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AGRICULTURAL CHEMISTRY. v7 
of oxide of copper; and soils formed from mica- 
ceous schist contain some metallic fluorides. 
Now, small quantities of these substances also 
are absorbed into plants, although we cannot 
affirm that they are necessary to them. 
De Saussure remarked that plants require un- 
equal quantities of the component parts of soils 
in different stages of their development; an ob- 
servation of much importance in considering the 
growth of plants. Thus, wheat yielded -,43, of 
ashes a-month before blossoming, ,249 while in 
blossom, and , 33 9 after the ripening of the seeds. 
| It is therefore evident that wheat, from the time 
of its flowering, restores a part of its organic 
constituents to the soil, although the phosphate 
of magnesia remains in the seeds. The fallow 
time is that period of culture during which land 
is exposed to a progressive disintegration by 
' means of the influence of the atmosphere, for the 
| purpose of rendering a certain quantity of alka- 
lies capable of being appropriated by plants. 
It 
is evident, that the careful tilling of fallow-land 
must increase and accelerate this disintegration. 
Now many plants in the family of the legumino- 
se are remarkable on account of the small quan- 
tity of alkalies or salts in general which they 
contain. They belong to those which are termed 
fallow-crops, and the cause wherefore they do not 
| exercise any injurious influence on corn which is 
cultivated immediately after them is, that they 
do not extract the alkalies of the soil, and only 
a very small quantity of phosphates. 
Two plants growing beside each other will 
mutually injure one another, if they withdraw 
the same food from the soil. Hence it is not 
surprising that the wild chamomile (Matricaria 
chamomilla) and Scotch broom (Spartium scopa- 
rium) impede the growth of corn, when it is con- 
sidered that both yield from 7 to 7°43 per cent. 
of ashes, which contain ,% of carbonate of pot- 
ash. Plants will, on the contrary, thrive beside 
each other, either when the substances necessary 
for their growth which they extract from the 
soil are of different kinds, or when they them- 
selves are not both in the same stages of develop- 
ment at the same time. 
On a soil, for example, which contains potash, 
both wheat and tobacco may be reared in suc- 
cession, because the latter plant does not require 
phosphates, salts which are invariably present in 
wheat, but requires only alkalies, and food con- 
taining nitrogen. According to the analysis of 
Posselt and Reimann, 10,000 parts of the leaves 
of the tobacco-plant contain 16 parts of phos- 
phate of lime, 8'8 parts of silica, and no magne- 
sia; whilst an equal quantity of wheat straw 
contains 47°3 parts, and the same quantity of the 
grain of wheat 99°45 parts. of phosphates. Now, 
if we suppose that the grain of wheat is equal to 
half the weight of its straw, then the quantity 
of phosphates extracted from a soil by the same 
weights of wheat and tobacco must be as 97°7: 
16. This difference is very considerable. The 
eee ree 
roots of tobacco, as well as those of wheat, ex- 
tract the phosphates contained in the soil, but 
they restore them again, because they are not 
essentially necessary to the development of the 
plant. 
Alternation of Crops.—Experience has shown 
that the same crop cultivated on the same soil 
through successive years, deteriorates and will 
finally cease to yield profitably; that certain 
plants will thrive better after others, and that 
these last will then again become productive. 
The experiments of Macaire-Princep prove 
that substances are excreted from the roots of 
plants, some of which he termed acrid and re- 
sinous, others mild like gum. The former he 
regarded as injurious, the latter nutritious. 
Hence the opinion that the same plant will not 
thrive in a soil where its excretions accumulate. 
Decandolle supposes that plants absorb soluble 
matter of every kind from the soil, and thus re- 
ceiving much matter unnecessary for nutrition, 
return it as excrement to the soil. 
The excretion consists of two parts, that which 
is returned to the soil in an unaltered state and 
that arising from transformations which have 
taken place within the plant. The former, al- 
though useless to a particular plant, may be 
nutritive to another. The latter appears to 
change into humus by a more or less gradual 
change, and then yielding carbonic acid, forms 
the nutriment of young plants. 
This artificial production of humus constitutes 
one advantage of the alternation of crops, and 
such plants are employed as excrete abundantly. 
Another advantage lies in the different kinds 
of inorganic matter required by different plants. 
Thus two plants requiring the same, and grown 
successively on the same ground, gradually ren- 
ders it incapable of producing them profitably ; 
but where one follows another requiring different 
inorganic constituents, the decomposing action 
of atmospheric agents during the lapse of time 
prepares the soil again for the production of the 
ATS 
Manure-—We may regard organic and many 
inorganic substances as manures; but we find 
them varying much in their value both practi- 
cally and by an analysis of their constituents. 
Thus the solid excrements of the cow and horse 
contain but little nitrogen, human feeces more; 
urine contains a large proportion. But the ex- 
crements of animals contain much silicate of 
potash and phosphates, human feces the latter, 
while urine is rich both in nitrogenous matter 
and phosphates. Too much cannot be said on the 
employment of human excrements both liquid 
and solid, for while they constitute fertilizing 
manure of the highest value, they are usually 
rejected in British agriculture. See Manure. 
We have presented an outline of the newer 
views in agricultural chemistry, chiefly due to 
Liebig, whose essay we have followed and freely 
extracted from, not, however, from a conviction 
