Washing-out or acidification are part of an entirely natural, ongoing process: nutrient translocation. It begins on bare stone in the mountains and comes to a provisional end in the oceans. This dynamic and unstoppable process has continued for billions of years on Earth.
Acidification is part of natural processes
The solvent water is natural rainwater with its pH value of 5.6. The pH value is affected by the atmospheric carbon dioxide content, i.e. carbonic acid.
Who does not remember the stalagmites and stalactites in the dripstone caves we visited with our parents, or later with our own children? A condition for the formation of dripstone caves is a high content of minerals dissolved in water, which are deposited in the caves.
The water of the oceans is determined by the chemism of the dissolved elements. It contains 88.6 % chlorine compounds, 10.8 % sulphates and 0.6 % carbonates or other elements. Gases are physically bound in seawater, e.g. oxygen, carbon dioxide and nitrogen. The oxygen content makes life possible.
At a depth of 0 – 2 m, our soils are the largest terrestrial stores of carbon, holding 2,400 gigatonnes. The Earth, a water planet, lost 40 – 60% of its humus due to changes in land use from forest and meadow to farmland. This resulted in an acceleration of the process of nutrient translocation. And this is increasingly being affected by humans. Ecosystems are a complex circular economy with a self-regulating dynamic.
Plant growth is only possible with acidification
Plants release protons via their roots in order to be able to absorb minerals in return. As a result, the soil becomes more acidic. The higher the concentration of hydrogen protons, the lower the pH value around the roots. This is the beginning of all succession. Plants remove more cations than anions from the soil, resulting in a slow process of acidification.
CO2 is added to the soil by rain, forming carbonic acid. The respiration of soil-dwelling organisms and plant roots supply CO2, which reacts with water to form carbonic acid and also makes the soil more acidic. This goes some way towards explaining the yearly cycle of the soil pH value due to mineralisation.
In addition, other natural substances and chemical reactions occur in the soil which contribute to soil acidification. Moreover, the soil pH value depends on the parent rock: Some types of rock naturally form more acidic soils.
Lasting acidification only occurs when the proton supply exceeds the acid neutralisation capacity of the soil.
The significance of human influences
The human influences on soil acidification processes are of a long-lasting nature. They mainly occur on a much wider scale and can accelerate these processes. Normally, all substances taken in by plants are returned to the soil when the plants die. This whole process is a cycle. However, if plants are harvested and removed by humans, this cycle cannot take place and an imbalance occurs. The amount of protons emitted by plants exceeds the amount of neutralising substances from dying plant parts that can be put back into the soil. Year-round vegetation slows down translocation processes.
Fertilizers used in excess of demand in agriculture lead to acidification. By using locally adapted liming, conditions can be achieved which enable all nutrients to be highly effective.
The “acid rain” which persisted into the late 1980s caused harmful substances from the atmosphere, caused by the burning of fossil fuels, to be absorbed and incorporated into the soil. The high deposits of sulphuric acid are currently significantly lower than the NOx deposits. Visible from the lichen growth on trees and bushes.
Logging and monocultures such as coniferous forests play a major role in soil acidification. While mainly deciduous forests originally grew in our latitudes, the forests have been restocked predominantly with conifers. As a result, the soil becomes more acidic. The future conversion of our forests to mixed woodlands will fulfil ecological requirements better.
Making use of the positive side of acidification
Plants extract nutrients from the soil at different levels. This enables them to withstand extreme weather conditions better and to make use of diverse levels of nutrient solubility. As is well known, salts are formed with anorganic anions (carbonate, nitrate, chloride, sulphate) and cations. Depending on soil humidity, for example NO3 (wet soils) or Cl (dry soils) cations are released for the nutrition of the plants or for transfer to lower soil levels. In long dry conditions, plants live off the nutrients from the damp subsoil.
The use of ammonium fertilizers in agriculture leads to a decrease in the pH level. This is a natural trick to release nutrients and make them more available to the plants. The extent of the reduction in the pH level depends on the remaining nitrate created by nitrification. So we have a degree of efficiency of e.g. 100% or just 50% of the nitrogen used for the plants. Nitrogen not used by plants is stored as nitrate during the cold season by calcium ions. Similar relationships underlie leguminous monocultures.
With placed nitrogen fertilization, no long-lasting decrease in pH value occurred outside the fertilizer depots. A very high degree of efficiency for nitrogen does not give rise to acidification potential due to the transfer of calcium nitrate.
The same applies for sulphur as for nitrogen. Only the amounts of sulphate not taken up by the plants can transfer calcium or magnesium. The transfer of magnesium sulphate has a stronger effect on the pH value.
The solubility of lime depends strongly on the pH value and particle size. Lime can supply the cations calcium, magnesium, silicon and trace elements. How quickly their positive effects can be seen in our soils depends on the flexible supply of inorganic anions. Neutral or acidic fertilizers used alone or in combination promote a high degree of nutrient efficiency. Human nutrition can be achieved ecologically and economically if we are aware of the natural complexity of nutrient translocation.