THE STUDY OF MICROORGANISMS IN THE SOIL 127 



Exercise 42 



Isolation of Specific Urea-decomposing Organisms 



Pour isolation plates of several dilutions of the enrichment 

 cultures of Exercise 41. Use urea agar or gelatin Medium 47. 



Incubate the plates at 28°C. 



Examine the plates every 48 hours for a period of 10 days. 

 The urea organisms are often characterized by a distinct halo 

 around the colonies. Under the low power of the microscope the 

 halo is composed of crystals. 



Make transfers to tubes of urea solution Medium 44 or 45 

 and incubate for two days. 



Now test the ammonia-producing power of the pure cultures 

 by inserting sterilized strips of Nessler's paper or tumeric paper 

 in the upper part of the tube. 



Prepare a stained mount of these organisms. 



Exercise 43 



Ammonia Production from Various Substances in Soil 



Prepare 500-gram portions of soil in small jars. The soil 

 should be mixed thoroughly. 



1. Control — no treatment. 



2. Soil plus 0.5 per cent of blood meal, casein, peptone, alfalfa 

 meal, soybean meal, or similar substances. 



After these substances are mixed with the soil bring the mois- 

 ture content of the soil to two-thirds saturation. In order to 

 secure the proper moisture content it is necessary to take into 

 account the water-holding capacity of the added substances. 



Incubate at room temperature and, after 2, 5, and 7 days, 

 determine the ammonia nitrogen. 



Ammonia may be estimated in a number of ways. The 

 aeration method is usually considered the most accurate although 

 good results may be obtained by the leaching of the soil with a 

 strong chloride solution and distilling with magnesium oxide. 



Determination of Ammonia Nitrogen. ^ — Weigh out 50 grams 

 of soil in a 750-cubic centimeter Pyrex Erlenmeyer flask. Add 

 500 cubic centimeters of a 10 per cent KCl solution (in case of neu- 

 tral or alkaline soils use a 20 per cent KCl solution) ; shake for 



1 Harper, H. J., Soil Sci., 18: 409, 1924. 



