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The following 8 theses are theoretically founded and experimentally quantified: 1. Rocks contain only bases and no acid precursors. Therefore, with the exception of sulfide containing rocks, soils cannot acidify as a result of atmospheric rock weathering.2. A consumption of protons in rocks and soils results in a decrease of their acid neutralizing capacity (ANC) and can result in the buildup of a base neutralizing capacity (BNC). Strong soil acidification leads to the formation of stronger acids from weaker acids in the solid phase; this may be connected with a decrease in the BNC.3. Weak acids (carbonic acid) lead in geological times to the depletion of bases without a larger accumulation of labile cation acids. Strong acids (HNO3, organic acids, HzS04) can lead within a few decades to soil acidification, i.e. to leaching of nutrient cations and the accumulation of labile cation acids.4. The acid input caused by the natural emission of SO2 and NO, can be buffered by silicate weathering even in soils low in silicates.

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With respect to soil acidification, two aspects must be considered:1. Soil acidification is the consequence of the formation or input of acids.Carbonate and silicate rocks are weak bases. Therefore soils cannot acidify as the consequence of rock weathering (exception: sulfide rocks, the content of sulfides in silicate rocks is usually negligible). Rainwater of pH > 5 possesses alkalinity and cannot therefore acidify soils. The main acid source remaining are the organisms due to their life processes. This chapter deals mainly with an approach to quantify the rate of formation of acidity due to the life processes in the ecosystem. Since man has changed the acid/base status of aerosols, cloud water and all types of precipitation from alkalinity to acidity, the rate of acid deposition has to be considered as well. 2. With respect to the life processes of the organisms existing in an ecosystem, the soil represents the reaction vessel. The second aspect of soil acidification is represented by the chemical reactions taking place in this reaction vessel. This aspect is not treated in detail in this chapter, but elsewhere in this volume.In textbooks, plants are listed under the soil-forming factors, and the soil or soil properties are listed under the factors determining plant growth. If both views are correct, and nobody doubts it, then we should view both the soil and the plant as components (subsystems) of the ecosystem.The flow of materials in an ecosystem, caused by life processes, can be expressed in the following material balance Eq. (1) (cf. Schnoor and Stumm 1985;:Photosynthesis, ion uptake and formation of organic substances -> (forward reaction) Respiration and mineralization of organic substances ~ (backward reaction) aC02 + bNOa + cPO!-+ dSO;-+ ... + gCa 2 + + hMg 2 + + iK+ + kNHt + yN2 + H20 + (b + 3c + 2d -2g -2h -i -k)H+ + hv ~ {CaNb+k+2yPcSd' .. CagMghKiH2xOx} biomass + (a + ... )02(1)