FIXATION OF FREE NITROGEN. 163 



example, grows luxuriantly on ordinary arable soil, common 

 experience can leave but little doubt. The evidence of fixa- 

 tion is, however, undoubtedly much the clearer in the case of 

 soils poor in nitrogen. Thus, in the cases of the experiments 

 with peas, vetches, and yellow lupins, growing in nitrogen- 

 free but duly infected sand, there being no other supply of 

 combined nitrogen excepting that in the seed sown, the pro- 

 portion of the total assimilation due to fixation was undoubt- 

 edly very large. It may safely be concluded, indeed, that 

 when luxuriant leguminous crops are obtained on soils 

 characteristically poor in available combined nitrogen, a large 

 proportion of the total nitrogen assimilated will be due to 

 fixation. It is, on the other hand, by no means so clear that Abundant 

 when such plants are grown in soil rich in available combined 9 ro J° th °f 



Till pit- i.li nodules not 



nitrogen, an abundant development ot nodules is to be taken always in- 

 as indicating that a correspondingly great proportion of the dicatwe of 

 total nitrogen assimilated is due to fixation. of nitrogen. 



There can, however, be little doubt that in the growth in 

 practical agriculture of leguminous crops, such as clover, 

 vetches, peas, beans, sainfoin, lucerne, &c, at any rate some, 

 and in some cases a considerable proportion, of the large 

 amount of nitrogen which they contain, and of the large 

 amount which they frequently leave as nitrogenous residue 

 in the soil for future crops, is due to the fixation of free 

 nitrogen, brought into combination by the agency of lower 

 organisms. Evidence is, however, obviously still wanting, to 

 enable us to judge decisively under what conditions a greater 

 or less proportion of the total nitrogen of the crop will be 

 derived — on the one hand from nitrogen-compounds within 

 the soil, and on the other from fixation. 



Incidentally the question suggests itself, How far the Causes of 

 failure of red clover, or of other leguminous crops, may be c ^ s er ' slck ~ 

 due to the exhaustion of the organisms necessary for nodule- 

 development, and for the coincident fixation of free nitrogen ; 

 how far to the exhaustion of combined nitrogen, or of the 

 necessary mineral constituents, in an available condition, 

 within the range of the roots ; or, as is sometimes the case, 

 to insect ravages due to the condition of the soil indepen- 

 dently of an otherwise failing condition of the plant ? 



Assuming it then to be established that a greater or less, Sources of 

 and sometimes a considerable proportion, of the nitrogen of ^f^-^. 

 our leguminous crops will be due to fixation under the con- 

 ditions supposed, it is obvious that such a fact not only serves 

 to explaiu the source of the hitherto unaccounted for amount 

 of the nitrogen of those crops themselves, but that it also affords 

 an explanation of the source of the increased amount of ni- 

 trogen which other crops acquire when they are grown either 



