Causes of Fertility or Barrenness of Soils. 
205 
VI. — Necessity or otherwise for the presence of 
Vegetable Matter in Soils. 
That vegetable life is capable of existence in the absence of 
decayed humus, is proved by the growth of the lowest forms of 
vegetable life in the crevices of bare rocks. That the decay of 
these, by adding humus to the disintegrated rock, greatly assisted 
in tlie subsequent fertility of the soil, is beyond doubt, and we 
may conclude that soils would lose much of their fertility from 
the total absence of vegetable matter, supposing such a thing 
possible, which it is evident could not be, when we consider the 
origin and gradual formation of all soils. The dark colour of 
soils is in geneial due to the presence of vegetable and animal 
matter ; such soils are always more fertile than white, light red, 
or brown soils, consequently it is fair to presume that the fertility 
is due, at least in part, to the presence of vegetable matter. The 
advantages which follow heavy applications of vegetable manures, 
such as sea-weeds, &c., are a further confirmation of this. From 
the fact of vegetable matters existing in all soils, it was supposed 
in earlier days of scientific investigation that the amount of 
humus determined the relative value of soils — an erroneous con- 
clusion, as many of the most valuable soils only contain from 
2 to 3 per cent., while peats, containing from 80 to 90 per cent., 
are often quite barren. Humus is decomposed vegetable matter, 
and its nature and qualities depend upon the circumstances under 
which the decay was conducted : thus brown peat is usually the 
result of decay under water, whereas black peat has been formed 
by free oxidation, and is much more valuable or rather less acid 
and noxious in quality than brown peat. The researches of 
some foreign chemists, especially Mulder, have shown that the 
changes which vegetable matter undergoes in the presence of 
oxygen are numerous and peculiar, consisting in the formation of 
a succession of organic acids and the elimination of a portion 
of carbonic acid, until, if the process be carried out, the last of 
the series resolves itself into carbonic acid and water. Mulder 
believed that each of these acids, uniting with lime or alkalies, 
was suitable food for plants, being decomposed in the cells of the 
plant, thus affording unlimited supplies of carbonic acid and 
oxygen. Chemists, however, are now generally agreed that such 
views are erroneous, and confine the value of peat in the soil to 
its power of absorbing heat and ammonia, and supplying by 
gradual decomposition carbonic acid, and possibly small quan- 
tities of ammonia or nitric acid ; qualities which are undoubtedly 
of great importance, but which cannot render a soil fertile unless 
proper mineral matters are present. I believe it is not at all 
certain whether ammonia is produced during the changes which 
