286 MICROBIOLOGY OF SOIL. 



reducing bacteria from the corresponding nitrates and sulphates. As 

 pointed out by Nadson ammonium carbonate produced in the decom- 

 position of protein compounds may react with calcium sulphate as follows: 



(NH 4 ) 2 CO 3 + CaSO 4 = CaCO 3 + (NH 4 ) 2 SO 4 



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

 with carbon dioxide as follows: 



CaS + CO 2 + H 2 O= CaCO 3 + H 2 S 



Magnesium would be subject to similar reactions and Nadson has observed 

 the formation of a mixture of calcium and magnesium carbonates 

 (corresponding to dolorite 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 acids 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 decomposition of 

 carbohydrates, fats and proteins all react with basic substances 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 man- 

 ner enormous amounts of lime are annually carried to the ocean as bi- 

 carbonates, and to an appreciable extent also as nitrate and sulphate 

 Thus soil 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 COMPOUNDS 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 Omelianski that magne- 

 sium carbonate is especially useful in facilitating the isolation and culture 

 of nitrate bacteria. Heinze and others have noted the favorable action 

 of calcium carbonate on the growth of Azotobacter, while the beneficial 



