312 
REVIEW OF LEIBIG’s VEGETABLE PHYSIOLOGY. 
plants must have excrements. He divides the lat¬ 
ter into two classes; these, namely, which have 
been absorbed by the roots, but not being adapted 
for the nourishment of plants, are again returned 
to the soil ; and secondly, such substances as hav¬ 
ing been transformed in the vegetable organism by 
the process of nutrition, are the result of the form¬ 
ation of starch, woody fibre, gluten, &c. Excre- 
mentitious matter of the first class may serve as' 
food for other plants; nay, they may even be essen¬ 
tial for that purpose. Those of the second, how¬ 
ever, can not be used by other plants in the form¬ 
ation of woody fibre, &c., until changed into humus, 
and decomposed into ammonia, carbonic acid, &c. 
This theory, says Dr. Mohl, is not only destitute 
of all reasonable foundation, but is directly contra¬ 
dicted by the experience of rotation. There is no 
known evidence in proof of the existence of such 
excrementitious matter. It is true, Liebig says, 
that such must be the case, but then he adduces 
no proof except an ambiguous analogy with the 
animal kingdom, and forgetting, as he so often 
does, what he said—“ that analogy is the parent 
of that unfortunate comparison between vegetable 
and animal functions, which places both on the 
bed of Procrustes, and is the cause of all error.” 
“ There is not,” concludes Dr. Mohl, “ the least 
necessity for assuming a secretion from roots. If 
substances formed by vital processes are of no fur¬ 
ther use to a plant, they are excreted in the form 
of gas through the leaves, or deposited in the form 
of secretion in the glands and other organs, or 
thrown off' with decaying leaves.” This theory is 
moreover at variance with the experience of what 
takes place in the shifting of crops. According to 
Liebig’s views, the excrementitious matter of the 
second class above mentioned, would not only in¬ 
jure the plants whence it is derived, but could not 
be assimilated by any others before it is trans¬ 
formed into humus. But experience points quite 
another way, because the stubble of clover, lucerne, 
or saintfoin, which is unfit for the growth of those 
species, will at once produce excellent crops of 
other plants. If Liebig should attempt to meet 
this objection by saying that such excrementitious 
matter can not be assimilated by the plants whence 
they are derived, but may be used by others, he 
will upset his whole doctrine of vegetable nutri¬ 
tion, according to which not only all the organic 
compounds which remain behind after the forma¬ 
tion of starch, sugar, &c.,but even starch and sugar 
themselves, (and thus all the organic substances, 
of plants,) are absolutely deleterious to other plants. 
It is impossible, therefore, not to arrive at conclu¬ 
sions entirely opposite to those of Liebig, especially 
if we consider the phenomena of rotation at greater 
length. The barrenness of. soil for the growth 
of one kind of plant, while it is still fertile for 
others, can only depend (says Dr. Mohl) on two 
causes. The first generation of plants may ex¬ 
haust the soil of such substances as are indispen¬ 
sable to growth, so that the second generation will 
be starved-—and this certainly takes place ; but it 
can not be the main cause of the failure of crops, 
else manure would again render the soil suitable 
for the same crop, which is only the case to a 
slight extent. We must, therefore, assume that 
the first crops do communicate to the soil sub¬ 
stances detrimental to the subsequent crops. 
These substances must be of an organic nature. 
It has been shown that these can not be excre¬ 
mentitious, and therefore it follows, that such de¬ 
leterious substances must consist of organic com¬ 
pounds, derived from the roots which have accu¬ 
mulated and remained behind in the land. If, 
then, in a soil filled with the remains of roots, the 
same crop will only succeed after a lapse of years, 
while other crops will thrive luxuriantly, we may 
conclude, that the organic compounds of such roots 
will be absorbed by plants previous to their being 
decomposed into inorganic substances ; and that, 
consequently, plants of a different kind will use 
them for food, although those of the same kind 
will be injured by them. 
After having assigned the utility of rotation to 
the formation of humus, Dr. Liebig states his views 
of vegetable nutrition at the different periods of 
growth. He says, that a plant returns just so 
much carbon to the soil, as it has absorbed from 
it in the form of carbonic acid, produced by de¬ 
composing humus. This supply of carbon is suf¬ 
ficient for many plants at the first period of their 
growth, but it is not sufficient to supply some of 
their organs with the necessary maximum of food. 
But the object of agriculture is to gain the maxi¬ 
mum of produce, and this, says Liebig, “ stands 
in a direct ratio to the amount of food which has 
been given to a plant during the first period of its 
development,” therefore all pains are to be taken 
to increase the amount of humus. 
The short and the long of these rather vague as¬ 
sertions (says Dr. Mohl) is, apparently, that a crop 
will be the greater the more food a plant has re¬ 
ceived from the soil before its period of flowering. 
But this axiom, although true in the main, is some¬ 
what contradicted by another, where it is stated 
that humus is useful to young plants by contribu¬ 
ting to the increase of their organs of atmospheric 
nutrition; but it is not indispensable, and its ex¬ 
cess may even be detrimental in the first stages of 
development. The food, namely, which a young 
plant receives from the air in the form of carbonic 
acid and ammonia, is restricted within certain 
limits—it can assimilate no more than the air con¬ 
tains. If, therefore, in the beginning of growth, 
the number of twigs, sprouts, or leaves, overpass 
this proportion in consequence of a superabundance 
of food obtained from the soil at that period, when 
the plant requires more food from the air for the 
completion of its development, and for its flower¬ 
ing and fruiting than the air can supply it with, 
blooming and fruiting will not take place. In 
many cases, such food will merely suffice for the 
development of leaves, stems, or branches. 
Here Dr. Mohl complains of the strange am¬ 
biguity of this part of Liebig*s theory. In one in¬ 
stance (says Mohl) the usual quantity of humus in 
the soil suffices merely to form leaves, and if we 
want an abundant harvest, we must get it by con¬ 
veying a maximum of food from the soil. On the 
other hand, humus adds nothing to the crops, but, 
on the contrary, is noxious, by conveying too much 
food, for it causes the production of too much 
foliage, a sufficient supply of food for which can 
