208 TILLEES OF THE GBOUND CHAP. 



question it did settle it for nearly thirty years. 

 But even this paper ends with a doubt, for the 

 authors say that if no plant can take free nitrogen 

 from the air, then it is not clear how the supply of 

 combined nitrogen in the soil is kept up. 



The air is full of free nitrogen, but if it is no use 

 to plants then they must depend entirely upon the 

 combined nitrogen of the soil. But every year our 

 rivers are carrying out to the sea a large, amount of 

 combined nitrogen, which has been taken from the 

 land and is swept out to the sea where it is useless 

 to man. Is the soil then every day growing 

 poorer ? Will there come a day when there is no 

 longer enough manure to grow the crops necessary 

 to keep man alive ? Will the earth grow poorer 

 and poorer, and the crops smaller and smaller till 

 we come nearer and nearer starvation ? This was 

 the problem that was left, or at least the practical 

 side of it. 



Then there was also the theoretical side ; this 

 curious mystery of pea and clover, which seemed 

 sometimes able to fix free nitrogen, but always lost 

 their power of doing so when they were tested under 

 the rigid conditions of an experiment. When 

 would the key to this mystery be found ? . 



Although for many years it was generally 

 believed that no fixation of free nitrogen occurred, 

 yet experiments' went on, many different people 



