144 PLANT-ANIMALS [CH. 



rendered amenable to cultivation by planting with 

 lupins. The roots of these plants, beset with nodules, 

 decay in the ground, release nitrogen-compounds, 

 hitherto deficient in the soil, and thus, by their decay, 

 admit of the growth of plants which rely entirely on 

 "fixed" or combined nitrogen. It is computed by com- 

 petent authorities that in Germany alone no less than 

 500 million pounds of nitrogen are secured annually 

 from the air through the activity of the root-tubercle 

 bacteria associated with leguminous crops. 



It is a grim commentary on the mode and rate of 

 progress of agricultural science that these discoveries 

 of the men of science yesterday were among the 

 accepted commonplaces of the ancients. Thus Pliny 

 observes that "the bean ranks first among the legumes 

 and it fertilizes the ground in which it has been sown 

 as well as any manure." 



Man solves the nitrogen problem by including 

 legumes in his crop-rotations, by transporting nitrates 

 from Chili to his European fields and more re- 

 cently by effecting a combination of the nitrogen 

 of the air with oxygen or other elements, utilising 

 for this purpose electrical energy. Where water- 

 power is available for the generation of electricity, 

 factories, destined to play an increasingly important 

 part in the solution of the nitrogen problem, are at 

 present at work turning out large quantities of cal- 

 cium nitrate or other nitrogen-containing compounds. 



