Soil acidification
 

In principle, soil acidification is a natural process. The acidifying nitrogen and sulphur compounds which are deposited to the forest, however, accelerate this process considerably. In Switzerland, 70% of the acidifying substances are nitrogen compounds. Model calculations confirm the importance of the anthropogenic inputs for the rapid soil acidification which presents one of the major threats for forest health. It is causing  a depletion of the important plant nutrients calcium, potassium, magnesium (base cations, abbreviated BC). Their proportion on the ion exchangers of the soil, together with sodium, is called base saturation. A base saturation below 40% is regarded to be low, below 15% critically low and below 5% very critically low. For a sustainable management and forest stability, a base saturation of >50% is thought to be sufficient for deciduous forest and >40% for coniferous forest. 

 
  Sampling of soil water for chemical analysis.

 
With increasing soil acidification the availability of phytotoxic aluminium ions (Al) in soil solution increases. The loss of base cations on the one hand and the increase of aluminium avalability on the other hand leads to a decrease of the ratio between base cations and aluminium, the BC/Al ratio. A ratio of below 1 is regarded as critical for plant growth. Between 1997 and 2003 the BC/Al ratio has significantly dropped in 15 out of 18 investigated plots, with some values below the critical limit. Experiments show that the BC/Al ratio decrease faster with additional nitrogen input.
The ratio between base cations and aluminium in soil solution (BC/Al ratio) which is an indicator for acidification has decreased significantly in 15 out of 18 investigated forest observation plots between 1997 and 2004. Six examples are presented here, of which five showing a decrease. The dashed line represents the critical limit.

 
The significance of nitrogen input for soil acidification is shown by this experiment from Zugerberg where the BC/Al ratio decreases the more rapidly the higher the nitrogen application is (given as ammonium nitrate). The atmospheric nitrogen input at this site is estimated to be 20 kg per hectare and year.   
 The soil acidification has consequences for roots and for the earthworms.