NUMBER OF BACTERIA ACCORDING TO PLATE METHOD I9 



ances sensitive to lo milligrams arc satisfactory for this 

 work. Analytic balances may be used, but are not neces- 

 sary. 



1. Weigh 20 to 30 grams of soil on a piece of sterile 

 paper or scoop, or weigh the entire soil sample, bottle, 

 and contents; then remove about 20 grams with a sterile 

 spatula. Reweigh sampling bottle and contents and record 

 loss in weight. Transfer the soil to a 200-c.c. sterile water 

 blank. 



Note. — ^Two hundred c.c. of water in a $qo~c.c. Erlenmeycr flask allows 

 ample space for shaking. Tap-water will be found very satisfactory for 

 soil counts. The water blanks may be sterilized in the autoclave for fifteen 

 minutes at 15 pounds' pressure. For ordinary work, provided blanks are not 

 stored for a long time, thirty minutes in the steamer will be sufficient. 



2. Shake this suspension vigorously for five minutes 

 and allow the coarse particles to settle. 



3. Add 10 c.c. of this first dilution, equivalent to i gram 

 of soil, to a 90-c.c. sterile water blank. One ex* from this 

 dilution is equal to o.oi gram of soil. 



4. After shaking, add i c.c. to a go-c.c. sterile water 

 blank. (Dilution r:xo.ooo.) 



5. Transfer i c.c. of the above to a 9-c.c. sterile water 

 blank. As a rule, this dilution, which represents i : 100,000 

 of a gram of soil to each cubic centimeter, is the one from^ 

 which to pour plates. If the soil is very poor, use a dilution 

 of I : 10,000; if very rich, i : 1,000,000. The number of 

 dilutions will depend on the type of soil Garden or well- 

 cultivated soil rich in organic matter requires a higher 

 dilution than poor, sandy soil. 



6. Pour plates from the following dilutions in triplicate: 

 I : 10,000, I : 100,000, and i : 1,000,000. 



7. Add about 10 c.c. of an agar medium, melted and cooled 



