different colorimetric measurements. Furthermore, the colorimetric method does
not distinguish fulvic from humic. Thus, a fulvic product that has even small amounts of humic contaminants will be darker color and as such will report higher colorimetric readings than a purer fulvic sample. Other organic and inorganic contaminants also impact color, and if present in the sample will be measured and reported as humic or fulvic. Fragments and derivatives of lignin, a very common organic polymer in plants are also dark-brown. Lignin is solubilized and removed from wood and other biomass in pulp and paper operations. The wastewater from pulp and paper often contains high concentrations of lignin, and lignin derivatives that will be indistinguishable by this method from humic substances. Finally, by using this method producers could claim 30% or more fulvic acid concentration in solutions, but does it mean anything? What is the customer really getting? Without more detailed analysis can they really know?
The California Department of Food and Agriculture (CDFA) stepped in to create a standard test for the quantification of humic acid. The method is more accurate than colorimetric and can result in more consistent concentrations. For typical samples the quantification using the CDFA method often resulted in concentrations that were less than half of what was measured using the colorimetric method. In, 1990 J.M Verploegh and L.A Brandvold published additional steps to the CDFA which would enable quantification of the fulvic acid fraction as well. The method does not account for the influence of ash content due to the presence of inorganics such as potassium (K), sodium (Na), and calcium (Ca). In addition, once again any acid insoluble organic species such as Klasson Lignin, which makes up the majority of lignin in biomass would also be insoluble in acid and contribute to humic acid levels quantified by the CDFA method. On the other hand, most common organic compounds such as sugars, polysaccharides, amino acids, proteins, carbohydrates, and other organic acids are soluble in acidic solutions, and as such would if they are present in the sample they would be quantified as fulvic acid by this method.
It was concerns around the short-comings of these methods that led the Humic Products Trade Association (HPTA) to embark on the development of a new standardized method. After eight years of dedicated research by the HPTA, the organization was successful in establishing an international standard for the analysis of humic acids and hydrophobic fulvic acids in commercial humic products. The new international standard (ISO 19822:2018) was published by the International Organization for Standardization (ISO) on August of 2018. As of 2014, the method was already approved by the Association of American Plant Food Control Officials (AAPFCO).
The new standard method can still be susceptible to the presence of Klasson lignin, or other acid insoluble organic matter for the quantification of humic acid, however, it is a great improvement as it removes the influence of ash on the quantification of humic acid. Additionally, the ISO method can only be used to quantify the 30 – 50% (by weight) of Fulvic acid complexes that exhibit hydrophobic interactions. Individual fulvic acid complexes contain both hydrophobic and hydrophilic moieties. As a result, the behavior of a fulvic acid complex is a function of probability. Fulvic acid complexes that have greater concentration of hydrophobic moieties have a greater tendency to exhibit hydrophobic behavior. Furthermore, lignosulfonates (common derivatives of lignin produced by pulp and paper production) also behave hydrophobically and would be indistinguishable from fulvic acid by this method. For this reason, the new ISO method proposes to test for the presence of sulfur, and if sulfur concentrations are found in excess of 0.75%, further characterization using Fournier Transform Infrared Spectroscopy (FTIR) analysis to be used to confirm the absence of lignosulfonates.
Although it has its limitations the new ISO method is the best method that we have encountered to date that addresses the inflation of the quantity of fulvic acid reported by inorganics as well as most non-fulvic acid organic additives or adulterants. As a member of the Humic Products Trade Association (HPTA) we are highly supportive of the development of a new standard method aimed at enabling a more accurate means to measure and compare products in the marketplace.
Visit http://www.humictrade.org for more information.