COMPARISON OF METHODS FOR DETERMINATION OF HUMIC SUBSTANCES IN COMMERCIAL LIQUID FERTILIZERS

The beneficial effects of humic substances (HS) on crop development have been evidenced in the last decade, leading to the commercialization of HS products with fertilizing properties. Currently, three methods are frequently used for HS determination: method described in the manual of the Brazilian Ministry of Agriculture, Livestock and Supply (MAPA); method employed by the United States Humic Products Trade Association (HPTA) and method used for the determination of soil HS (“HS-Soil”). This study aimed to evaluate and compare the quantification of HS in commercial liquid fertilizers, by the mentioned methods. For HA-rich fertilizers the studied methodologies presented results close to the HA and FA contents informed in the labels. For products rich in FA, the methodologies that use dichromate to oxidize organic carbon overestimated the results of FA, taking as reference the HPTA method, that determines gravimetrically the purified FA and HA. The time of analysis varied between the methodologies studied, as well as the type and volume of the produced waste. Based on the chemical principle of HS determination of the method and the environmental impact of the waste generated, we propose that the most appropriate method for determination of HS in liquid fertilizers is that of HPTA.