24 PRINCIPLES OF SOIL MICROBIOLOGY 



The total number of organisms developing on the plate is recorded 

 either on the basis of moist soil, giving thereby the moisture content of 

 the soil, or on the basis of dry soil. The moisture is determined by 

 drying 10 grams of soil (50 or 100 grams may also be used, especially 

 in the case of non-homogeneous soil, like peat) at 100°C. to constant 

 weight. If N is the total number of organisms developing on the plate 

 from a given soil, a the average number for each plate, u the amount 

 of dilution, x the moisture content of the soil, then, 



a • u • 100 



N = 



100 - x 



Mathematical interpretation of results. The determination of numbers 

 of microorganisms by the plate method involves a number of manipula- 

 tions, each of which involves a certain error. The final error is a re- 

 sultant of all these errors. The constant errors should be eliminated, 

 but the occasional errors depend on the "Law of Chance" and these 

 tend to counterbalance one another rather than to fall in one direction. 

 A study of the variability of the number of bacteria, actinomyces, and 

 fungi on their respective media is given in table 2. The results were 

 calculated as follows: 



If n plates are used for making a quantitative bacteriological count 

 of a given soil, a the number of colonies developing on the plates, 

 while z n is the most probable value obtained from counting n-plates, 

 or the arithmetic mean value, then we have 



2 a 



Zn = 



n 



In other words, the mean (z n ) is found by dividing the sum of all the 

 determinations by the number of determinations. 



The following formulae are commonly used for calculating the average 

 deviation and most probable error of the determination. 



a a • + - J S (Zn - a) 2 

 Average deviation, a = \ 



j n — 1 



, ,, /S (Zn - a) 2 



Most probable error = ± 0.6745 -4/ 



C.V. = 



n(n — 1) 



the average deviation X 100 

 mean 



most probable error X 100 



Em = 



mean 



