BACTERIA IN NATURAL PROCESSES. 115 



theless, in this fundamental process of rock disin- 

 tegration bacterial action plays a part, though 

 perhaps a small one. Some species of bacteria, 

 as we have seen, can live upon very simple foods, 

 finding in free nitrogen and carbonates sufficient- 

 ly highly complex material for their life. These 

 organisms appear to grow on the bare surface of 

 rocks, assimilating nitrogen from the air, and car- 

 bon from some widely diffused carbonates or from 

 the COa in the air. Their secreted products of 

 an acid nature help to soften the rocks, and thus 

 aid in performing the first step in weathering. 



The soil is not, however, all made up of dis- 

 integrated rocks. It contains, besides, various 

 ingredients which combine to make it fertile. 

 Among these are various sulphates which form 

 important parts of plant foods. These sulphates 

 appear to be formed, in part, at least, by bacterial 

 agency. The decomposition of proteids gives 

 rise, among other things, to hydrogen sulphide 

 (H 2 S). This gas, which is of common occurrence 

 in the atmosphere, is oxidized by bacterial growth 

 into sulphuric acid, and this is the basis of part 

 of the soil sulphates. The deposition of iron 

 phosphates and iron silicates is probably also in 

 a measure aided by bacterial action. All of these 

 processes are factors in the formation of soil. 

 Beyond much question the rock disintegration 

 which occurs everywhere in Nature is chiefly the 

 result of physical and chemical changes, but there 

 is reason for believing that the physical and chem- 

 ical processes are, to a slight extent at least, as- 

 sisted by bacterial life. 



A more important factor of soil fertility is its 

 nitrogen content, without which it is complete- 

 ly barren. The origin of these nitrogen ingre- 



