148 SOIL BACTERIOLOGY 



7. Connect the flask with the condenser (having prepared 

 the acid to receive the ammonia). Mix the contents thor- 

 oughly by shaking, and distil until all the ammonia has 

 passed over into the standard acid. 



(8) Humus: 



1. Extract 10 grams of air-dry soil in a Gooch crucible 

 with i per cent, hydrochloric acid until the nitrate gives 

 no precipitate with ammonium hydroxid and ammonium 

 oxalate. 



2. Wash until all the acid is removed. In the case of 

 clay soil, the washing should be done chiefly by decantation 

 from a cylinder or tall beaker. 



3. Wash the contents of the crucible (including the 

 asbestos filter) into a glass-stoppered cylinder, with 500 

 c.c. of 4 per cent, ammonium hydroxid. (Mix 300 c.c. of 

 water with 200 c.c. of ammonia (sp. gr. .90) and add more 

 water or ammonia until the hydrometer reads .9604, which 

 is exactly 20 per cent, solution of ammonium hydroxid 

 (NH 4 OH). Dilute this to 4 per cent, with water.) 



4. Allow to remain, with occasional shaking, for twenty- 

 four hours. During this time the cylinder is inclined as 

 much as possible without bringing the contents in contact 

 with the stopper, thus allowing the soil to settle on the side 

 of the cylinder and exposing a very large surface to the 

 action of the ammonium hydroxid. 



5. Place the cylinder in a vertical position and leave for 

 twelve hours to allow the sediment to settle. 



6. Draw off 300 c.c. of the supernatant liquid with a 

 pipet, without stirring up the sediment, place in a stoppered 

 5oo-c.c. flask, and let stand for forty-eight hours. 



7. Carefully pipet off 200 c.c. of the liquid, free of clay 

 particles, into a 3oo-c.c. beaker, evaporate it on a steam- 

 bath, and let the residue heat on the bath for two hours. 



