THE MAIN TYPES OF SOIL 67 



Although the two subjects are not really 

 connected, one's mind naturally turns, on 

 thinking of nitrification, to the peculiar 

 relationship of leguminous plants to the free 

 nitrogen of the air. It is many years since 

 observers suspected that the nutrition of 

 leguminous plants differed in some respect 

 from that of plants belonging to other 

 natural orders, but it was only some thirty 

 years ago that exact experiment proved that 

 this class of plant could utilize for their 

 nutrition the free nitrogen of the atmosphere. 

 As is well known, the atmosphere contains 

 some seventy-nine per cent, of uncombined 

 nitrogen, and it is sometimes said that the 

 main function of this nitrogen is to act a& 

 a diluent of the oxygen which constitutes 

 the bulk of the remainder of the atmosphere. 

 For most plants and all animals, so far as 

 we know, this atmospheric nitrogen is of na 

 direct account, but an exception to this 

 rule is furnished by the Leguminosse, and a 

 few other less-important orders, which, 

 through the agency of colonies of bacteria 

 that establish themselves in outgrowths 

 (nodules) on the roots, are able to draw 

 upon the supplies of free atmospheric 

 nitrogen. In some way or another, these 

 organisms can evidently lay hold of, and 

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