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. Suppose 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 nitrogenous 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 following year.
An interesting illustration of these considerations has been furnished 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 stations 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
