EFFECT OF MANURES ON NUMBER OF BACTERIA 25 



2. Place 200-gram portions of the air-dry soil into four 

 small glass jars. One pint Mason jars may be used. 



3. Adjust the moisture content as follows: 



(a) Air dry. 



(6) IS per cent, moisture. 



(c) 30 per cent, moisture. 



(d) 45 per cent, moisture. 



4. Incubate the soil samples at room temperature. 



5. After intervals of one and three weeks determine the 

 number of bacteria according to the plate method. Pour 

 triplicate plates from the dilution of i : 100,000. 



6. Arrange results in tabular form (see p. 24). 



Exercise 7 



Effect of Manures on Humber of Bacteria 



1. Prepare three tumblers or beakers with 100 grams 

 each of field soil. 



2. Arrange as follows: 



(a) Control.! 



(b) Treat with i per cent, of finely chopped green clover. 



(c) Treat with i per cent, of well-rotted stable manure. 



3. Since these substances contain great numbers of 

 bacteria, especially the stable manure, plate counts should 

 be made of the manures at the time the soils are treated. 

 Jor this purpose shake 20-gram portions of the manures 

 with 200 c.c. of sterile water. Dilute as given in the 

 previous exercises. Pour plates from the dilutions 

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



4. After mixing thoroughly the soil and manure in 

 tumblers, raise the moisture to two-thirds saturation. 



^ Control or blank is equivalent to no treatment. 



