1890.] On the Question of the Fixation of Free Nitrogen. 113 



with nitrate. Again, certainly the evidence of the experiments which 

 have been described, so far as it goes, seems to indicate a less develop- 

 ment of nodules when the soil contained an abundance of combined 

 nitrogen. If this indication should be confirmed, and the inference 

 be generally applicable, it would be concluded that the agency of the 

 symbiotic growth supposed, in fixing free nitrogen, will, other things 

 being equal, be the less the more the soil itself is in a condition to 

 supply an abundance of combined nitrogen ; whilst its capability in 

 this respect will depend not only on the richness in combined nitrogen 

 of the soil within the range of the roots, but on its state of combina- 

 tion, and on the character of the soil as to porosity and aeration. On 

 the other hand, the development of the supposed nitrogen-fixing 

 organisms obviously depends on the infection of the soil with the 

 organism essential to symbiotic life with the particular leguminous 

 crop to be grown. It would also seem that it is, at any rate in some 

 cases, dependent on the due porosity and aeration of the soil. 



Should these assumptions be borne out by the results of future in- 

 vestigation, we may conclude that the proportions in which any par- 

 ticular leguminous crop will derive its nitrogen from soil-supplies of 

 combined nitrogen on the one hand, and from fixation under the 

 influence of the symbiotic growth on the other, will be very different, 

 according to the characters of the soil, as to available supply of com- 

 bined nitrogen, mechanical condition, and due infection. We should 

 farther conclude that, in such cases as those in which poor sandy 

 soils will not grow fair crops of cereals, but will nevertheless yield 

 enormous crops of some leguminous plant — lupins, for example — the 

 leguminous crop will depend for a large proportion of its nitrogen on 

 fixation, under the influence of the symbiotic growth. Again, in such 

 cases as those of the growth of lucerne for many years in succession, 

 as in some Continental countries, it may be supposed that such fixa- 

 tion would be the source of a considerable proportion of the very large 

 amounts of nitrogen assimilated over a given area under such condi- 

 tions. 



In the case of beans, there is evidence that there is nodule-formation 

 when the plant is grown under ordinary conditions, in the garden or 

 in the field ; it has also been seen that nodules were formed on the 

 roots of the peas and the vetches experimentally grown in garden- 

 soil ; and the inference so far is, that wherever there is such formation, 

 there is more or less fixation. 



Then, as to the important case of the growth of red clover in our 

 rotations. There can be no doubt that red clover does avail itself of 

 soil supplies of combined nitrogen. On the other hand, the so-called 

 leguminous nodules have frequently been observed on the roots of red 

 clover growing in the field. Farther, although Hellriegel in his 

 earlier experiments did not get definite results with clover, he has 



