172 Agricultural Bacteriology. 



hydrogen which are as essential as the other chemical 

 elements mentioned are always abundantly present in an 

 available form in the water and in the air. 



The elements obtainable from the soil are a greater 

 source of worry. to the farmer. While the soil, as a rule, 

 contains large amounts of these different elements, they 

 may be present in such forms that they cannot be utilized 

 by the green plant. "While they represent plant food, 

 it is of no immediate value because unavailable. This 

 material is, however, undergoing a change in form that 

 renders it more and more available. A large number of 

 agencies are at work, causing this change and amongst 

 them biological factors are very important. 



As indicating the relatively large amount of unavail- 

 able plant food that may be present, the following analy- 

 sis of 49 American soils are presented. The total nitro- 

 gen, phosphorus and potash content of the first eight 

 inches was as follows : 2600 pounds of nitrogen per acre, 

 2090 pounds of phosphorus, and 7400 pounds of potas- 

 sium. The other essential elements are usually present 

 in larger amounts and therefore are not so likety to be 

 depleted in the soil as are the three mentioned. 



The amount of any of these elements that are available 

 at any one time is usually very small indeed. Often, it 

 is not enough to furnish what is needed for a single crop. 

 The potential but immediately unavailable supply must 

 be constantly changed into available food at a rate suffi : 

 ciently rapid to supply the plant so that growth may go 

 on rapidly or otherwise the crop will be a poor one. A 

 fertile soil may, in one sense, be defined as one in which 

 unavailable plant food is being changed to available food 

 at such a rate that there is an abundant supply at all 



