142 Bacteria in Relation to Country Life 



well as by the turning under of green-manures. When 

 any of these manurial substances are introduced into the 

 soil, there are changes produced in its content of soluble 

 salts, and, likewise, modifications in its moisture and 

 aeration conditions. An additional factor is introduced 

 in the case of barnyard manure, since the latter is a 

 material rich in bacteria. An application of several 

 tons of manure per acre introduces into the soil many 

 millions of bacteria, and not only adds thus to the num- 

 bers already present there, but also influences the rate 

 of subsequent increase. 



The kinds and numbers of bacteria in the soil bear 

 also a certain relation to the processes of irrigation and 

 drainage, to subsoiling, to the once prevalent practice 

 of paring, burning, sanding, or claying, and, more par- 

 ticularly, to the important processes of marling and 

 liming. The application of lime, or of lime marl, exerts 

 a far-reaching effect on the numbers and species rela- 

 tionships of the soil bacteria. The effect of such appli- 

 cation may be observed for years in the size and quality 

 of the crops grown. 



Distribution of bacteria in the soil. — After the middle 

 of the last century, when the universal presence of bac- 

 ter a began to be more widely recognized, bacteriologists 

 turned their attention to the soil as a breeding-place for 

 various microorganisms. Miquel in France showed in 

 1879 that the soil, at a depth of several inches below the 

 surface, may contain very large numbers of bacteria. 

 Several German investigators who studied the subject 

 in the eighties of the last century confirmed Miquel's 

 results. It was demonstrated by them that the greatest 



