64 AGRICULTURAL BACTERIOLOGY. 



it may enter into combinations outside of the plant. It is an 

 active chemical agent and readily unites with phosphorus or 

 silica which may be in the soil, to form phosphates or silicates 

 of iron. These salts are important soil ingredients and con- 

 tribute to mineral foods of plants. It is believed that the great 

 deposits of bog iron ore and other iron compounds are to be 

 attributed to this action of iron bacteria. Occasionally these 

 bacteria have been the cause of great trouble in water pipes in 

 regions where the water is largely impregnated with iron com- 

 pounds. Such water furnishes the organisms with abundance 

 of food and they consequently grow in great abundance in the 

 water mains, sometimes almost closing them. 



Other Mineral Ingredients. It is not unlikely that other 

 mineral ingredients in the soil are to a greater or less extent 

 modified, and perhaps produced, through the agency of bac- 

 terial life. In regard to all these matters we are yet in great 

 ignorance. Up to the present time these problems have hardly 

 been touched by bacteriologists. The mineral ingredients 

 have been so generally looked upon as the result of pure 

 chemical and physical forces that they have not seemed to 

 offer fields for bacteriological research. Moreover, the more 

 important question of nitrogen has been so prominent as to 

 have attracted most of the attention of bacteriologists interested 

 in soil bacteria. It is only a few of the other points which are 

 most patent, like those of sulphur deposition and iron fermen- 

 tations, that have as yet been studied by bacteriologists. The 

 problem of the exact relation of soil bacteria to the mineral in- 

 gredients of the soil remains as a subject for future investiga- 

 tion. We know that the rocks undergo disintegration and we 

 know that numerous chemical changes are constantly occur- 

 ring in the mineral ingredients. To what extent these are 

 explained by chemical and physical phenomena and to what 

 extent vital forces have contributed and are constantly con- 

 tributing, cannot to-day even be conjectured. 



