102 SOIL BACTERIOLOGY 



The ammonium sulphate or the calcium carbonate should 

 be sterilized separately and added after cooling. In place 

 of 2 grams of ammonium sulphate, 7.4 grams of magnesium 

 ammonium phosphate may be used. This solution is 

 suited to a quantitative study of nitrification. 



Medium 30 

 Silicate Jelly for the Nitrifying Bacteria 



A. Undialyzed 



1. Prepare a solution of sodium siHcate (Na2Si03) of 

 approximately 8 per cent. Weigh out the sodium silicate 

 and boil in water for thirty minutes, filter through cotton 

 and coarse grained filter-paper. It is more convenient to 

 use Merck's concentrated solution of sodium sihcate and 

 dilute to the desired strength. This must be kept tightly 

 stoppered. 



2. Prepare a solution of hydrochloric acid (HCl) of such 

 a strength that i c.c. of the acid neutralizes i c.c. of the 

 sodium silicate, using methyl-orange as an indicator; or 

 use normal hydrochloric acid and determine the amount of 

 sodium silicate required to neutralize the acid. 



3. To 120 c.c. of the hydrochloric acid solution add, with 

 stirring, 100 c.c. of the sodium silicate solution. If normal 

 acid is used, be sure there is 20 c.c. excess of acid in each 

 220 c.c. of the mixture. 



4. Tube 12-C.C. portions of the mbcture and sterilize in 

 the autoclave for ten minutes at 15 pounds' pressure. If 

 the tubes are sealed tightly this mixture may be kept for 

 several weeks. In case the mixture becomes a milky color 

 or solid when taken from the autoclave, it indicates an 



