Humic substance

Humic Substances

Humic acid sample

Humic substances are a complex mixture of many different acids containing phenolate and carboxylate groups, produced by the biodegradation of dead organic matter. They are a major component of humus, the organic component of soil, peat, and coal, and also a constituent of many upland streams, dystrophic lakes, and ocean water.

Composition and Structure

Humic substances are generally categorized into three main fractions: humic acid, fulvic acid, and humin. These fractions are defined based on their solubility in water at different pH levels.

Humic Acid

Structure of humic acid

Humic acid is the fraction of humic substances that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH. It is a dark brown to black substance and is the most complex of the humic substances. Humic acid is known for its ability to chelate metal ions, which can enhance the availability of nutrients in the soil.

Fulvic Acid

Fulvic acid sample

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Structure of fulvic acid

Fulvic acid is the fraction of humic substances that is soluble in water under all pH conditions. It is lighter in color than humic acid, ranging from yellow to yellow-brown. Fulvic acids have a lower molecular weight and are more chemically reactive than humic acids. They are known for their ability to improve the uptake of nutrients by plants.

Humin

Humin is the fraction of humic substances that is not soluble in water at any pH. It is the least studied of the humic substances due to its insolubility, but it is believed to play a role in the structure and stability of soil.

Formation and Sources

Humic substances are formed through the microbial degradation of dead plant and animal matter. This process involves the breakdown of complex organic molecules into simpler compounds, which are then polymerized into the larger humic substances. The exact composition and structure of humic substances can vary widely depending on the source material and environmental conditions.

Functions and Importance

Humic substances play a crucial role in soil fertility and plant growth. They improve soil structure, increase water retention, and enhance the availability of nutrients. In aquatic environments, humic substances can affect the color and acidity of the water and influence the bioavailability of metals and other pollutants.

Related Pages

• Soil science
• Organic matter
• Biodegradation
• Nutrient cycling