MEASURING THE HUMIC ACIDS CONTENT OF COMMERCIAL LIGNITES AND AGRICULTURAL TOP SOILS IN THE NATIONAL SOIL PROJECT
Radiocarbon dating studies have demonstrated that soil organic matter (OM) consists of two pools: a labile, short-lived pool that is an energy source for microbes, and a much longer-lived humic substances (HS) pool that is protected from microbial attack and represents the sequestered carbon in a soil. The longer-lived pool has three operationally-defined fractions: fulvic acids (FA, soluble at all pH), humic acids (HA, insoluble below pH ≈ 2) and humins (HU, insoluble at all pH). HA and FA can be extracted from soils and low-rank coals with aqueous bases and fractionated by acidification of the basic solutions. Several other procedures have been published but, to our knowledge, none have been shown to determine quantitatively the HA content of lignites and soils. This paper describes a gravimetric protocol that measures the HA content of four lignites used as commercial sources of HS products and 24 agricultural top soil samples from 13 counties in southern Idaho. No HU was found in the samples. The lignites and soil samples contain from 2-41 %HA and 0.31-9.8 %HA, respectively. Analytical precision limits are attributed to sample heterogeneity.