THE NITROGEN OF THE AIR 239 



make their homes on the roots of leguminous plants, such as the pea, 

 bean and clover, which have the power of absorbing nitrogen from the 

 air and of converting it within the roots of the plant into organic 

 nitrogen compounds. 



This discovery explains for the first time the fact long known to 

 farmers that the richness of the soil can be increased by rotation of 

 crops — a fact so extraordinary, till its explanation was understood, 

 that one might well have wondered whether it was not one of the 

 fallacious traditions which are so common among farmers and sailors. 

 This increased fertility is now readily accounted for as follows. Sup- 

 pose that a crop of wheat is first grown on a piece of land, and that 

 thereby the nitrogen compounds in the soil are largely consumed in 

 producing the nitrogen compounds contained in the grain. Suppose 

 now that the next year the same land is planted with clover. As it 

 , grows, the bacteria referred to develop upon its roots, absorb nitrogen 

 from the air, and store up in the roots an abundant supply of nitro- 

 genous compounds. After the clover crop is harvested, these roots 

 decay in the soil, yield up to it their nitrogen-content, which becomes 

 available for the nourishment of a new wheat crop during the follow- 

 ing year. 



An interesting illustration of these considerations has been fur- 

 nished within recent years by the vegetation of the island of Krakatoa. 

 It will be remembered that this island was overwhelmed in the year 

 1883 by an eruption of its volcano, which destroyed all vegetation and 

 buried the original soil beneath a thick layer of volcanic ashes. It 

 might have been expected that this new soil of ashes, which was of 

 course free from all nitrogenous organic matter, would not be able to 

 support plant life ; yet the island soon became covered with an abundant 

 growth. This vegetation was found, however, to be of an unusual 

 character, in that it consisted very largely of leguminous plants — that 

 is, of those plants which, with the aid of bacteria, can take their 

 nitrogen directly from the air. 



These facts suggest that the problem of supplying plants with the 

 nitrogen needed by them may ultimately be solved most simply and 

 directly by the biologist. For through further study of the conditions 

 determining the activities of different species of nitrogen-absorbing 

 bacteria, considered in relation to the kind of crop, the character of the 

 soil and other agricultural conditions, it may prove practicable, by 

 inoculating the soil with the proper kind of bacteria and by treating 

 it in such ways as will best regulate bacterial growth, to secure all the 

 needed nitrogen from the air. Already, government agricultural sta- 

 tions are furnishing pure cultures of nitrogen-absorbing bacteria which 

 have a limited value in the case of certain soils. 



Until such a perfect solution of the problem can be worked out by 



