50 THE SPIRIT OF THE SOIL 



elapse to enable them to grow. To test the accuracy 

 of this view Chloroform was passed through the 

 filter. The change of Ammonia to nitrate at once 

 ceased, but on removing the Chloroform and adding 

 a little turbid water extract of dry soil to the filter 

 the action started again. The experiment proved 

 conclusively that bacterial action was responsible for 

 the change. 



The publication of this paper furnished Warring- 

 ton, who was working at Rothamsted on the problem 

 of soil Nitrogen, with the key to the solution. He 

 proved 



1. Nitrification in the soil was stopped both by 

 Chloroform and by Carbon bisulphide. 



2. Solutions of Ammonium salts could be nitri- 

 fied by adding a trace of soil. 



3. The change occurred in two stages, two 

 different bacteria being involved in the process, the 

 Ammonia becoming first of all nitrite (NO 2 ) and 

 then nitrate (NO 3 ). 



So was solved the mystery as to how non-legu- 

 minous plants obtained their nitrogenous food. It 

 became generally (if not quite accurately) recognized 

 that in whatever form of manure Nitrogen might be 

 added to the soil, the plants got practically nothing 

 but nitrates as the source of their nitrogenous food. 



It was nearly ten years before the problem as 

 affecting leguminous plants was solved. In 1886 the 

 classical paper clearing up the difficulty was pub- 

 lished by Hellriegel and Wilfarth. They grew 

 several series of plants in sand, adding the food con- 

 stituents as they desired, and they found that the 



