Studies have shed light on the chemical nature of humic and fulvic acid chelation reactions (Chen and Stevenson, 1986). An important property of any chelate is the strength with which it holds the micro-mineral. The Log K of a chelate is a way of quantifying its strength. Humic and fulvic acids that have higher oxygen content can have higher Log K values because they possess a greater number of carboxyl and hydroxyl functional groups. This Log K value for humic and fulvic acids: A moderate value is more to be desired.