DEVELOPMENT OF SOIL BACTERIOLOGY 9 



conceivable that they were the active agents in changing these 

 within the soil. Pasteur had expressed the opinion that nitrifica- 

 tion, the changing of ammonia into nitrates, was a bacterial 

 process. Nothing definite was known on the subject until 1877, 

 when Schloesing and Miintz were surprised by the pecularity in 

 action of sand filters used in purifying sewage. 



A continuous stream of sewage was allowed to trickle down a 

 column of sand and limestone at a rate such that it required eight 

 days to pass. The first twenty-one days the ammonia in the sew- 

 age was unaffected. Then it began to appear as nitrates, and in 

 time only nitrates issued from the filter. If it were a chemical 

 process, nitrates should be found at once. Why must twenty-one 

 days elapse before the ammonia was transformed into nitrates? 

 In attempting to answer this, they found that the process could 

 be stopped by the addition of chloroform and started by adding 

 the extract of fresh soil. Nitrification was thus shown, to use 

 their expression, to be due to organized ferments. 



The succeeding thirteen years represent the history of a race 

 among the leading scientists of Europe and America to deter- 

 mine to whom should go the honor of first obtaining and studying 

 in pure culture the microorganisms which possessed this important 

 property of transforming ammonia into nitrates. 



Warington of England proved that no matter what com- 

 pounds of nitrogen are supplied to a plant as manure, they are 

 rapidly transformed within the soil into nitrates. Furthermore, 

 he found that ammonia is first changed to nitrite and then into 

 nitrate, but was unable to obtain pure cultures of the organism 

 bringing about either reaction. 



However, up to the time Winogradski took up the subject, 

 all attempts to isolate such organisms had failed. After long 

 trying experiments which demonstrate the keen, untiring ingeni- 

 ous, nature of this investigator, he gave to the world nitrifying 

 organisms. He found they would not grow in ordinary media as 

 had been used by previous workers, but the media must be free 

 from organic matter. 



It has been known for generations that uncropped soil increases 



