HUMIC ACIDS
Humic acids are specific natural high-molecular-weight acidic compounds formed during the transformation of mainly plant residues outside living organisms under the influence of mesofauna, microorganisms, and abiotic factors.
The chemical and biological activity of humic acids is due to the high content and a large set of double bonds (including aromatic C=C bonds), quinoid and phenolic groups, oxygen-containing functional groups such as carboxyl groups (–COOH), phenolic and alcoholic hydroxyl groups (–OH), aldehyde groups, and amino groups (–NH₂).
The ability of humic acids to bind heavy metals is assessed by the stability constants of the corresponding complexes, which significantly depend on pH. As pH increases, the stability of heavy metal humates generally increases. For example, the logarithms of stability constants for humates are: copper — 7–12, nickel — 5–9, cadmium — 5–9, zinc — 5–10. The stability of iron(III) humates decreases with increasing pH, since Fe³⁺ ions form very difficult-to-dissolve hydroxides. Humic acids can adsorb or chemically bind various pesticides and other organic compounds.
In addition to directly affecting the availability and mobility of pollutants, humic acids have an overall beneficial effect on soil properties, thereby reducing the negative (toxic) effects of pollutants on plants and soil biota. The positive effect of humic acids is due to the fact that they contribute to the formation of soil structure, and create a long-term reserve of nitrogen, phosphorus, and other plant nutrients, including trace elements.
Source: Chemical Pollution of Soils and Their Protection: A Dictionary / D.S. Orlov, M.S. Malinina, G.V. Motuzova, et al. — Moscow: Agropromizdat, 1991.
The chemical and biological activity of humic acids is due to the high content and a large set of double bonds (including aromatic C=C bonds), quinoid and phenolic groups, oxygen-containing functional groups such as carboxyl groups (–COOH), phenolic and alcoholic hydroxyl groups (–OH), aldehyde groups, and amino groups (–NH₂).
The ability of humic acids to bind heavy metals is assessed by the stability constants of the corresponding complexes, which significantly depend on pH. As pH increases, the stability of heavy metal humates generally increases. For example, the logarithms of stability constants for humates are: copper — 7–12, nickel — 5–9, cadmium — 5–9, zinc — 5–10. The stability of iron(III) humates decreases with increasing pH, since Fe³⁺ ions form very difficult-to-dissolve hydroxides. Humic acids can adsorb or chemically bind various pesticides and other organic compounds.
In addition to directly affecting the availability and mobility of pollutants, humic acids have an overall beneficial effect on soil properties, thereby reducing the negative (toxic) effects of pollutants on plants and soil biota. The positive effect of humic acids is due to the fact that they contribute to the formation of soil structure, and create a long-term reserve of nitrogen, phosphorus, and other plant nutrients, including trace elements.
Source: Chemical Pollution of Soils and Their Protection: A Dictionary / D.S. Orlov, M.S. Malinina, G.V. Motuzova, et al. — Moscow: Agropromizdat, 1991.
HUMUS
Humus is the totality of all organic compounds found in the soil, but not part of living organisms or formations that preserve the anatomical structure, and not involved in the construction of tissues of plant and animal remains. According to this definition, humus contains humic substances (specific organic compounds of soil origin), which include humic acids, himatomelanic acids, fulvic acids (according to Forsyth), and humin; various individual organic compounds of biological origin (both low-molecular-weight compounds and biopolymers), as well as man-made organic compounds that enter the soil with the application of fertilizers, pesticides, tillage, and technogenic pollution.
The role of humus in soil formation, soil fertility, and soil protection is extremely important. Humus is associated with the main reserves of plant and microbial nutrients; it contains many physiologically active substances (enzymes, antibiotics, and humic substances), and serves as a source of carbon and energy for soil microorganisms. Humus contributes to the formation of a favorable water-air regime, high buffering capacity of soils, and their resistance to adverse external conditions and chemical pollution.
The most effective influence of humus on crop productivity in agricultural use and soil protection can be achieved only with an optimal combination of humus content, composition, and qualitative characteristics for each soil (zone) and crop.
In general, the optimal humus content can be considered as that which is favorable for obtaining the planned yield, subject to the mandatory condition of the most effective use of applied fertilizers and the maximum effectiveness of agrotechnical measures. Soil with optimal humus content is maximally resistant to destructive factors (erosion, deflation), the influence of chemical pollutants, droughts, or waterlogging.
Agricultural use of soils with low crop cultivation usually leads to loss of humus from arable soils. The fastest losses occur when plowing virgin land. With subsequent use, the humus content gradually stabilizes at a certain reduced level, depending on the farming system. With a high level of production and the application of organic fertilizers, the humus content in soils may increase.
The main causes of humus loss from arable soils include the following: reduction of the amount of plant residues entering the soil; increased mineralization of humus as a result of increased aeration and intensification of microbial activity; increased mineralization during drainage of waterlogged soils; and erosion and deflation losses.
There is information in the literature that, with permanent cultivation of grain crops, annual humus losses in typical chernozem reach 0.5–1 t/ha, and with row crops — up to 1.5 t/ha. According to some researchers, over the past hundred years, intensively plowed chernozems have lost up to 25–30% of their total humus content.
Source: Chemical Pollution of Soils and Their Protection: A Dictionary / D.S. Orlov, M.S. Malinina, G.V. Motuzova, et al. — Moscow: Agropromizdat, 1991.
The role of humus in soil formation, soil fertility, and soil protection is extremely important. Humus is associated with the main reserves of plant and microbial nutrients; it contains many physiologically active substances (enzymes, antibiotics, and humic substances), and serves as a source of carbon and energy for soil microorganisms. Humus contributes to the formation of a favorable water-air regime, high buffering capacity of soils, and their resistance to adverse external conditions and chemical pollution.
The most effective influence of humus on crop productivity in agricultural use and soil protection can be achieved only with an optimal combination of humus content, composition, and qualitative characteristics for each soil (zone) and crop.
In general, the optimal humus content can be considered as that which is favorable for obtaining the planned yield, subject to the mandatory condition of the most effective use of applied fertilizers and the maximum effectiveness of agrotechnical measures. Soil with optimal humus content is maximally resistant to destructive factors (erosion, deflation), the influence of chemical pollutants, droughts, or waterlogging.
Agricultural use of soils with low crop cultivation usually leads to loss of humus from arable soils. The fastest losses occur when plowing virgin land. With subsequent use, the humus content gradually stabilizes at a certain reduced level, depending on the farming system. With a high level of production and the application of organic fertilizers, the humus content in soils may increase.
The main causes of humus loss from arable soils include the following: reduction of the amount of plant residues entering the soil; increased mineralization of humus as a result of increased aeration and intensification of microbial activity; increased mineralization during drainage of waterlogged soils; and erosion and deflation losses.
There is information in the literature that, with permanent cultivation of grain crops, annual humus losses in typical chernozem reach 0.5–1 t/ha, and with row crops — up to 1.5 t/ha. According to some researchers, over the past hundred years, intensively plowed chernozems have lost up to 25–30% of their total humus content.
Source: Chemical Pollution of Soils and Their Protection: A Dictionary / D.S. Orlov, M.S. Malinina, G.V. Motuzova, et al. — Moscow: Agropromizdat, 1991.
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