THE POSSIBILITIES OF ARTIFICIAL MANURES IN FORESTRY. 247 
neutralising and rendering harmless certain poisonous by-products 
produced by chemical changes in the interior of the plant. 
Each individual substance has a function to fulfil, and must not 
only be available to the plant, but it must be available in 
sufficient quantity, otherwise the development of the plant will 
suffer in proportion to the scarcity of any one of the essential 
elements. At the same time, the tree only requires a certain 
amount, and its total production is determined by those food 
materials which are present in minimum. This law of the 
minimum is the crucial point in all questions relating to the 
nutrition of plants; nor must we forget that the same law applies 
equally well to all factors which regulate the growth of trees, 
such as light, heat, and moisture. In order to satisfy this law of 
the minimum, it is not only essential that the requisite food 
materials should be present in the soil, but they must be present 
in a form in which they can be absorbed by the roots, hence we 
must know in which form these substances can be most easily 
made use of by the tree. This last point is a very important 
one, and is worthy of close attention by all growers of plants. 
The mere presence of any of these essential minerals in the soil 
is not sufficient, as it may occur, combined with other elements, 
in a form unavailable or even injurious to plant-growth, and 
herein lies the danger in relying too much upon a chemical 
analysis of the soil in determining its capability of sustaining 
different kinds of crops. We may liken the tree to a living 
machine with, it is true, an infinitely complex mechanism. 
Nevertheless, no matter how complex that mechanism may be, 
this living machine is incapable of doing any work unless certain 
external conditions are fulfilled. The external conditions, then, 
which cause the “machine” to work, in other words to grow, 
are, as already mentioned, a sufficient supply of light, heat, 
moisture, air, and mineral substances. These are the things 
which the tree requires in the formation of the combustible 
organic portions, living and non-living, of which its body is 
composed. This may all seem somewhat beside the point as 
regards the possibilities of artificial manures in forestry, but it is 
mentioned to emphasise and to direct attention to the fact that 
the raw food materials taken in by trees must undergo a process 
of building up into organic compounds or food within the plant, 
before it can be assimilated by the living substance. This all 
means a considerable expenditure on the part of the tree, and 
