512 Daw soit. — On the Economic 
many experiments upon plants in soils inoculated with arti- 
ficial cultures of the organisms or with decoctions of soils 
in which the plant in question had been already cultivated. 
Such work as this led to the introduction by Nobbe and 
Hiltner of the substance ‘Nitragin/ i. e. pure cultures of the 
nodule-organisms derived from different Leguminous plants, 
on a commercial scale for use in practical agriculture. 
Although so much work has been done upon the relation 
existing between the presence or absence of Leguminous 
nodules and the proportion of nitrogenous material found in 
the culture-soils, but few workers have used the pure culture 
‘ Nitragin’ in exact experiments of this character, and so far 
as I am aware, in all cases, in which this material has been 
used, the conditions of the experiment have been very un- 
certain, owing chiefly to non-sterilized soils only being 
employed as culture media. Though not wishing to under- 
value such experiments, I may point out that with such very 
complex conditions, it is impossible to fix upon any one 
variable condition as the certain cause of any observed effect 
produced in the crops, and indeed recent work upon this 
subject has tended to prove more and more clearly that not 
only is this problem of modern agriculture one of special 
difficulty, but that we can only hope to come to an adequate 
explanation of the facts involved by bringing to bear upon 
them evidence derived from the study of many apparently 
side issues. 
Since ‘Nitragin’ was intended for use on a commercial 
scale and on any kind of soil, it seemed advisable that careful 
test experiments should be undertaken in a large number of 
different districts, and it was in order to furnish one such set 
of experiments that the work, to be briefly described in this 
paper, was undertaken during the summers of 1898-1900 in 
the Cambridge University Botanical Laboratory and Gardens. 
Throughout the work I had the privilege of the invaluable 
help and advice of Professor Marshall Ward, and I wish to 
take this opportunity of acknowledging my indebtedness to 
him. The results of some of this work have already been 
