35} LABORATOEY BACTERIOLOGY. 



EXBECISE XXIII. 



THE AVAILABILITY OF NITKOGENOUS ORGANIC MATTER AS IN- 

 DICATED BY ITS NITRIFICATION. 



1. Weigh off sixteen 100 gm. portions of air-dry, sieved 

 soil, in tumblers. 



2. Make additions of the following materials in such 

 amounts that the nitrogen present in all cases shall be the 

 same as is supplied by 100 mgs. ammonium sulfat. 



3. Arrange thus : 



1 & 2— nothing. 



3 & 4 — 100 mgs. ammonium sulfat. 



5 & 6 — dried blood. 



7 & 8 — concentrated tankage. 



9 & 10 — cottonseed meal. 

 11 & 12 — bone meal. 



13 & 14 — horse manure (dried and ground). 

 15 & 16 — cow manure (dried and ground). 



4. Stir all thoroughly by means of a sterile spatvila. 



5. Add sterile water to bring the moisture conditions up 

 to the optimiim for the soil. 



6. Cover with Petri dish covers; weigh and record the 

 weight on the tumblers. 



7. Incubate 4 weeks at 20° C. 



8. Weigh every ten days and with sterile water replace 

 that lost by evaporation. 



9. At the end of the incubation period add water and 

 a. little lime ; shake, filter, make up to some convenient vol- 

 ume and determine the nitrates present in aliquots, by the 

 phenol-sulfonic acid method.' 



