368 MICROBIOLOGY OF SOIL 



Actinomyces may reach 1,000,000 or more per g. of soil. The fungi 

 almost disappear below 20 to 30 cm., while the actinomyces do not 

 decrease rapidly at depths lower than 30 cm. 



Distribution at Different Depths. Most of the soil bacteria are found 

 in the stratum in which the organic residues are concentrated, that is, 

 in the surface soil. Immediately at the surface the rapid evaporation 

 and the germicidal effect of direct sunshine act as disturbing factors, 

 hence the numbers in the uppermost 25 to 50 mm. (i to 2 inches) are 

 smaller than in the layer of soil immediately below. Beyond the 

 depth of 20 cm. or 22 cm. (8 or 9 inches) the numbers diminish rapidly. 

 Material from a depth of .6 m. to .9 m. (2 to 3 feet) is nearly sterile in 

 humid regions. Differences occur, however, in keeping with the 

 mechanical composition of the soil. In light, open soils the bacteria 

 are not only carried down to greater depths by the percolating water, 

 but can also multiply there, thanks to better aeration. On the con- 

 trary, fine-grained compact soils are more effective in holding back 

 suspended material and do not allow the bacteria to pass downward as 

 readily. Moreover, the less thorough aeration of these soils and the 

 accumulation of toxic reduction products in the subsoil serve as an 

 effective check in the increase of bacteria in the deeper layers. 



In irrigated soils of the arid and semi-arid regions bacteria are dis- 

 tributed at much greater depths. Their occurrence 2 m. to 3. m. 

 (8 or 10 feet) below the surface is made possible not only by the better 

 aeration of these soils, but by the penetration of roots to great depths 

 and the accumulation there of considerable amounts of organic matter. 

 The practical significance of distribution appears, among other things, 

 in the use of soil for inoculation purposes; for instance, it is reported by 

 Salstrom that in making peat soils arable the addition of small amounts 

 of fertile loam increases to a very marked extent their crop-producing 

 power. The efficiency of the inoculating material decreases as it is 

 taken from the deeper soil layers. Similarly, in the use of alfalfa soil 

 for the inoculation of new fields the most efficient material is secured at 

 a depth between 7.62 cm. and 17.78 cm. (3 and 7 inches). 



Seasonal Variations of Bacterial Numbers and Activities. Conn has 

 reported an apparent increase of bacteria in frozen soil. This increase 

 seems to be due to an actual multiplication of the organisms rather than 

 to a mere lifting of the bacteria from lower depths by capillary action. 

 The greatest increase was found to occur during the winter in the slow- 



