576 PROCEEDINGS OF SECTION G. 
the constituents of the soil. If we could learn, 
for instance, how to make soluble some of the 
insoluble silica in the soil, we might promote 
the formation of zeolitic substances, which are 
valuable for reasons of the same character as 
are the double humates. As some of the ingre- 
dients of soils are important, rather for the part 
they play in promoting chemical changes than 
because of any value they possess as plant-foods, 
it is possible that an exhaustive study of them 
may show us how best to make use of them, and 
that some soils may have their fertility per- 
manently increased by adding to their natural 
content of some of these substances, either by 
applying them as manures, or by giving the soil 
such treatment as leads to their production in it. 
We have seen that in taking in hand this task, we have a 
comparatively unworked field before us; that the soil is 
of most complicated constitution, and that its composition 
is constantly changing, mainly in the amount of vegetable 
matter it contains; that the physical conditions (and par- 
ticularly of temperature and moisture) to which it is exposed 
are also continually changing, and that in consequence chemi- 
cal changes of a most complicated and obscure character are 
taking place in it incessantly, and that it is to these changes 
that the release in small quantities of assimilable plant-foods 
from unavailable (potential) forms, which is constantly 
going on in a fertile soil, is due; also, that not only are we 
to a large extent ignorant of the nature of these changes, 
but unable even to tell what the combinations are in which 
the constituent elements of a soil exist in it. We have also 
seen that we have good grounds for thinking that we may 
increase the activity of the forces to which the release of 
available plant-foods is due, by attending to the tilth of the 
soil and its content of vegetable matter, and not improbably 
by adding to it suitable substances, or by special treatment 
which favours their production in it. With these data 
before us, it remains to consider how we ought to set to 
work in order to secure our end. 
As our knowledge of the changes in the soil which follow 
from any steps we may take in cultivating it, especially 
if in taking such steps we leave the well-trodden path of 
established practice, is so imperfect, it is plain that we are 
very far from being in a position to say what specific course 
of treatment will give us the results we are seeking. It is 
true, indeed, that our knowledge of the principles on which 
