Of soil acidification occurs when from the outside or through the bottom internal processes more protons (H + ions) are introduced by acids than the soil can neutralize. As a result, oxonium ions (H 3 O + ) are formed and the soil pH drops. Soil acidification is increased when the basic reaction products are washed out by neutralization reactions .
Natural influences on the soil pH
- Due to high CO 2 concentrations due to the oxidation of biomass and the respiration of roots and soil organisms, carbonic acid (H 2 CO 3 ) is produced, which adjusts the pH value to 5.6. H 2 CO 3 is the largest source of protons for agricultural soils (pH between 5 and 7), since below pH 5 no more carbonic acid is formed.
- Organic acids , particularly fulvic acids , which are either excreted by plant roots or are intermediate products in the breakdown of organic matter (therefore easily degradable) can contribute to acidification.
- If Fe 2+ is oxidized to Fe 3+ and then hydrolyzed in soils containing iron sulfide, hydronium or oxonium ions (H 3 O + ) are released.
- When plants take in more cations than anions for their diet , they release protons to the soil so that electrical neutrality is maintained. At the same time, they form salts of weak organic acids inside. If the plants rot on site, salts and protons combine again. There is only a permanent pH drop over a large area if the plants are removed by harvesting / felling (alkali removal).
- The buffer capacity of the starting substrate has a decisive influence on the rate of acidification. Above all, a high proportion of carbonates and three-layer clay minerals (loaded with cations such as Ca 2+ , Mg 2+ ) ensures that the pH value remains at a relatively constant level as long as these buffer substances are present in the soil. When these buffer systems are "used up" or missing from the outset, others take their place, and the pH value drops.
- Acid entry from the atmosphere ( acid rain ) also causes the pH value of many soils to drop, and the higher the initial pH value, the more so.
- Excessive ammonium input from organic fertilizers (liquid manure), mineral fertilizers and via the atmosphere also accelerates acidification. The ammonium is microbially oxidized:
- NH 4 + + 2O 2 ↔ NO 3 - + 2H + + H 2 O.
- Take the plants, the nitrate does not occur, it is with the leachate discharged and acidification effects are even more serious (Basenauswaschung).
- In the immediate vicinity of mining areas with sulphidic ores, the weathering of pyrite (an iron sulphide) can lead to a significant drop in pH due to the formation of sulphurous acid .
Many plants are not tolerant of increasing acidification. Yields decrease on agriculturally used soils. Plant growth is stopped at pH values below 3. In the range of low pH values, the mobility of toxic heavy metals also increases. Water that seeps through acidic soils also has a low pH. This has an impact on the surface waters fed from it. One example is the acidic lakes in Scandinavia . In addition, the photosynthesis rate of many plants is greatly reduced by too high soil acidity. This has major consequences both for the forest ecosystem and for the life of many other organisms.
With the permanent use of litter, snowfall every 50 years as well as liming of fields, gardens and forests as well as stricter requirements for exhaust air purification, attempts are being made to curb soil acidification. The lime can neutralize the acidic effect of the wastewater up to a certain point.