xvn UTILISATION OF THE SOIL 203 



it does not need to be supplied in manure. He 

 believed, further, that plants could also take 

 ammonia from the air, and use it to build up 

 proteid. 



The practical conclusions which Liebig drew from 

 his experiments, then, were that farmers need not 

 trouble about giving their plants nitrogenous manure. 

 The air, he said, contains plenty of nitrogen, just as 

 it contains plenty of carbonic acid gas. Plants can 

 get as much of these substances as they want. They 

 cannot get salts from the air these must be given 

 in manure, but nitrogen need not be given. 



These statements of Liebig gave rise to fierce 

 discussion and to not a little quarrelling. Farmers 

 had always been in the habit of giving their plants 

 some form of nitrogenous manure, and they said 

 that they were quite certain that wheat, for instance, 

 would not grow without such manure. Liebig, on 

 the other hand, said that, as there was plenty of 

 nitrogen in the air, it was a pure waste of money 

 to give nitrogenous manures to the plants. We 

 know now that the farmers were right, at least for 

 most plants, and that Liebig was wrong ; but never- 

 theless he did a great deal for scientific agriculture, 

 though he happened to be partly wrong on the 

 question whether plants took their nitrogen from 

 the air or from the soil. 



One step of great importance which he took was 



