28 PRINCIPLES OF SOIL MICROBIOLOGY 



reproductive stage, while the mycelium is the active stage of the 

 organism, and since some fungi, like the Mucoraceae, produce a myce- 

 lium which does not readily break up into fragments, the results ob- 

 tained by plate count may be far from representing the actual abun- 

 dance of the particular organisms in the soil. 



Numbers of bacteria in the soil. In view of the fact that the methods 

 used in the past for the determination of numbers of bacteria in the soil 

 varied greatly and most of the media were not synthetic in composition, 

 the results cannot be readily compared. However, certain funda- 

 mental facts have been established, which throw interesting light upon 

 the nature of the soil population. By the use of the microscopic method 

 Conn found, in one gram of soil, 140 to 390 millions of small rod-shaped 

 bacteria less than 0.5 micron in diameter, 1.5 to 12 million rods 0.5 

 to 0.8 micron in diameter and 500,000 to 5,000,000 spores per 1 gram 

 of soil. Very few of the large rods (over 1 micron in diameter) were 

 found in all the soil samples. In manured soils, the number of bacteria 

 were found to reach 800,000,000 per gram, as determined microscopically. 

 Still larger numbers were reported by others (p. 11). On the other 

 hand, by the use of nutrient agar plate, Adametz 49 found 320,000 to 

 500,000 bacteria per gram of sandy soil, and 360,000 to 600,000 in loamy 

 soils. The numbers of bacteria in the soil obtained by the numerous 

 investigators fall somewhere between these two sets of figures. These 

 tremendous differences and variable results led Lohnis 50 to the conclu- 

 sion that a determination of bacterial numbers in the soil is worthless 

 as an attempt of interpreting soil phenomena. However, when the 

 results are considered critically and compared carefully, it is found 

 that the information obtained from the study of numbers of micro- 

 organisms gives not only an interesting insight into the microbiological 

 population of the soil, but also throws light on soil fertility, particularly 

 when this information is combined with that obtained from the study of 

 physiological activities of soil microorganisms (p. 711). 



In general the numbers of bacteria, as determined by the plate 

 method, are found to range in normal soils well supplied with organic 

 matter from 2 to 200 millions per gram, as determined by the plate 

 method. These numbers vary with the soil type, soil treatment, 

 season of year, depth, moisture content and various environmental 



49 Adametz. Untersuchungen iiber die niederen Pilze der Ackerkrume. Diss. 

 Leipzig. 1886. 



10 Lohnis, F. Handbuch der landwirtschaftlichen Bakteriologie. Berlin, 

 1910, p. 513. 



