63 



have found for measuring fixation. Some preliminary tests were made at 

 the beginning of the work, using plates with 400 grams of soil to which 

 were added 4 grams of mannite and kept at 28 C. for 21 days. But the 

 fixation by this method was so slight that it gave no index to the compara- 

 tive power of the soils to fix nitrogen. The method finally used employed 

 100 c. c. of Ashby's solution in 1 litre Erlenmeyer flasks to which were added 

 10 grams of soil. The Ashby's solution was made according to the follow- 

 ing formula: 



Mannite 20 grams 



Nad 2 " 



Mg SO 4 ; 2 " 



Ca SO 4 1 " 



K H 2 P0 4 2 " 



Water 1000 c. c. 



The mono-potassium phosphate was dissolved separately and neutral- 

 ized with NaOH. 



Four 10 gram portions from each soil were weighed out on a dry basis. 

 Two of the Erlenmeyer flasks, containing 100 c. c. of solution, each received 

 10 grams and the remaining portions were analyzed for total nitrogen by 

 the Kjeldahl method. The flasks were incubated for 21 days at approxi- 

 mately 28 C. at the end of which time their entire content was analyzed 

 for total nitrogen by the method previously employed. The average from 

 the two analyses was taken. There was always close agreement in the two 

 results from the soil itself and seldom any wide variations in the fixation 

 flasks. "Where this did occur, the tests were repeated. The slight variations 

 which always accompany fixation tests are impossible to avoid. In Table I 

 is given the soil type as classified by the Bureau of Soils, the original nitro- 

 gen content of the virgin and cultivated soils, and the increase in each after 

 21 days incubation of 10 grams in 100 c. c. of Ashby 's solution. The first 

 of the two numbers refers to the virgin soil. 



DIVISION OF 



FORESTRY 



f I ^CULTURE 



COLLEGE Of 



