SOILS AND FERTILIZERS. 383 



amounts f. e. of potash and phosphoric acid a crop may contain, and 

 by plowing under, yield up to the soil, it is plain that it merely returned 

 what it borrowed, and no art in manipulation can alter the fact. 



The two bodies, however, have certainly become, by green manur- 

 ing, more available to plants than they were before; but the same 

 thing might have been effected by plowing and cultivating the land 

 without any crop, and even, though in a minor degree, by permitting it 

 to lie fallow for a season ; so that the practice could scarcely be either 

 wise or profitable, were it not for the power, formerly suspected and 

 now known to exist in certain plants, of fixing the elementary nitrogsn 

 of the air and converting it to their uses. This power the great ma- 

 jority of our cultivated plants lack. They need and depend upon nitro- 

 genous manures for their nitrogen supply, and require of the farmer 

 either watchful care in saving and utilizing all vegetable and animal 

 refuse of the farm, or the purchase of chemical fertilizers in open 

 market. 



We distinguish thus between two classes of plants : The nitrogen 

 consumers just now described, and the nitrogen producers, possessing 

 the economically most important property of being able to live upon 

 the nitrogen of the air ; they produce not nitrogen, not something that 

 did not exist before ; but they convert the non-serviceable free nitrogen 

 of the air into serviceable combined nitrogen of plant and soil. All 

 nitrogen producers so far discovered are leguminous plants, and it is 

 probable that every member of this family possesses this power. Peas, 

 beans, vetches, clover, serradella and lupines have in turn been the 

 objects of experimentation, and while much yet remains to be explained, 

 the following facts are definitely known : 



1. The power of leguminous plants to assimilate atmospheric 

 nitrogen depends upon their roots showing numerous, irregular, small 

 swellings called tubercles. 



2. The tubercles are caused by minute living organisms, existing 

 in great numbers in the soil. (They are different from the bacteria 

 spoken of before as the cause of the conversion of nitrogenous sub- 

 stances into nitrates.) 



3. Leguminous plants grow in a sterilized soil : that is, a soil in 

 which these organisms have been destroyed ; like all other plants, they 

 demand assimilable nitrogen as f. e. nitrates. 



4. A sterilized soil in which leguminous plants are grown by the 

 aid of nitrates, and in which they would perish if these were cat off, 

 may acquire the power of producing in them root tubercles by coming 

 in contact with a minute portion of earth possessing it, or with water, 

 even only a few drops, that had percolated it ; the plants then become 

 able to live without nitrates and upon the free nitrogen of the air. 



