Sept. 15, 1925 
Plate Counts of Soil Microorganisms 
515 
It remained to be seen whether duplicate series of dilutions would 
give concordant results. Four composite samples were taken from 
the plot used for the frequent platings, and two series of dilutions 
were made from each. From the highest dilution of each series, 
four plates were poured with soil extract agar and incubated for the 
usual seven days at 28° C. In Table VIII are given the averages 
of the four plates made from duplicate dilutions of four samples. 
The irregularity of the counts indicates that it is necessary to make 
parallel dilutions if a fair average is to be obtained. The same end 
may be attained, of course, by plating two or more samples in the 
usual way and taking the averages as single determinations. This 
is especially to be recommended if the field has been heavily fertilized 
and the soil is therefore of uneven structure. 
Table VIII .—Plate counts from duplicate dilutions of composite samples 
Sample 
Dilution 
Per cent 
deviation 
a from b 
a 
b 
1 X ......... 
54.0 
60.5 
-11 
104.0 
80.5 
+22 
3 _____ 
73.5 
78.0 
-6 
4______ _ 
120.0 
91.0 
+24 
From a plot acre) which had received each year 320 pounds 
sodium nitrate per acre in addition to minerals, Waksman {12) took 
51 single samples at the same time. The counts varied from 8.6 
to 22.6 millions per gram (average 12.9 millions), which result illus¬ 
trated the well-known fact that one plating of a single sample is 
very unreliable. On the other hand, 5 composite samples consisting 
of 3 borings, each taken from a check plot which had not been fer¬ 
tilized for 14 years, gave very consistent counts, ranging from 7.39 
to 9.07 millions (average 8.35). The close agreement of these counts, 
and the wide variation of those made on the single samples, should 
not be attributed to the method of sampling alone, as the different 
treatment of the plots has evidently influenced the results obtained. 
That plate counts made from a very uniform soil may show fluc¬ 
tuations from the average of about 20 per cent plus or minus, even 
though composite samples are used, may be seen from the following 
test. In a greenhouse experiment, corn was raised in buckets filled 
with 25 pounds of an evenly mixed unfertilized soil. The average 
dry weights recorded for eight rows, each containing eight buckets, 
together with the plus and minus percentage deviations from the 
average, as given in Table IX, clearly show the uniformity of the 
soil. 
Table IX. —Fluctuations in plate counts of uniform greenhouse soil 
Row No.--- 
5 
9 
11 
17 
23 
27 
29 
35 
Dry weight, gm. 
Per cent ± average_ 
Total counts, millions. 
Per cent ± average__— 
13.0 
-1.5 
13.3 
+0.8 
13.4 
+1.5 
13.5 
+2.3 
13.1 
-0.8 
12.8 
-3.0 
13.1 
-0.8 
/ 
13.0 
-1.5 
75.0 
+14.0 
68.0 
+4.0 
62.0 
-7.0 
75.5 
+ 14.5 
53.0 
-19.0 
68.5 
+4.0 
59.5 
-9.0 
64.0 
-4.0 
