Chemical soil condition (2.2)

Changes in the chemical composition of the soil, such as acidification⁶ and eutrophication⁷, affect trees either directly or indirectly, through the process of decomposition of organic matter by organisms in the soil.

The soil in coniferous forests is acid by nature. The most favourable pH range for conifers is 4.7–5.5. If the soil is any more acid, the rate of growth of trees slows down as their nutrient uptake becomes more difficult. When soil pH falls below 4, the leaching of nutrients vital to trees increases. On the other hand, acidity changes contribute to the leaching of substances toxic to tree roots such as aluminium.

The decomposition of organic matter also slows down in acid soils, reducing the nitrogen supply of plants. Increased soil acidity destroys many easily decomposing plants which produce nitrogen-rich litter in the forest. However, acidification reduces the productivity of forest land only very slowly. The eutrophication of forests is not a problem for the health of trees and vegetation in the short term. If eutrophication is widespread and prolonged, the biological diversity and species composition of forests change as a result of a reduction in the area of nutrient poor sites and their species.

The deposition of acidifying and eutrophying compounds has been monitored since 1996 on sample sites for intensive monitoring under the Forest Focus programme. No changes in the concentrations of sulphur or nitrogen in soil water have been observed during the monitoring period. In other studies conducted by the Finnish Forest Research Institute, only weak signs of acidification in forest soils have been observed. The decrease of sulphur concentrations in the organic layer of soils is linked to the decrease in sulphur deposition that started in the 1980s. The stability of nitrogen concentrations in the soil or their decrease over the monitoring period implies that the current nitrogen deposition will not constitute a health risk for forests in southern Finland in the near future.

⁶ Acidification = In acidification, the capacity of the soil to resist, or neutralise, acid deposition deteriorates. Acidifying compounds can be deposited in the soil with rain (wet deposition) or as particles or gases (dry deposition).

⁷ Eutrophication = Eutrophication means the increase of primary productivity caused by increased amounts of nutrients. In a soil ecosystem, eutrophication can manifest as increased growth of standing stock, for example. Increased growth is caused especially by nitrogen deposition, but increased concentrations of CO₂ in the atmosphere also increase the growth of trees.