HISTORICAL AND INTRODUCTORY 15 



genous food, a small part of which was further converted into nitric 

 acid, which apparently also served as a plant nutrient (174). During 

 the sixties and seventies great advances were being made in bacteri- 

 ology and it was definitely established that bacteria bring about 

 putrefaction, decomposition and other changes ; it was therefore con- 

 ceivable that they were the active agents in the soil and that the process 

 of decomposition there taking place was not purely chemical as 

 Liebig had asserted. Pasteur himself had expressed the opinion that 

 nitrification the curious change of ammonia to nitrate known to 

 take place in soils was a bacterial process. The new knowledge 

 was first brought to bear on agricultural problems by Schloesing 

 and Miintz (244) in 1877 during a study of the purification of 

 sewage water by land filters. A continuous stream of sewage was 

 allowed to trickle down a column of sand and limestone so slowly 

 that it took eight days to pass. For the first twenty days the 

 ammonia in the sewage was not affected, then it began to be con- 

 verted into nitrate ; finally all the ammonia was converted during its 

 passage through the column and nitrates alone were found in the issuing 

 liquid. Why, asked the authors, was there a delay of twenty days 

 before nitrification began ? If the process were simply chemical, oxi- 

 dation should begin at once. They therefore examined the possibility 

 of bacterial action and found that the process was entirely stopped by 

 a little chloroform vapour, but could be started again after the chloro- 

 form was removed by adding a little turbid extract of dry soil. Nitri- 

 fication was thus shown to be due to micro-organisms " organised 

 ferments " to use their own expression. 



Warington (296) had been investigating the nitrates in the Rotham- 

 sted soils, and at once applied the new discovery to soil processes. He 

 showed that nitrification in the soil is stopped by chloroform and car- 

 bon disulphide ; further, that solutions of ammonium salts could be 

 nitrified by adding a trace of soil. By a careful series of experiments 

 described in his four papers to the Chemical Society he found that there 

 were two stages in the process and two distinct organisms : the am- 

 monia was first converted into nitrite and then to nitrate. But he 

 failed altogether to obtain the organisms in spite of some years of study 

 by the gelatin plate methods then in vogue. The reason was dis- 

 covered later : the organisms will not grow in presence of nitrogenous 

 organic matter. Not till 1890 did Winogradsky (311) succeed in iso- 

 lating them, and thus complete the evidence. 



Warington established definitely the fact that nitrogen compounds 

 rapidly change to nitrates in the soil, so that whatever compound is 



