INFLUENCE OF ENVIRONMENTAL CONDITIONS 787 



poorer in carbon but richer in nitrogen, due to the greater liberation of 

 carbon dioxide and perhaps to increased nitrogen fixation. 



The nature of the crop rotation also influences the activities of 

 microorganisms in the soil; a soil under a four year rotation, while 

 the fourth crop was still on the land or before the cycle was completed 

 for the first time, gave increased number of organisms over that of 

 the soil under continuous cropping. 76 Soil under continuous corn and 

 wheat contained relatively low numbers of bacteria, in comparison 

 with the timothy and rotation plots. 77 



Influence of salt concentration upon the activities of microorganisms in 

 the soil. The addition of mineral nutritive elements to the soil in- 

 fluences the activities of microorganisms in the soil in various ways. 

 (1) It stimulates the growth of higher plants, thus leading to an in- 

 crease in crop residues, greater supply of available energy and, there- 



TABLE 90 



Influence of soil treatment upon the numbers of microorganisms in the soil 



fore, an increase in microbial activities. (2) In the presence of an 

 excess of available energy, the mineral elements are often limiting 

 factors in the activities of microorganisms; this is true especially 01 

 nitrogen, phosphorus, potassium, calcium and magnesium. (3) It 

 tends to produce a more favorable balance in the concentration of the 

 soil solution and colloidal condition of the soil, for these activities. 

 However, an increase in the concentration of salts, as in the case of 

 saline and alkali soils, tends to injure these activities, although most 

 soil microorganisms are capable of withstanding high concentrations 

 of salts. 



76 Brown, P. E. Bacteriological studies of field soils. II. The effect of con- 

 tinuous cropping and various rotations. Iowa Agr. Exp. Sta., Res. Bui. 6. 

 1912. 



77 Gainey, P. L., and Gibbs, W. M. Bacteriological studies of a soil sub- 

 jected to different systems of cropping for twenty-five years. Jour. Agr. Res., 6: 

 953-975. 1916. 



