which can be considered as properly the nutri- 
tious parts of plants. * * * If afield on 
which a rotation of cultivated plants has been 
carried on for a series of years contain an ordi- 
nary proportion of alkaline silicates, clay, lime, 
and magnesia, it will possess an almost inex- 
haustible provision of alkalies, alkaline earths, 
and silica; with this difference, however, that 
they are not all in a fit state to be used by the 
plant at the same time. By the mechanical 
operations of the farm, and by chemical means 
(by the use of lime, &c.), we may shorten the 
time in which these obtain a form fitted for the 
vital functions of the plant; but these matters 
do not suffice for its complete maturity. When 
phosphates and sulphates are absent from the 
soil, the plants growing on it cannot form seeds, 
because all seeds, without exception, contain 
compounds in which phosphoric acid and sul- 
phur are invariable constituents. Although all 
the other ingredients of plants be present in 
superabundance, the soil will become completely 
sterile, when the period arrives at which it can 
no longer furnish phosphates or sulphates to a 
new vegetation. We must suppose that, for the 
formation of the stem and herb, for the fixation 
of carbon, and for the production of sugar, 
starch, and woody fibre, a certain amount of 
alkalies (in the case of the potash-plants), or an 
equivalent of lime (in the case of the lime plants) 
is necessary. But we must bear in mind, at the 
same time, that the constituents of blood can be 
formed in the organism of the plant only in 
quantity corresponding to that of the phosphates, 
however abundantly ammonia or carbonic acid 
may be supplied. The production of the con- 
stituents of the juice containing sulphur and 
nitrogen is inseparably connected with the pre- 
sence of these salts. Every soil upon which a 
weed attains maturity is fitted for culture if that 
weed yields, on incineration, alkaline ashes. The 
alkalies of these ashes arise from silicates, so 
that in addition to the alkalies, soluble silica 
must exist in the same soil. Such a soil may 
contain a quantity of phosphates of lime and 
magnesia sufficient for potatoes and turnips, 
without on that account being rich enough for 
crops of wheat. These considerations must show 
the great importance which ought to be attached 
to phosphates in the practice of agriculture. 
These salts are present in the soil only in small 
quantity, and therefore the greater attention 
should be paid to prevent its exhaustion. 
* * * “Tt follows, then, from the preceding ob- 
servations, that the advantage of the alternate 
system of husbandry consists in the fact, that 
the cultivated plants abstract from the soil un- 
equal quantities of certain nutritious matters. 
A fertile soil must contain in sufficient quantity, 
and in a form adapted for assimilation, all the 
inorganic materials indispensable for the growth 
of plants. A field artificially prepared for cul- 
rm enter ti Sage Np hee, 
ROTATION OF CROPS. 
dients, and also of ammoniacal salts and decay- 
ing vegetable matter. The system of rotation 
adopted on such a field is, that a potash-plant 
(turnips or potatoes) is succeeded by a silica 
plant, and the latter is followed by a lime-plant. 
All these plants require phosphates and alkalies 
—the potash-plant requiring the largest quantity 
of the latter and the smallest quantity of the 
former. The silica plants require, in addition 
to the soluble silica left by the potash plants, a 
considerable amount of phosphates; and the 
succeeding lime plants (pease or clover) are capa- 
ble of exhausting the soil of this important in- 
gredient to such an extent, that there is only 
sufficient left to enable a crop of oats or of rye 
to form their seeds. The number of crops which 
may be obtained from the soil depends upon the 
quantity of the phosphates, of the alkalies, or of 
lime, and the salts of magnesia existing in it. 
The existing provision may suffice for two suc- 
cessive crops of a potash or of a lime plant, or 
for three or for more crops of a silica plant, or it 
may suffice for five or seven crops of all taken 
together; but after this time, all the mineral 
substances removed from the field in the form . 
of fruit, herbs, or straw, must again be returned 
to it; the equilibrium must be restored if we 
desire to retain the field in its original state of 
fertility.” 
The causes of the exhaustion of soil by plants, 
or the natural agencies which render a constant 
rotation of crops in farm cultivation essential to 
profitable management, are so obvious from the 
six general laws of vegetation which have been 
illustrated in the six preceding paragraphs, that 
they scarcely need to be mentioned. One is the 
natural dispersion, partly by the plants them- 
selves and mainly by birds and land animals who 
feed upon them, of some of the saline principles 
which all nutrimental plants more or less freely 
abstract from the soil. Another is the rapid re- 
moval from the farm, in the form of seeds and of 
the carcases of live stock, both of the scarcest and 
most valuable of the saline principles which are 
naturally contained in the soil, and of all the es- 
sential and most characteristic principles which 
are artificially supplied, from time to time, in the 
form of manure. A third is the removal from one 
field to another, or sometimes the removal from 
all the arable area of the farm, of such portions 
of the principles of nutrition as are abstracted 
by weeds. A fourth is the difference of range 
and depth in the cubic space which the roots of 
different plants occupy during the progress of 
their development and growth. A fifth is the 
diversity of inorganic food appropriated by dif- 
ferent groups of cultivated plants,—such that 
when a field has lost the materials for feeding 
one group, it still possesses those for feeding an- 
other, and when it has lost the materials for 
feeding a second, it still possesses those for feed- 
ingathird. A sixth is the very different amount » 
and composition and value of the residuary 
| ture, contains a certain amount of these ingre- L 
Beteliy yaad SRA LR TLE TTS TT OA OES SE a TSS 
