3S6 MICROBIOLOGY OF SOIL 



ties of denitrifying and other reducing bacteria from the corresponding 

 nitrates and sulphates. As pointed out by Nadson ammonium car- 

 bonate produced in the decomposition of protein compounds may react 

 with calcium sulphate as follows: 



(NH4)2C03 + CaS04 = CaCOs + (NH4)2S04 



Moreover, calcium sulphate may be reduced to sulphide and may react 

 with carbon dioxide as follows: 



CaS + CO2 + H2O = CaCOa + H^S 



Magnesium would be subject to similar reactions and Nadson has 

 observed the formation of a mixture of calcium and magnesium car- 

 bonates (corresponding to dolomite in composition) in media inoculated 

 with a pure culture of B. {Proteus) vulgaris. 



Lime as a Base. — The carbon dioxide generated in vast amounts in 

 the life processes of most soil bacteria, the nitrous and nitric acids 

 formed by the nitro-bacteria, the sulphuric acid produced in the 

 oxidation of hydrogen sulphide and of sulphur by the so-called sulphur 

 bacteria, and the great variety of organic acids formed in the decom- 

 position of carbohydrates, fats and proteins all react with basic sub- 

 stances in the soil. Of these basic substances calcium carbonate is by 

 far the most prominent. Combining with the different acids it 

 maintains a favorable reaction for microorganic life in the soil. 



The calcium salts thus formed are more or less soluble. In this 

 manner enormous amounts of lime are annually carried to the ocean 

 as bicarbonates, and to an appreciable extent also as nitrate and 

 sulphate. Thus soU bacteria help to furnish shell fish and other forms 

 of marine life, the material necessary for the building of their skeletons. 

 In the course of ages the latter become a portion of the solid land and as 

 coral reefs, chalk cliffs and marl beds offer to microorganisms a new 

 opportunity to start calcium carbonate on its migrations. 



Effect of Calcium and Magnesium Compoiinds on Bacterial 

 Activities. — Being basic in character calcium and magnesium car- 

 bonates are of great service in maintaining a suitable reaction in the 

 soil-. But somewhat apart from this service calcium and magnesium 

 compounds seem to be particularly important for the growth of certain 

 organisms. It has already been observed by Winogradski and Ome- 

 lianski that magnesium carbonate is especially useful in faciUtating the 



