THE POSSIBILITIES OF ARTIFICIAL MANURES IN FORESTRY. 253 
manures in agriculture warrants its application in the cultivation 
of trees, and that in the afforestation of heaths and sand-dunes 
the desired results are more quickly obtained by such means. 
Past results justify the opinion. In fact, it is said that in this 
connection Belgium is already past the experimental stage, and 
that the results already obtained indicate future procedure. The 
State aid given for this purpose has been thoroughly justified by 
the results achieved, especially by communities with large tracts 
of unfertile waste land. Their success has further stimulated 
private owners to follow their good example. 
It may not be out of place to recall a few facts regarding 
the physical and chemical properties of the soil. The soil on 
which the higher plants vegetate is produced by the disintegra- 
tion of rocks of different kinds. Rain, frost, the carbonic acid 
gas of the air and general weathering, together with the action 
of lower forms of vegetation, as lichens and mosses, bring about 
this disintegration. The soil therefore consists of small, solid 
particles formed by the breaking down of various kinds of rock 
in which the decaying remains of vegetable substances occur 
as humus. Each little particle of soil is surrounded by a film 
of water called the hygroscopic film, and the root-hairs of the 
higher plants come closely in contact, and even fuse with these 
solid particles, so that they cannot be detached without 
injuring the hair. The hygroscopic water, the carbon-dioxide 
of the soil, and the vital action of the root-hairs, tend to bring 
these solid mineral substances into solution, and it is only in 
this form that they can be taken in by the tree, since they have 
to pass through the membrane of the root-hair before gaining 
the interior. 
Between the solid soil particles are spaces filled with air. 
This air is necessary not only for the roots, but also for the 
other living things which inhabit the soil, especially certain very 
important micro-organisms, which will be referred to presently. 
Any excess or loose water in the soil occupies these spaces and 
drives out the air, nor is this loose water of any use to the 
plant. The water which is absorbed is the hygroscopic water, 
or that influenced by the force of adhesion to the soil particles. 
Having opened up the subject so far, let us next turn our 
attention to the property which the soil possesses of absorbing 
and retaining certain solutions, as this is a point of great 
importance, especially as regards the application of the various 
