82 BACTERIA AND SOIL FERTILITY 



this. They act on hydrogen sulfide and produce sulfur. Yel- 

 low granules of sulfur may often be seen in their bodies. If 

 transferred to fresh water these granules disappear and sulfuric 

 acid is produced. This would render available more plant-food. 

 Today some workers are advocating the mixing of sulfur, in- 

 soluble phosphates, and bacteria. The bacteria oxidize the sul- 

 fur which in turn liberates the phosphorus. Probably consider- 

 able of the benefit resulting from the addition of sulfur to 

 some soils comes from this reaction. Aeration and optimum 

 moisture favor the change, whereas carbohydrates depress it. 



Iron. — The iron bacteria resemble the sulphur bacteria greatly 

 in their metabolic processes. They oxidize iron. They probably 

 play a great part in the formation of bog iron. The water of 

 some regions appears normal on being brought to the surface, but 

 on standing it takes a yellowish color. The yellowish scums and 

 deposits one sometimes sees around artesian wells are due to bac- 

 terial action. The water in both cases contains great numbers of 

 iron bacteria. On coming in contact with the air, the iron of 

 their body is oxidized, thus giving the yellow color. 



Potassium. — ^This element is required by all plants in com- 

 paratively large quantities, and the total supply in soils is com- 

 paratively large as compared to crop requirements. Yet potas- 

 sium is used extensively as a fertilizer, and this with beneficial 

 results. This is due to the fact that its addition to a soil well 

 supplied with available potassium results in the liberation of more 

 deficient plant elements. Moreover, it may be applied to a soil 

 having a large quantity of total potassium but small quantities 

 available to plants. Hence, shall we add potassium to soils or 

 render that which they already possess available? If the latter: 

 How can it be rendered available? We have seen that bacteria 

 produce nitrous, nitric, sulfuric, acetic, lactic, butyric, and car- 

 bonic acids. These would act upon the potassium containing sili- 

 cates and render the potassium available to the plant. We have 

 learned that cultivation, the addition of animal manure, green 

 manures, and commercial fertilizers, all increase bacterial activ- 

 ity. Hence, this is the method by which the potassium can be 

 rendered available. 



