142 



SOILS AND MANURES 



Fixation of Free Nitrogen by Leguminous Plants. 

 From time immemorial it has been customary to take wheat 

 after clover because it was found that better crops were 

 obtained than in any other order of rotation. It has also 



FIG. 17. 



been known for a long time that, when used as green 

 manures, clover vetches and lupines gave better results 

 than most other crops. More recently it was discovered 

 that, although these plants contain a very large amount 

 of nitrogen (p. 169), the proportion of nitrogen in the soil 



