Brief: For practical purposes, Humic acid, Humus, and Soil Organic Matter (SOM) can all be used interchangeably when discussed in soil science; however, solubilized humic substances can take on enhanced characteristic based on the extraction process employed.
When discussing humic substances, there are various terms thrown around by different groups: soil organic matter, humic acid, humus, humates, and fulvic acid. The definitions of which are not wholly agreed upon through a general lack of standardization throughout the industry (HPTA is working to sort out a large number of standardization issues).
Soil organic matter (SOM) is composed primarily of humus; however, living organisms and more defined dead ones are also included. So using humus and SOM interchangeably is reasonable, especially when referencing humic material in the soil. A slightly longer discussion of soil organic matter can be found in a previous post.
In a majority of research publications, ‘humic substances’ is the general term applied to all humic compounds. This is due to the enormous complexity of humic substances, where chemical characterization is impossible (Troeh 2005). Any subdivisions of this humic material can never be completely characterized, so academics speak of humic compounds as the whole class of compound. Basically, this means that the terms humic substances and humus can be used interchangeable. However, humates typically refer to humus in dry bulk, non-solubilized form and humin represents the completely insoluble faction left over after alkaline extraction.
Some scientists and regulatory agencies (CDFA) do not draw a distinction between humic and fulvic acid. Fulvic acid being the smaller molecular weight, acid soluble components of humic material that can be separated and applied independently. In California, fulvic acid is not allowed to be listed as a separate active ingredient in agricultural input products. However, fulvic acids have the reputation of being a highly active portion of humus that is more readily taken up by plant systems, due to their small size.
Humic substances can be extracted into soluble forms of salt complexes (sodium, potassium, and even ammonium) that bind to the large, complex organic structures of the humic material – contributing to the unraveling of the tightly packed humus structures and activating previously hidden functional groups ready for binding. Thus even while the accepted naming conventions do not distinguish well between various components/formulations of humic acids, Ocean Agro LLC is confident that material differences exist based on the extraction process and the activation profile of final formulations. These activated humic acid formulations are more reactive within soil systems than traditional solid humate amendment products and can be applied in smaller amounts.
For everyone’s convenience, I have provided a table of terms and definitions for Humic Substances from Stevenson’s seminal work, Humus Chemistry: Genesis, Composition, Reactions:
| Organic residues | Undecayed plant and animal tissues and their partial decomposition products. |
| Soil Biomass | Organic matter present as live microbial tissue |
| Humus | Total of the organic compounds in soil exclusive of undecayed plant and animal tissues, their “partial decomposition” products, and the soil biomass. |
| Soil Organic Matter | Same as humus |
| Humic substances | A series of relatively high-molecular-weight, brown to black colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to the soil (or sediment) environment in that they are dissimilar to the biopolymer of microorganisms and higher plants (including lignin). |
| Non Humic Substances | Compounds belonging to known classes of biochemistry, such as amino acids, carbohydrates, fats, waxes, resins, organic acids, etc. Humus probably contains most, if not all, of the biochemical compounds synthesized by living organisms. |
| Humates/Humin | The alkali insoluble fraction of soil organic matter of humus |
| Humic acid | The dark-colored organic material which can be extracted from soil by various reagents and which is insoluble in dilute acid |
| Fulvic acid | The colored material which remains in solution after removal of humic acid by acidification |
| Hymatomelanic acid | Alcohol soluble portion of humic acid |
Table 2.3: Terms and definitions of various Humic substances (Stevenson 1982)
Reference:
Stevenson, F. J. “Extraction, Fractionation and General Chemical Composition.” Humus Chemistry: Genesis, Composition, Reactions. New York: Wiley, 1982. 33-36. Print.
Troeh, Frederick R., and Louis M. (Louis Milton) Thompson. Soils and Soil Fertility. 6th ed. Ames, Iowa: Blackwell Pub., 2005.