Relation of Bacteria to Fertility. 173 



times to allow of a rapid and luxuriant growth of the 

 plants. 



The food to be available to the plant must be in solu- 

 tion so that it can pass into the roots of the plant. Such 

 soluble material is easily lost in the drainage water that 

 leaches from the soil during the wet times of the year. 

 Thus, if more of any one element is made available than 

 the crop can use, it is very likely to be lost during the 

 winter and spring, and the soil thus robbed of its fer- 

 tility. An ideal condition is to have enough of each ele- 

 ment rendered available so as to ensure an abundant 

 crop, but not to have an excess. This ideal condition 

 cannot be realized but much can be done by the farmer 

 to conserve the fertility of his soil by methods which will 

 be mentioned later. 



If any one of the essential elements is lacking, or is 

 present only in small amounts in available form, it will 

 act as a limiting factor to the yield. One element is as 

 essential as another. Any may be the limiting one. If 

 this be added to the soil in the form of a fertilizer, an 

 immediate increase in yield is noted. For this reason the 

 use of commercial fertilizers is so extensive. The fer- 

 tility can also be restored by treating the soil in such a 

 way as to render the unavailable plant food available. 

 For example, it ; has been shown in certain of the wheat 

 fields of the Rothamsted Experiment Station in England 

 that the amount of available phosphorous present in the 

 soil was so small as to limit the yield, yet this same soil 

 contained a large amount of the element in an insoluble 

 form. By treating the soil in a proper manner so as to 

 allow the growth of certain kinds of bacteria, the phos- 

 phorus would be made soluble and the fertility of the 

 soil improved. 



