The Smirces of the Nitrogen of otir Leguminous Crops. 699 
It is to be supposed that when nodules develop abundantly 
I on the roots of leguminous plants growing in soil rich in readily- 
available combined nitrogen, the nitrogen assimilated will be 
partly due to soil supplies of combined nitrogen, and partly to 
fixation. That there is gain when red clover, for example, 
grows luxuriantly on ordinary arable soil, common experience 
' can leave but little doubt. The evidence of fixation is, however, 
' undoubtedly much the clearer in the case of soils poor in nitro- 
■ gen. 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 than that 
I in the seed sown, the proportion of the total assimilation due to 
' fixation was undoubtedly very large. It may safely be concluded, 
I indeed, that when luxuriant leguminous crops are obtained on 
I 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, 
I when such plants are grown in soil rich in available combined 
I nitrogen, an abundant development of nodules is to be taken as 
( indicating that a correspondingly great proportion of the total 
I nitrogen assimilated is due to fixation. 
There can, however, be little doubt that in the growth in 
I 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 
failure of red clover, or of other leguminous crops, may be 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 neces- 
sary mineral constituents, in an available condition, within the 
range of the roots; or, as is sometimes the case, to insect 
ravages depending on the condition of the soil independently of 
an otherwise failing condition of the plant. 
Assuming it then to be established that a greater or less, 
and sometimes a considerable proportion, of the nitrogen of our 
