THE GERM LIFE OF THE SOIL 141 



The phenoldisulphonic acid is prepared as follows : To 37 grams 

 of concentrated sulphuric acid add 3 grams of pure crystalline phenol. 

 Heat for six hours in a lightly stoppered flask set in boiling water. 



Procedure. — To 50 grams of soil in the 500 c.c. container add 

 250 c.c. of distilled water. Add 1 gram of hydrate of lime to floccu- 

 late the soiL Stir three minutes and allow to stand 20 minutes. 

 Pipette off 25 or 30 c.c. of the clear supernatant liquid and filter it. 

 Evaporate 10 c.c. of the filtrate to dryness over a water bath in an 

 evaporating dish. Moisten with a few drops of phenoldisulphonic 

 acid and stir well. Allow to stand a few minutes. Dilute with a 

 few cubic centimeters of water and neutralize with ammonia. The 

 development of a yellow color is an indication of the presence of 

 nitrates and its intensity is a measure of the amount. 



Exercise II. — Test for ammonia in soil. 



Materials. — A small portion of the soil solution obtained in 

 Exercise I, and Nessler's solution. 



The Nessler's solution is made as follows : To a 250 c.c. 

 solution of potassium iodide (made by dissolving 63 grams in 250 

 c.c. of ammonia-free water) add a saturated solution of mercuric 

 chloride until the precipitate nearly all redissolves. Now add 250 

 c.c. of a solution of potassium hydrate (150 grams to 250 c.c. of 

 water). Make up the whole solution to one liter. Allow to stand 

 until any precipitate has settled before using. Keep in well-stop- 

 pered bottle in the dark. 



Procedure. — To ten cubic centimeters of the soil extract add a 

 few cubic centimeters of Nessler's solution. The development of a 

 light yellow is an indication of ammonia. 



Exercise III. — Factors affecting nitrification. 



Materials. — Same as Exercise I plus four 100 c.c. graduated cyl- 

 inders. Use moist acid soil from beneath sod. 



Procedure. — Place four 50-gram portions of a moist soil from 

 beneath sod in 8-ounce wide-mouth bottles. Bring soil of bottle 

 No. 1 to optimum moisture. Saturate soil of bottle No. 2 to give 

 poor aeration. Thoroughly mix one gram of carbonate of lime to 

 bottle No. 3 and one gram of lime plus one-tenth gram of ammonium 

 sulfate with soil of bottle No. 4. Raise both to optimum moisture. 

 Stopper all bottles lightly with cotton and allow to stand in a warm 

 room for a week or ten days. 



Develop nitrates from these samples as directed in Exercise I. 

 Pour developed solutions into 100 c.c. graduates and dilute to a con- 



