Bacteria and Crop Rotation 141 



influence the numbers and kinds of bacteria in the soil. 

 There is reason to believe, however, that a decided in- 

 fluence is thus exerted by the growing crops. These 

 affect the growth of soil bacteria in still another way. 



It is a well-known fact that different crops do not 

 take out of the store of available constituents in the soil 

 the same amounts and proportions of plant-food. For 

 this reason, they affect the composition of the soil to 

 an unequal extent and unequally change the numbers 

 and character of the soil bacteria. The effects of differ- 

 ent systems of cropping are clearly distinguishable both 

 in the size and quality of the harvests and in the endur- 

 ance of soil fertility. The pernicious effects of the con- 

 tinuous growing of cereals were noted generations ago, 

 and led gradually to the introduction of rotation systems. 

 It seems that the evil effects of continuous grain-growing 

 are due in part to the one-sided and wasteful changes 

 in the soil-humus caused by bacteria. On the other hand, 

 a succession of different crops, including members of the 

 legume family, creates conditions favoring an economical 

 transformation of the soil-humus. It will thus be seen 

 that there is a direct relation between the crops on the 

 soil and the bacteria in the soil. 



The number of soil bacteria varies likewise with the 

 methods of tillage. All of the mechanical operations, 

 such as plowing, harrowing, disking, hoeing, and rolling, 

 which affect the evaporation from the soil, the penetra- 

 tion of air, or the supply of moisture from the subsoil by 

 capillary action, affect the rate of increase of the soil 

 .bacteria. The numbers of bacteria in the soil are readily 

 affected by the application of manures and fertilizers as 



