NITROGEN" FIXATION BY PUEB CULTURES. 47 



EXEECISB XXXVI. 



NITROGEN FIXATION BY PURE CULTURES OE AZOTOBACTEE IN 

 SOLUTIONS AND IN SOILS. 



1. Measure off six 100 cc. portions of mannite solution 

 into 500 cc. Jena flasks. 



2. Plug and sterilize in the autoclave as usual. 



3. Weigh off sis 100 gram portions of air-dry, sieved soil, 

 in tumblers. 



4. Add 1 gram of mannite to each. 



5. Stir thoroughly by means of a sterile spatula. 



6. Inoculate vi^ith 1 cc. of vigorous cultures thus : 



1 & 2 — Mannite solution, Az; 'beijerincki. 

 3 & 4 — Mannite solution, Az. chroococcum. 

 5 & 6 — Mannite. solution, Az. viuelandii. 

 7 & 8 — Soil, Az. beijerincki. 



9 & 10 — Soil, Az. chroococcum. 



11 & 12 — Soil, Az. vinelandii. 



7. Add sterile water to the soil portions to bring the moist- 

 ure conditions up to the optimum for the soil. 



8. Cover with Petri dish covers. 



9. Incubate soils and solutions 7-8 days at 20° C. 



10. Determine the nitrogen in the solutions by the regular 

 KJeldahl method. 



11. Dry and grind the soil portions and determine the ni- 

 ti'ogen present. 



12. Grind a sample of the air-dry soil used as the medium 

 and determine the nitrogen present. 



4 



