BACTERIA OF THE SOIL 



63 



3. Keep one flask for a control and inoculate the other with 

 a few grams of fresh garden soil. Incubate the flasks at 30°-37° C. 

 for a week. 



4. Moisten strips of clean filter paper with Nessler's reagent 

 and pour upon them a few drops from each of the flasks. A 

 reddish-yellow color indicates the presence of ammonia. Another 

 method is to remove a portion, say 50 cc, 

 make alkaline with NaOH, and boil it in a 

 small flask, holding a strip of Nessler paper 

 at the mouth of the flask. 



Exercise 91. Ammonification of Nitrogenous 

 Substances in Soil 



1. Place 100 g. samples of freshly gathered 

 soil in each of four tumblers or beakers. Add 

 5 g. of cottonseed meal or dried blood to two 

 tumblers, leaving the others for controls. Stir 

 with a sterile spatula. 



2. Add water to give optimum moisture 

 content as nearly as possible. Cover the tum- 

 blers loosely with a glass plate. Incubate at 

 room temperature for one week. 



3. At the end of the incubation period 

 transfer the soil to a flask, preferably a copper 

 flask. Add a little paraffin and some magne- 

 sium oxide and distill off the ammonia. 



4. For a quaUtative test a piece of filter 

 paper moistened with Nessler reagent may be 

 held at the mouth of the flask as in Exercise 90. 

 For quantitative work a condenser is attached 

 and the distillate is collected in standard acid 

 and titrated against standard alkali. 



5. Compare the amount of ammonia liberated in the control 

 and in the experimental tumblers. What part do bacteria play 

 in this process ? 



Fig. 29. Burettes ar- 

 ranged for titration 



