344 Canadian Record of Science. 
required for a particular crop, but contains an abundance 
of other elements required by other crops. If these latter 
are now planted, the soil, in course of time, suffers special 
exhaustion with reference to their requirements, while it 
regains its ability to produce the crop of the first kind. 
Thus, by a judicious system of rotation, land may be kept 
in a constant state of productiveness. It is only when food 
elements are so completely withdrawn that no one class of 
plants can be brought to perfection, that the soil is said to 
be generally exhausted. Therefore, when we speak of the 
fertility of a soil, or the exhausted condition of a soil, it 
must always be with direct reference to the particular 
requirements of the plants we wish to cultivate. And I 
cannot let this part of my subject pass without pointing 
out that a large part of the difficulty in successfully com- 
bating some of the most destructive diseases of the orchard 
and garden, arises from a failure to properly appreciate 
and apply the principles stated. 
It is impossible to give more detailed consideration to 
these aspects of our subject in the brief space allotted to us, 
important though they are. There are, nevertheless, two 
features of this question to which I would particularly draw 
your attention, and from their very important bearing upon 
the economic side of horticulture, I feel that their somewhat 
detailed statement will not be out of place. I refer, in the 
first place, to the relation of nutrition to conditions of 
health and disease ; and in the second place, to the relation 
of nutrition to improved qualities of fruits. 
For many years, the Germans have been among the fore- 
most investigators in efforts to determine the special 
functional value of the various food elements of plants. The 
method usually selected has been that of water culture 
already described, through the medium of which the effect of 
eliminating any given element, or of varying its proportion 
and particular chemical combination in the food supply, 
could be accurately ascertained. From a series of such 
experiments made as long ago as 1871, in which buckwheat 
was the particular plant employed, it was observed that in 
