69 



The soils used in the experiments, and their manner of collection, have 

 all been set forth in the preceding paper. Suffice it to say that the soils 

 retain the same number throughout these reports and the nitrification tests 

 were made from the same sample and simultaneous with those which meas- 

 ured fixation. 



It was decided that soil itself should be the medium in which nitrifica- 

 tion would be measured, and the * ' nitrifying efficiency ' ' method as proposed 

 by Stevens and Wither 1 was adopted and used throughout the series of tests. 

 The soil was first carefully sieved and then 400 grams weighed on a dry 

 basis was placed in one litre Erlenmeyer flasks. To this was added 10 c. c. 

 oJ sterile ammonium sulphate solution containing 240 milligrams nitrogen 

 and the soil brought to 18 percent moisture content. Two flasks were set up 

 for each soil and were kept in the incubator room at approximately 28 C. 

 for six weeks, the moisture content being kept at 18 percent. At the end 

 of this time nitrate determinations were made, usually by the Iron-Zinc Re- 

 duction method, or in cases where the nitrate content was low, the phenol- 

 disulphonic acid method was employed. The results are reported on the 

 basis of milligrams of nitrate nitrogen per 100 grams of soil. 



All of the soils were subjected to another series of tests which may be 

 termed " nitrate accumulation." Four hundred grams of soil were weighed 

 out as before, but now into ordinary quart fruit jars. These samples were 

 kept in the incubator room for six months at 28 C. and 18 percent moisture 

 content. No ammonium sulphate was added and the nitrate present in the 

 soils at the end of six months reflected the power of the soils to convert 

 their own organic nitrogen into nitrate. The analyses at the end of this 

 period were made by the phenol-disulphonic acid method. 



Determination of the original nitrate content was made for all soils im- 

 mediately after they were brought to the laboratory. These analyses showed 

 extremely small quantities of nitrates present in the soils and in many cases 

 only a trace was to be found. Yet these quantities, however negligible, were 

 deducted from the amount found at the conclusion of the tests and all the 

 figures represent increases in nitrate over that originally present. In 

 Table I the results from all nitrification tests are presented. As in the fixa- 

 tion tests, 93 soils were examined, 88 of which were in pairs of a virgin and 

 cultivated sample. In the table is shown the nitrifying efficiency of the 

 various soils with 60 milligrams nitrogen as ammonium sulphate per 100 

 grams of soil and the accumulation of nitrate by the soils when kept under 

 optimum conditions for six months. The figures are the averages from two 

 flasks for each soil. The blank spaces indicate no nitrate. 



'Stevens and Withers. Studies in Soil Bacteriology. III. Cent, f. Bakt., Bd. 25, pp. 64-79. 

 1910. 



