Black Earth Blog
Roles of Humic Substances in Soil Structure
Posted by Tesfay Teklay
Jul 22, 2013 11:30:00 AM
Effects of Humic Substance on Soil Structure
Humic substances, including Black Earth’s Humalite and Leonardite, are naturally occurring materials that are enriched in humic, fulvic and other organic acids.
Their major uses are as a soil amendment in agriculture and reclamation, as a drilling fluid additives and in municipal wastewater treatment applications (1,3). Soil structure is the arrangement of soil particles into secondary units called aggregates. Soil aggregates are cemented clusters of sand, silt, and clay particles.
Why Soil Structure Matters
Soil structure is a vital soil property that affects water retention, aeration, water infiltration/percolation and drainage. Most soils rely on aggregation of particles to maintain favourable conditions for soil microbial and faunal activity and plant growth. Certain components of soil organic matter such as polysaccharides, humic substances, root material and fungal hyphae have an important role in soil structural stabilisation.
Numerous studies have linked organic carbon levels with aggregate stability, infiltration and soil strength, so a decrease in organic carbon is often associated with degrading soil physical conditions. Soil aggregation and aggregate stability are the main factors affecting the susceptibility of soil to rain drop impact and consequently surface sealing and soil erosion (2).
Maintaining friable soil structure
Humic substances are key components of a friable (loose) soil structure. Various carbon containing humic substances are key components of soil crumbs (aggregates). Complex carbohydrates synthesized by bacteria and humic substances function together with clay and silt to form soil aggregate. As the humic substances become intimately associated with the mineral fraction of the soil, colloidal complexes of humus-clay and humus-silt aggregates are formed.
These aggregates are formed by electrical processes which increase the cohesive forces that cause very fine soil particles and clay components to attract each other. Once formed, these aggregates help create a desirable crumb structure in the top soil, making it more friable. Soils with good crumb structure have improved tilth, and more porous openings. These pores allow for gaseous interchange with the atmosphere, and for greater water infiltration (1).
Helping plants set down strong roots
Humic substance can also affect soil structure by regulating the dispersion or flocculation properties of soils. Soil clay particles are considered dispersed when they are unattached to one another or flocculated when they clump together in aggregates. Flocculation is a desirable property because it allows water move and roots grow easily between aggregates. On the other hand, dispersed clays plug soil pores and impede water infiltration and soil drainage.
Dispersed soil particles have a negative impact on soil structure and contribute to soil erosion and contaminant movement. In alkaline or saline/saline-sodic soils, humic substances modulate the effects of cations that either flocculate or disperse soil particles. For instance, sodium is a poor flocculator while calcium and good flocculators. Soil particles will flocculate if concentrations of Ca2+ + Mg2+) are increased relative to the concentration of Na+.
The value of humic substances in soil fertility and plant nutrition relates to the many functions these complex organic compounds perform as a part of the life cycle on earth (1). They play vital roles in: improving uptake of vital plant nutrients, promoting microbial activity, improving water holding capacity of soil, improving soil structure, aggregation, friability and tilth, reducing toxic effects of heavy metals and pesticides, buffering pH, stabilizing soil temperature, stabilizing soil enzymes, improving germination of seeds and seedling development, root growth, and overall improved plant metabolism (2, 3 )
Want to know more about the effects of Black Earth on soil structure? See the results of our applied research studies.
1. Hoffman, G.L.; Nikols, D.J.; Stuhec, S.; Wilson, R.A. Open File Report 1993-18 Evaluation of Leonardite (Humalite) Resources of Alberta, Retread Resources Ltd.;
2. Dr. Robert E. Pettit http://www.calciumproducts.com/articles/Dr._Pettit_Humate.pdf
3. Saharinen MH, Dinel H, and Schnizer M (2000), Effect of humic substances on plants and soil properties: A literature Review. Agriculture and Agri-Food Canada
4. A.M. Whitbread, Soil Organic Matter: Its Fractionation and Role in Soil Structure, Department of Agronomy and Soil Science, University ofNew England, Armidale, NSW 2351, Australia. http://aciar.gov.au/files/node/334/soil_organic_matter_management_for_sustainable_agr_20481.pdf
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