SOLVENT ACTION OF ACIDS 157 



cium. Compounds of iron are generally somewhat more resistant 

 than any of these. 



Among the inorganic acids produced by bacteria, carbonic, 

 nitric, and sulfuric acids are of principal importance. Nitrous 

 acid is a precursor to the formation of nitric acid, but it is gen- 

 erally so rapidly converted to nitric acid that it seldom accumu- 

 lates in sufficient amounts to exert pronounced effects. Carbonic 

 acid is produced by all mircoorganisms in appreciable amounts in 

 their growth processes; nitric acid is the end product of oxidation 

 of ammonia by nitrifying bacteria; and sulfuric acid is the product 

 of oxidation of sulfide and sulfur by sulfur bacteria. The group of 

 autotrophic bacteria as a whole is particularly active in solvent 

 activities, since strong acids are so frequently produced by their 

 development. Chief among these are nitric and sulfuric acids. 



Algae are frequently recognized as corrosive agents acting upon 

 stone and rock surfaces. As individuals and as associated with 

 certain fungi in the form of lichens they represent some of the first 

 plant forms to develop upon naked rocks. They exert effects 

 through respiration products, carbon dioxide in particular, and also 

 indirectly by supplying organic food materials to bacteria which in 

 turn may bring about solvent effects in different ways. 



These inorganic acids vary in strength and in the amounts pro- 

 duced in soils, and consequently in their solvent action. Sulfuric 

 and nitric acids are much stronger than carbonic acid, but are 

 formed in small amounts compared to this weaker acid. Under 

 most soil conditions nitric acid is produced in much more abun- 

 dance than sulfuric acid. Exceptions to this may appear in cases 

 where sulfur is applied to soils in considerable quantities to create 

 acid conditions. 



There may be as much as 7,300 kilograms of carbon dioxide 

 produced each year per acre of fertile soil. This would represent 

 about 10,300 kilograms of carbonic acid. Considering that all 

 of the nitrogen removed by a heavy crop and lost in drainage 

 waters had been produced as nitric acid in the soil, this would 

 represent at a maximum close to 250 kilograms of nitric acid per 

 acre. Similarly, it may be assumed that there would be as much 

 as 100 kilograms of sulfuric acid produced annually in the same 

 amount of soil. Fortunately, carbonic acid is a weak acid, exerting 

 a mild solvent effect and readily disappearing by the volatihzation 

 of carbon dioxide under soil conditions. The amounts of carbon 



