NUMBER OF BACTERIA IN SOIL 161 



conditions, the numbers are far below the number actually occurring 

 in the soil. Some of the reasons for this are as follows : 



1. Even many of the peptone-decomposing bacteria fail to grow 

 on the gelatin plates. This^may be due to overcrowding or to the 

 sudden change of conditions and the resulting osmotic disturbances. 



2. The nitrifiers do not grow on the ordinary organic laboratory 

 media; moreover, they have not been found in soil in sufficient 

 numbers to occur on plates dilute enough to show the more abundant 

 organisms. The nitrogen-fixing organisms both symbiotic and 

 non-symbiotic usually occur in soil in too small a number to be 

 noted by the ordinary plate method. 



3. The strict anaerobes which occur in soils in vast numbers do 

 not grow on the gelatin plate under the ordinary conditions of 

 aerobic culture. 



4. No medium yet devised resembles the soil in composition and 

 structure, and hence the plate does not necessarily reflect the flora 

 active in the soil. Moreover, it is impossible to tell which of the 

 forms developing on the plate are active and which are spores in the 

 soil. 



A third method for the determination of the bacteria in the soil 

 is the direct microscopic count. This method, however, has not 

 been used sufficiently as yet to permit a conclusion as to its relative 

 value. 



Value of Bacterial Counts. The methods for determining the 

 number of bacteria in soil are admittedly faulty; yet they have the 

 advantage of showing whether the number is high or low and whether 

 they are increasing or decreasing. The counts show fairly accurately 

 whether any given treatment of the soil has raised or lowered the 

 number of bacteria in the soil. But numbers alone furnish only 

 meager information, for, as pointed out by Remy, the number of 

 colonies of aerobic soil bacteria appearing on plates show no direct 

 relationship to the ammonifying, nitrifying, or denitrifying powers 

 of the corresponding soil. Lohnis is even more emphatic than Remy 

 in designating mere quantitative methods as untrustworthy. He 

 points out that it is quite possible that two million very efficient 

 ammonia-producing bacteria present in 1 gram of soil will accomplish 

 more work than five million less efficient ones will in another soil. 



This same principle is brought out by Chester when he states 

 that a soil may be low in the total number of bacteria, but contain 

 such a bacterial flora, or combination of bacterial species, which 

 are known to be favorable to the rapid digestion of plant-food, as 

 to give it what might be termed a high bacterial potential. In other 

 words, he holds that we should consider not alone numbers but also 

 physiological efficiency. 



Number of Bacteria in Soil. The number of bacteria, as deter- 

 mined by the plate method, in good arable soil well supplied with 

 11 



