Length of Time to Incubate Petri Plates. 105 
The variations between the per cent of the colonies that developed 
is from 7.4 to 43.4 per cent for the three days’ incubation, 34.5 to 88.4 
per cent for the five days’ incubation, and from 53.6 to 100.0 per cent 
for the seven days’ incubation. The figures are taken from the 1—400,000 
bacterial dilutions, where the number of colonies was small enough to 
allow for all organisms to develop into colonies. The plates for the 
1—40,000 bacterial dilutions in many cases had tco many organisms for 
satisfactory counts, and this is shown in the general averages for this 
diJution as compared to those for the 1—400,000 bacterial dilution. The 
cropping system, the aeration of the soil and soil temperature very 
evidently influence the rate at which the organisms of soil develop into 
discernable colonies on petri plates. 
One contention for the use of the bacteriologist’s soil sampler* was 
that it sampled the soil accurately to the depth desired and kept the 
sample under field conditions of aeration until analyzed. Table II gives 
data showing how the methods of sampling can be compared by the 
relative distribution of the rapid and slow growing organisms present 
in the different samples. 
TABLE JI. 
Percent 3, 5, and 7-Day Counts of 10-Day Counts **. 
(Gravelly soil sodded and containing about 7% moisture.) 
TiME oF INCUBATION | 3 5 7 10 Days* 
Bacteriologist’s Soil Sampler 1 15.3% 20.8% 53.8% 100.0% 
2 11.8 39.4 49.0 100.0 
3 ; Uta | 26.1 46.3 100.0 
PAV CLAZ CA Uel idc tras ayaere eiee 1 PAeT 28.8 AQT AR elltcsaccre cee 
P. I. Brown's Method 1 11.5 46.0 72.8 100.0 
2 29.2 43.8 65.7 100.0 
3 14.1 26.4 45.0 | 100.0 
ASV ETAL Ol ean. cic teealiae eee 18.3 | 38.4 7 OP, ety eee 
¥ | 
Slice Method 1 20.5 48.2 64.0 100.0 
2 4.8 42.8 63.2 100.0 
AVETAge. ©. os5.0 2: |ocroacesesee 12.7 45.5 6356 les etree cise 
Atyrerareronall teialloveocsisrisyetore-c 14.8 37.7 Bide Silbeoe ccc 
*Counts after 10 days incubation at 20°C taken as 100% those at other times are stated as parts of 
this. Bacterial Dilution 1-400,000. 
**Counts were about 3.0 million per gram of dry soil. 
