CHAPTER XVI. 

 THE SOIL FLORA. 



Two methods are in general use for the quantitative determina- 

 tion of bacteria in the soil: the Koch gelatin-plate method and the 

 Hiltner and Stormer dilution method. 



Koch Gelatin-plate Method. The determination by the gelatin- 

 plate method is made as follows : Samples of soil are carefully mixed, 

 usually by shaking, and 100 grams of the mixed soil weighed on sterile 

 paper or watch-glasses into 200 c.c. of sterile water. This is shaken 

 for one minute. Ten c.c. of this is transferred to 90 c.c. of sterile 

 water. This is continued until the proper dilution is obtained, 

 usually 1 to 20,000 and 1 to 200,000, then they are plated on gelatin 

 or some other solid media. The number of colonies developing is 

 counted at the end of four or seven days. The longer period is pref- 

 erable for many of the important soil organisms are missed in the 

 shorter incubation period. 



Hiltner and Stormer Dilution Method. ~ The Hiltner and Stormer 

 method uses solutions prepared to favor in each case the develop- 

 ment of one of the various groups of bacteria (ammonifying, nitrify- 

 ing, nitrogen-fixing, etc.). The soil is inoculated into the appro- 

 priate media in constantly decreasing quantities usually 100, 10, 1, 

 0.1, 0.01, 2, and 0.001 mg. of soil. The material is incubated and 

 after the appropriate time the gain in ammonia, nitrites, nitrates, etc. 

 determined. By this procedure a point is finally reached where the 

 small quantity of soil employed contains none of the specific organ- 

 isms, and hence fails to give rise to the specific physiological reaction. 

 By this method a fairly accurate estimation may be made of the 

 number of different microorganisms in the soil. 



Lohnis clearly demonstrated that the Hiltner and Stormer method 

 is more exact than is the gelatin-plate method. He found that 

 a soil which yielded 1,270,000 bacteria per gram by the gelatin- 

 plate method, by the Hiltner and Stormer method yielded 3,750,000 

 peptone-decomposing bacteria, 50,000 urea-decomposing bacteria, 

 50,000 denitrifying bacteria, 7000 nitrifying bacteria, and 25 

 nitrogen-fixing bacteria, making a total of 3,857,025, which is over 

 three times the number obtained by the plate method. 



Defects of Plate Method. The number of organisms found in soil 

 by this method varies with the media used and the period elapsing 

 between plating and counting. But even under the most favorable 



