The comparative evaluation of humic acid determining methods in humic-based commercial fertilizers

The increased use of humic acid (HA) as a plant-growth stimulant in recent years has led to an intense interest in finding an accurate and reliable method to quantify it in an acceptable manner among producers and consumers. Today, there are four common laboratory methods for the determination of HA, including the CDFA, Colorimetric Method, ISO 5073, and NSM. To date, there has been no comparison among these methods to evaluate the quantity of HA across a wide range of commercial fertilizers containing various additives. In the present study, the four aforementioned methods were used to determine the HA content in 22 samples containing a wide range of physical and chemical properties. According to NSM's principles the method and their consistency with the classical method, it was used as a reference method to make comparison among the other methods. Compared to the NSM, CDFA, and ISO 5073 methods that underestimate the HA content (13.8% and 1.5%, respectively), however, it has been demonstrated that the colorimetric method overestimated the HA content by 64.2%. The low ratio of extractant to sample and the presence of soluble organics are the main reasons for under and overestimation in CDFA and colorimetric method, respectively.