Analytical Characteristics of Humic and Fulvic Acids Extracted from Tropical Volcanic Soils

Certain tropical soils formed on materials of volcanic origin on islands in the West Indies, tend to accumulate substantial amounts of organic matter but are so infertile that the addition of lime and fertilizers does not increase crop response. To obtain a better understanding of this unusual behavior, humic and fulvic acids were extracted from problem soils on the island of Dominica, and characterized by chemical and spectroscopic methods. Elementary and functional group analyses as well as optical and IR data for humic and fulvic acids extracted from the tropical soils were similar to those of humic and fulvic acids from soils from widely differing climatic zones. The only major difference was the tenacity with which the tropical fulvic acids retained (in water‐soluble form) relatively large amounts of ash (between 9.4 and 12.6% of oven‐dry weights), rich in silica. Our data suggest that the unusually high resistance to degradation of organic matter in these soils is, at least in part, related to the formation of stable silica‐fulvic acid “complexes.”