2QO MICROBIOLOGY OF SOIL. 



sulphide according to him may react with carbon dioxide and water, 

 giving rise to the formation of hydrogen sulphide. Thus: 



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



The hydrogen sulphide derived from sulphates or from proteins 

 becomes a source of energy to the sulphur bacteria as already noted in 

 the preceding pages. 



POTASSIUM. 



THE TRANSFORMATION OF POTASSIUM COMPOUNDS IN THE SOIL. 

 Potassium occurs in the soil largely in the form of silicate minerals. 

 Smaller amounts occur as nitrate, carbonate and in organic compounds. 

 The portion present as silicates is often very large in clay loam soils, 

 amounting not infrequently to 22,679 kg. to 34.019 kg. (50,000 to 

 75,000 pounds) per acre foot. Unfortunately for the farmer, the grow- 

 ing crops fail, in many cases, to secure sufficient quantities of available 

 potash for their rapid development, notwithstanding these enormous stores 

 of potassium compounds. However, when sufficient quantities of readily 

 fermentable organic matter are present and the generation of carbon 

 dioxide is rapid the silicates weather sufficiently fast to meet the demands 

 of maximum harvests. The part played by carbon dioxide in the trans- 

 formation of inert potash compounds may be illustrated by the following 

 reaction: 



A1 2 O 3 K 2 O 6SiO 2 +CO 2 +2H 2 O = Al 2 O 3 2SiO 2 2H 2 O+K 2 CO 3 +4SiO 2 



Under actual conditions it is the aim of the farmer to stimulate 

 bacterial activities (and, therefore, the production of carbon dioxide) in 

 his land by the use of animal manures or green manures and of commercial 

 fertilizers. Apart from the influence of carbon dioxide available potash 

 compounds may likewise be formed on account of nitric, sulphuric, 

 acetic, lactic, butyric and other acids produced by different soil bacteria. 



OTHER MINERAL CONSTITUENTS. 



IRON. The investigations of Ehrenberg, Winogradski, Molisch, 

 Adler, Ellis and others have accumulated a mass of data relating to the 

 so-called iron bacteria. These organisms belong to the class of higher 

 bacteria and have a marked ability to precipitate iron oxide out of solutions 

 of iron salts. Winogradski believed that the reaction is a physiological 



