30 
water blank or Petri dish. Be sure each dish receives the dilution for which it 
is labeled. Prepare the blank as shown. 
5. Melt 9 tubes of plain agar and cool them to 45° C. 
6 . Pour the plates, being very careful to get the dilution water well mixed 
with the agar. Place the poured plates on a level surface while the agar 
hardens. Rinse out the tubes with hot water while; the agar remaining in 
them is still soft. 
7. When the agar is solid, incubate the dishes at room temperature. Ex¬ 
amine the plates in 4 to 6 days. 
8 . Assuming that each colony has grown from a single organism, count the 
colonies on the best plates and estimate the number of organisms per gram of 
soil. 
Plates containing between 70 and 200 colonies give the most accurate 
results and incidentally are the most easily counted. 
9. Record the results on the regular note paper in tabular form like the 
following: 
TABLE II.—Bacterial Content of Soil 
Dilution 
Number of 
Colonies 
on Plate 
Number o^ 
Bacteria 
Per Gram 
Average 
of two 
plates 
Apparent 
Number of 
Kinds from 
Gross 
Appear¬ 
ance 
Number of 
Molds 
1/1000 g. 
1 /10,000 g. 
1 /100,000 g. 
1 /1,000,000 g. 
Blank 
(a) From the results, can you say that there are many bacteria in 
average soils? 
( b ) Are there many kinds present? Did all of the organisms 
present develop? Why? Give two good reasons for the an¬ 
swer. 
