SOIL ORGANISMS 393 



ditions of the soil affect the growth of bacteria. Among the 

 most important of these are the supply of oxygen and mois- 

 ture, the temperature, the presence of organic matter, and 

 the acidity or the basicity of the soil. 



All soil bacteria require for their growth a certain amount 

 of oxygen. Some bacteria, however, can continue their activ- 

 ities with much less oxygen than can others. Those requir- 

 ing an abundant supply of oxygen have been called aerobic 

 bacteria, while those preferring little air are designated as 

 anaerobic bacteria. This is an important distinction, because 

 those bacteria that are of greatest benefit to the soil are, in 

 the main aerobes, and those that are injurious in their action 

 are chiefly anaerobes. However, it seems likely that an 

 aerobic bacterium may gradually accommodate itself within 

 certain limits to an environment containing less oxygen, and 

 an anaerobic bacterium may accommodate itself to the pres- 

 ence of a larger amount of oxygen. It is quite possible that 

 the aerobic and anaerobic organisms function in the soil at 

 the same time, since a portion even of a well aerated soil is 

 always highly charged with carbon dioxide. It is not improb- 

 able, also, that there exists a more or less beneficial inter- 

 relation between the two general groups. 



Bacteria require moisture for their growth, optimum water 

 for higher plants seemingly being the best moisture for the 

 development and activity of favorable soil organisms of all 

 kinds. With a decrease of moisture the soil becomes well 

 aerated, while an excessive water supply tends to encourage 

 anaerobic conditions. Moisture, when aeration and tempera- 

 ture are favorable, seems to be the main control of biological 

 changes within the soil. 



Soil bacteria, like other plants, continue life and growth 



Seed-Bea upon Bacterial Activity in the Soil; Soil Sei., Vol. II, No. 2, 

 pp. 193-204, 1916. 



Greaves, J. E., and Carter, E. G., Influence of Moisture on the Bac- 

 terial Activities of the Soil; Soil Sci., Vol. X, No. 5, pp. 361-387, 1920. 



