ROLE OF BACTERIA IN NATURE 33 



to till the soil. In the formation of soil from the primitive rock, 

 bacteria played no small part, the changes wrought by the elements 

 first giving them a foothold. 



Changes in temperature tear loose huge rocks and break them 

 into fragments. It is well known that most substances when heated 

 expand and contract in such varying degrees that parts are put 

 under a strain. This strain at times is sufficient to cause cracks of 

 various sizes to occur in the rock, a result which may be illustrated 

 by the sudden cooling of hot glass or the sudden heating of cool 

 glass. Throughout the long, hot days of summer rock is heated to 

 a comparatively high temperature, as the boy who has chased bare- 

 foot over their surface in quest of grasshoppers or butterflies will 

 testify. At night they cool. This is repeated day after day. This 

 continued heating and cooling gradually causes small crevices to 

 appear in even the most resistant. These become filled with water 

 and dust; when the cold nights of autumn come the water freezes. 

 In freezing, the water expands and the rocks are broken into pieces. 

 So it continues day after day and year after year, until the rock 

 becomes a fine powder. Even then, however, the plant-food is still 

 insoluble and cannot be taken up by the plant. Long before it has 

 reached the form of powder, bacteria begin to grow upon the surface 

 of the rock and in the crevices. In their growth they form acids 

 which act upon the insoluble plant-foods, rendering them soluble. 

 Bacteria continue their work long after the rocks have been changed 

 to soil, each day liberating a little more plant-food for the growth 

 of plants during that day. During the year the bacteria are able in 

 a fertile soil to liberate enough plant-food for the production of a 

 good crop. When manure is applied, it not only supplies food for 

 the growing crop, but it also supplies food for the microorganisms, 

 and they in turn liberate more of the insoluble constituents of the 

 fine rock particles of which the soil is mainly composed. There are 

 millions of them in every ounce of soil, struggling, to be sure, for 

 their very existence, but always rendering a little more mineral 

 plant-food available. 



One of the essential elements for crop production, and the one 

 which is usually in the soil in the smallest quantities, is nitrogen. 

 This, unless it be applied to the soil in the form of the costly fer- 

 tilizer, nitrates, must be prepared for the plant by bacteria. The 

 farmer finds his crops are limited directly by the speed with which 

 these organisms prepare the food for his growing crop. If they 

 are active, other things being favorable, he will get a good crop; 

 but if they do not play their part, though everything else may be 

 ideal, yet there is no crop. 



Bacteriological examinations of cultivated soils have shown that 

 usually those that are riches^contain the greatest number of bac- 

 teria. The number in the soil is dependent upon the quantity and 



