B. P. I.— 64fi. 



BACTERIOLOGICAL STUDIES OF THE SOILS OF THE 

 TRUCKEE-CARSON IKRKiATION PROJECT. 



INTRODUCTION. 



In making a bacteriological study of any soil or group of soils there 

 are certain fairly well defined groups of micro-organisms whose func- 

 tions, although as yet imperfectly understood, are recognized as im- 

 portant factors in crop production and are more or less familiar to 

 everyone who has attempted to investigate the problems of soil 

 fertility. These groups of micro-organisms may be roughly separated 

 into four classes, depending upon their physiologic characteristics: 

 (1) Parasites, or organisms important chiefly because they are patho- 

 genic to animals or plants and are frequently found in soils; (2) the 

 cellulose-destroying organisms ; (3) the organisms associated with the 

 formation of humus; and (4) the organisms associated with the trans- 

 formation of soil nitrogen. Only those groups concerned with the 

 transformation of nitrogen, which in the form of ammonia or nitrate 

 is practically the most important of all plant foods, are reported upon 

 at this time. 



The data sought in studies of this character may be outlined as 

 follows: 



(1) Total numbers of saprophytic bacteria in measured quantities of soil. 



(2) Ammonification; the breaking down of nitrogenous organic matter into ammonia. 



(3) Nitrification; the oxidation of various compounds of nitrogen to nitrate. 



(4) Denitrification; the reverse of nitrification. 



(5) Nitrogen fixation, symbiotic and nonsymbiotic; the utilization of atmospheric 

 nitrogen in forming nitrogenous organic compounds. 



In the work conducted at Fallon, Nev., during the season of 1909, 

 in cooperation with the Oflice of Western Agricultural Extension, no 

 quantitative study was made of nitrogen fixation, and the data on the 

 subject of ammonification are very meager. Some preliminary inves- 

 tigations in arid regions had shown that nitrification takes place here 

 at considera])le depth. All studies, therefore, were made of a 3-foot 

 zone, keeping separate the samples of soils from difl^erent depths. 



The comparative nitrifying power of the difi'erent samples from the 

 various plats is shown by curves, the parts per million of nitrogen as 

 nitrate and nitrite being plotted as ordinates, and the difterent depths 

 as abscissae. These curves show only the gain in nitric and nitrous 

 nitrogen. Chlorids and sulphates are also shown, but seem to be of 



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