THE STUDY OF MICROORGANISMS IN THE SOIL 103 



Divide the sample of soil, taking one portion for plate count, 

 the other for moisture determination. For virgin field soil the 

 following dilutions have been found satisfactory. 



Take soil samples and plate as follows: 



(a) Surface soil 1 



(6) Soil 1 foot deep 1 



1 



(c) Soil 2 feet deep 1 



1 



(d) Soil 4 feet deep 1 



1 



Follow the method given in previous exercises. 

 Tabulate results. 



Exercise 13 



Effect of Manures on Number of Microorganisms 



Prepare five tumblers or beakers with 100 grams each of field 

 soil. 



Arrange as follows: 



(a) Control. No treatment. 



(6) Treat with 1 per cent of finely chopped green clover, 



(c) Treat with 1 per cent of well-rotted stable manure. 



(d) Treat with 1 per cent of ground wheat or rye straw. 



(e) Treat with 1 per cent of ground wheat or rye straw and 0.1 

 per cent (NH4)2HP04. 



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. For this purpose shake 

 5-gram portions of the manures with 5 cubic centimeters of 

 sterile water. Dilute as given in the previous exercises. Pour 

 plates from the dilutions 1:100,000 and 1:1,000,000 for bacteria 

 and 1 : 10,000 for fungi. 



After mixing thoroughly the soil and manure in tumblers, 

 raise the moisture to two-thirds saturation. 



Cover the soil with Petri dishes and incubate at room 

 temperature. 



Determine the number of bacteria and fungi after 1 and 3 

 weeks. 



