Tannin, a polyphenol compound commonly found in tree bark, leaves, fruits and seeds. In farming we will often encounter this substance because many growing media (substrate) are made from materials containing a lot of tannin.
1. Reduced ability to absorb nutrients
Tannins can combine with metal ions (iron, calcium, magnesium) or proteins in the soil, forming insoluble complexes. This reduces the availability of important nutrients that plants need to grow.
2. Inhibits the activity of beneficial microorganisms
Tannins have antibacterial and antifungal properties. When they accumulate in the soil, they can inhibit beneficial microorganisms such as nitrogen-fixing bacteria (Rhizobia) and mycorrhizae. This affects the ability of the soil to deliver nutrients to plants.
3. Change soil pH
Tannin is mildly acidic. When dissolved in soil, it can reduce pH, creating a more acidic environment. Acidic soil will limit the growth of many plants that cannot tolerate acidic conditions.
4. Impact on soil structure
Tannins can change the chemical properties of soil organic matter, reducing its porosity or facilitating the accumulation of compounds that are difficult to decompose. This reduces the drainage and aeration capacity of the soil, negatively affecting plant root growth.
5. Toxic to plant roots and seeds
At high concentrations, tannins can be directly toxic to plant roots and seeds, damaging plant tissue or inhibiting germination. Some plant species may have their growth inhibited by allelopathy (negative chemical interactions between plant species).
6. Slows down the decomposition of organic matter
Tannins have the ability to inhibit the activity of enzymes and microorganisms involved in the decomposition of organic matter. This causes organic material to decompose more slowly, affecting the supply of nutrients to plants.
7. Causes allelopathy
Some plants produce tannins to inhibit the growth of other nearby plants. This is a form of chemical competition (allelopathy), making it difficult for other plants to germinate or grow, especially in species that are not tannin tolerant.
8. Altering soil microflora
Tannins can alter soil microbial composition by killing sensitive species and favoring tannin-resistant species. This can lead to ecological imbalance in the soil.
9. Increased accumulation of toxic compounds in soil
When tannin accumulates over time, it can form by-product compounds (such as unbalanced humic and fulvic acids) that are toxic to plants or reduce soil quality, especially in areas with old growth or forests.
10. Limit the growth of symbiotic fungi
Tannins can inhibit the growth of some important symbiotic fungi, such as mycorrhizaeThis prevents the plant from taking full advantage of the fungi's assistance in absorbing nutrients and water from the soil.
11. Effects on soil water balance
Because tannins can reduce water infiltration in soils (due to their effects on soil structure and accumulation of difficult-to-degrade compounds), they can limit the soil's ability to retain or drain water, thereby affecting plant growth.
12. Causes odor and affects soil quality
When tannins break down in soil, especially under anaerobic (low oxygen) conditions, they can produce unpleasant odors and affect soil quality. This is common in waterlogged or poorly drained soils.
13. Increased toxicity to soil ecosystems
When tannin accumulates over time, it can react with other substances in the soil, creating toxic compounds such as quinones or derivatives phenolicThese compounds not only affect crops but also harm soil animals and beneficial microorganisms.
14. Causes accumulation of hard-to-decompose residue
Tannins can interact with other organic compounds, creating a difficult-to-decompose residue on the soil surface. This residue prevents transpiration, gas exchange, and reduces the ability of plant roots to absorb water.
15. Effects on seed germination
Tannins in soil can directly inhibit seed germination by reducing the ability of the seed to absorb water, or be toxic to the seed embryo. This reduces plant density or natural regeneration of plants in the ecosystem.
16. Degradation of groundwater quality
Water-soluble tannins can leach into groundwater, causing water pollution. This reduces the quality of irrigation water and even affects human health if this water source is used.
17. Increased risk of soil erosion
By changing the soil structure (making it harder or less able to drain), tannins can make soil more susceptible to erosion during heavy rains. This causes the topsoil to be washed away, leading to nutrient loss and long-term crop damage.
18. Inhibits the natural breakdown of toxins
In the soil there are many tannins, microorganisms and enzyme Inhibition can slow the breakdown of natural toxins or residual chemicals (e.g. pesticides). This increases the risk of toxic chemical residues in the soil.
19. Create differentiation between plant species
Tannins can give some plant species a competitive advantage (through allelopathy), but at the same time weaken other species, leading to an imbalance in the ecosystem. This can reduce biodiversity.
20. Inhibition of earthworm activity
Earthworms are an important component of improving soil structure and fertility. However, tannins can be toxic to earthworms or reduce their activity, leading to soil degradation.
21. Inhibits the ability to exchange gases in the soil
Tannins can increase soil compaction or create a layer of organic matter that is difficult to decompose. This limits the ability of gas exchange between plant roots and the air, leading to a lack of oxygen for plant roots.
22. Increased nutrient leaching
Water-soluble tannins can reduce the ability of soil to retain soluble nutrients. These nutrients are easily washed away during heavy rain or irrigation, causing malnutrition in plants.
23. Causes toxic accumulation over time
When tannin accumulates in the soil for a long time, it not only reduces soil quality but also causes the accumulation of related toxic substances such as phenol or tannin-degradable compounds, which has a long-term impact on arable land use.
24. Effects on the development of pollinating insects
If tannins are transferred from the soil to flowers or plants, they can reduce the attractiveness of the plant to pollinators such as bees or butterflies. This affects the plant's ability to reproduce, especially fruit trees.
25. Reduced fertilizer use efficiency
Tannins can interact with compounds in fertilizers (especially organic or nitrogen-rich fertilizers), reducing the effectiveness of the fertilizer by preventing plants from accessing these nutrients.
26. Harmful to sensitive plants
Some plants are more sensitive to tannins (e.g., vegetables, fruit trees, or non-native plants). The accumulation of tannins in the soil can cause these plants to fail to grow or to experience significant yield reductions.
27. Crop Rotation Issue
Tannin accumulation in the soil can affect crop rotation. Successive crops planted after harvesting of tannin-containing plants (e.g. acacia, eucalyptus) may have difficulty germinating or growing due to the altered soil environment.
28. Reduced effectiveness of biological soil treatment
Tannins can interfere with microbial soil amendment methods (e.g. bioremediation). This is because tannins inhibit the activity of important bacteria or enzymes in the soil treatment process.
29. Causes changes in soil color
Tannins can stain soil or water in the soil, especially in wetland areas. This not only affects the environment but also makes it difficult to identify soil quality.
30. Impact on aquatic plants near wetlands
If tannins in the soil are washed into nearby water sources, they can cause a lack of oxygen in the water (due to the organic decomposition process), affecting aquatic plants and animals.
Summary
Tannins, although they have natural roles in the ecosystem (such as protecting plants from pests), can cause many harmful effects when they accumulate in excess in the soil.
Impact of tannin The effects of tannins on plants depend on the type of plant, the type of soil, and the specific environment (sand, clay, or organic soil). Some plants, such as tea, chestnut, and acacia, have adapted to the tannins in the soil, while other plants may be severely affected. Soil management (organic fertilizers, pH adjustment, or microbial amendments) can reduce the negative effects of tannins.
Tannins cause a wide range of harmful effects on soils, ecosystems, and plants, depending on concentration, accumulation time, and environmental conditions.
These effects are particularly important in ecosystems or agricultural soils where tannin-producing plants (such as eucalyptus, acacia, or oak). To minimize negative impacts, measures such as soil improvement, intercropping with soil-improving plants (such as beans, peanuts), and control of tannin accumulation should be implemented.
See also: Tannins in Growing Media: Explanation, Effects and Treatment
