The Evaluation of Methods for Determination of Humic and Fulvic acids in Fertilizer Materials

Humic substances (HS) are complex and heterogeneous materials with different sizes produced through decomposition and transformation of plant and microbial residues by chemical and biochemical processes. HS producers, consumers and also regulators have a great interest to find a precise and accurate method for quantification of HS due to increasing use of them in agriculture.  Since there is no distinct definition and consistent chemical structure for HA, an accurate quantification method has not been developed. There are four laboratory methods for determination of HA, FA, humates, and HA derivations (e.g. humate extracts) in commercial fertilizers. These methods include; ISO 5073 titrimetric method that has been published as national standard 11094, a colorimetric method that has been developed according to Mehlich method (1984), and two gravimetric methods that one of them has been developed by California Department of Food and Agriculture (CDFA) and other one is called New Standardized Method (NSM). All of these methods are based on the solubility of HS in dilute alkali solutions.  Moreover, FA and other alkali soluble materials e. g. amino acids, proteins, sugars and fatty acids are extracted and measured in colorimetric and titrimetric methods as HA, hence these methods overestimate HA content. The NSM method has obtained by modification of classical method and is based on extraction of HA and FA from natural substances. In this method the separation of FA from other strong base extractable substances such as polysaccharides, amino acids, proteins, and lipids is possible. Also, the NSM method has been accepted by International Humic Substances Society (IHSS) and Humic Products Trade Association (HPTA) and it is the base of standard method that has been developed by International Organization for Standardization (ISO 19822). Therefore, the NSM method is recommended as a reference method.