What are Humic Acids?
Humic acids are naturally occurring organic materials derived from biological sources. They typically are mixtures of several types of chemical compounds, including humic acids, fulvic acids and humans and are generally characterised as being yellow to black in colour, of high molecular refractomery and molecular weight. Leonardite is a common source of humic acid. Humic acid (HA) is the major extractable component of soil humic substances and it is thought to be a complex mix of aromatic macromolecules with amino acids, amino sugars, peptides, aliphatic acids and other apiphatic compounds involved in linkages between the aromatic groups. (4)
Potential Benefits of Humic Acid Application
Organic matter provides the raw material for nutrient cycling processes; it is a major source of natural "glue" that results in the aggregation of soil separates to form secondary structure.
These aggregates are also critical to the development of microhabitats for beneficial soil microorganisms. Adding humic substances to the soil generally results in an increase in the number of soil organisms, which leads to an increase in soil organic matter levels. (5,12) In many instances, the build up of organic matter may be perceived as unfavourable, although there are situations where some controlled organic matter accumulation may be beneficial.
It has been recognised for centuries that soils containing adequate organic matter are typically more productive than sandy ones and that if sufficient humic substances are present, then up to 35% of the soluble nitrogen applied to soils as fertiliser can be retained in the soil in organic forms. (8)
The use of heavy sand applications to increase the drainage potential of sports turf root zones inevitably means that moisture holding capacity and organic matter levels are depleted. (1)
Drought resistance of grass may be poor, (due to inadequate root system growth and nutrient retention), resulting in potential leaching of soluble nutrients such as nitrogen and potassium during wet weather conditions or under heavy irrigation if organic matter is lacking. When combined with accepted cultural management practises, organic amendments can effect a long-term and perhaps permanent change in the soil environment to favour disease-suppressive microbes, making disease management more sustainable. (9)
Coupled with this, humic acids relatively low rate of decomposition and high nitrogen content may help to allow a steady supply of available nitrogen, phosphorus, potassium and micronutrients to the growing plant as well as to the microflora active in humus degradation. (5)
The soil organic fraction known as humus, serves both as a soil conditioner and as a repository for available plant nutrients. Soil microorganisms play a major role in both its formation and its decomposition. (7)
Humic substances may enhance the uptake of minerals through the stimulation of microbial activity. (8) Studies of the effects of humic substances on plant growth, under conditions of adequate mineral nutrition, consistently show positive effects on plant biomass. (2) Humic substances have been reported to influence plant growth both directly and indirectly. The indirect effects of humic compounds on soil fertility include:
- Increase in the soil microbial population including beneficial microorganisms
- Improved soil structure
- Increase in the cation exchange capacity and the pH buffering capacity of the soil.
Humic substances may possibly enhance the uptake of minerals through the stimulation of microbiological activity. (8) When adequate humic substances are present within the soil, the requirement for nitrogen, phosphorus and potassium fertiliser applications may be reduced. (10)
Humic substances actually coat mineral surfaces with a membrane-like bi-layer, which aids in the solubilisation of otherwise insoluble compounds by dissolving, complexing, and chelating the dissolved nutrients. (8)
Humic acids have been shown to improve phosphate availability and uptake by crop plants. Several mechanisms for this phosphorus uptake increase have been cited including (a) competing ions are complexed by humic acids, (b) amine groups complex phosphates, (c) root interception is increased due to greater root development and root area caused by humic acid application and, (d) humic acids encourage mycorhizal fungi symbiosis.(3)
Humic acids have the ability to transform solid phase forms of micronutrient cations such as Fe3+, Fe2+, Mn2+ and Zn2+ into soluble metal complexes, which are available to plants. (11) Humic substances have a very profound influence on the growth of plant roots. When humic acids and/or fulvic acids are applied to soil, enhancement of root initiation and increased root growth may be observed. (10)
About the author
Tim Butler is studying for a Ph.D. in Sportsturf Science at both University College Dublin, Ireland and Michigan State University, U.S.A. Contact Tim at firstname.lastname@example.org
1. Baker, S.W. 1984. Long-term effects of three amendment materials on the moisture retention characteristics of a sand-soil mix. Journal of the Sports Turf Research Institute 60: 61-65.
2. Chen, Y. and Aviad, T.A. 1990. Effects of humic substances on plant growth. P.161-186. In: McCarthy, P., Clapp, C.E., Malcolm, R.L. and Bloom, P.R. Humic Substances in Soil and Crop Sciences. American Society of Agronomy, Soil Science Society of America, Madison, Wisconsin.
3. Day, K.S., Thornton, R. and Kreeft, H. 2000. Humic acid products for improved phosphorus nutrient management. p. 321-325. In: Ghabbour, E.A. and Davies, G. (eds.). Humic Substances: versatile components of plants, soils and water. The Royal Society of Chemistry, Cambridge.
4. Hatcher, P.G., Schnitzer, M., Dennis, L.W. and Maciel, G.E. 1981. Ammaticity of humic substances in soils. Soil Science Society of America Journal 45: 1089-1094.
5. Henis, Y. 1986. Soil microorganisms, soil organic matter and soil fertility. p. 159-168. In: Chen, Y. and Avnimelech, Y. (eds.). The Role of Organic Matter in Modern Agriculture. Martinus Nijhoff, Boston.
6. Holl, B. 2003. Organic amandments & biosolids in turfgrass management [Online].Available at www.golfandsportsturf.com.au/article.asp?ArticleID=344
7. Lebedjantez, A.N. 1924. Drying of the soil as one of natural factors in maintaining soil fertility. Soil Science 18: 419-447.
8. Mayhew, L. 2004. Humic substances in biological agriculture [Online]. Available at www.acresusa.com/toolbox/reprints/Jan04_Humic%20Substances.pdf
9. Nelson, E.B. 1997a. Biological control of turfgrass diseases [Online]. Available at www.gscaa.org/gcm/1997./july97/07bio.html
10. Pettit, R.E. 2004. Organic matter, humus, humate, humic acid, fulvic acid and humin: their importance in soil fertility and plant health [Online]. Available at
11. Raviv, M., Chen, Yand Inbar, Y. 1986. Peat and peat substitutes as growth media for container grown plants. p. 257-287. In: Chen, Y. and Avnimelech, Y. (eds.). The Role of Organic Matter in Modern Agriculture. Martinus Nijhoff, Boston.
12. Visser, S.A. 1985. Physiological action of humic substances on microbial cells. Soil Biological Biochemistry 17: 457-462.