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ROTATION 
ents soluble in water. There are certain ashes 
of plants wholly soluble in water; others are 
only partially soluble; while certain kinds yield 
only traces of soluble ingredients. When the 
parts of the ashes insoluble in water are treated 
with an acid (muriatic acid), this residue, in the 
case of many plants, is quite soluble in the acids 
(as for instance the ashes of beet, turnips, and 
potatoes); with other plants, only half the re- 
sidue dissolves, the other half resisting the sol- 
vent action of the acid; while in the case of cer- 
tain plants only a third or even less of the re- 
sidue is taken up by the acid. The parts of the 
ashes soluble in cold water consist entirely of 
salts with alkaline bases (potash and soda). The 
ingredients soluble in acids are salts of lime and 
magnesia; and the residue insoluble in acids 
consists of silica. These ingredients being so 
different in their behaviour to water and to 
acids, afford us a means of classing the cultivated 
plants according to their unequal quantity of 
these constituents, Thus potash plants are those 
the ashes of which contain more than half their 
weight of soluble alkaline salts ; and we may de- 
signate as lime plants and as silica plants, those 
in which lime and silica respectively predomi- 
nate. The ingredients thus indicated are those 
which form the distinguishing characteristics of 
the plants which require an abundant supply of 
them for their growth. The potash plants in- 
clude the chenopodia, arrach, wormwood, &c. ; 
and amongst cultivated plants, the beet, mangel- 
wurzel, turnip, and maize. The lime plants 
comprehend the lichens (containing oxalate of 
lime), the cactus (containing crystallised tartrate 
of lime), clover, beans, pease, and tobacco. Silica 
plants include wheat, oats, rye, and barley. 4 
Salts of Salts of 
Potash Lime 
aid ata Silica, 
Soda. Magnesia. 
_( Oat-straw with seeds 34:00 4:00 62:00 
&= ) Wheat-straw : 22:00 7:20 61:05 
= ) Barley-straw with seeds 1900 25°70 55°03 
*( Rye-straw >. 18°65 16752 63°89 
Tobacco(Havannah) 24°34 67°44 8:30 
“© (Dutch) 23°07 62°23 15°25 
‘* (grown in an 
eg cra 29°00 59:00 12-00 
5 soil 
- | Pea-straw : ~ 27:82 63°74 7-81 
Potato-herb " 4°20 59°40 36°40 
Meadow-clover « 8920 56:00 4:90 
{ Maize-straw 71:00 6°50 18:00 
<4 | Turnips : . 81:60 18°40 
#5 4 Beetroot 5 : 88°00 12:00 
a | Potatoes (tubers)  . 85°81 14:19 
Helianthus tuberosus 84°30 15°70 
This classification, however, is obviously only a 
very general one, and permits division into a 
great number of subordinate classes; particu- 
larly with respect to those plants in which the 
alkalies may be replaced by lime and magnesia. 
The potato plant belongs to the lime plants, as 
far as regards the ingredients of its leaves; but 
its tubers (which contain only traces of lime) 
belong to the class of potash plants. With refer- 
OF CROPS. 
89 
ence to the silicious plants, this difference of 
their parts is very marked. Barley must be 
viewed as a lime plant, when compared with oats 
or with wheat, in reference to their ingredients 
soluble in muriatic acid; but it would be con- 
sidered as a silicious plant, if viewed only in re- 
ference to its amount of silica. Beet-root con- 
tains phosphate of magnesia, and only traces of 
lime; while the turnip contains phosphate of 
lime, and only traces of magnesia. When we 
take into consideration the quantity of ashes, 
and their known composition, we are enabled to 
calculate with ease, not only the particular in- 
gredients removed from a soil, but also the de- 
gree in which it is exhausted of these by certain 
species of plants belonging to the potash, lime, 
or silicious plants. This will be rendered ob- 
vious by the following examples. A soil, consist- 
ing of four Hessian acres, has removed from it 
by a crop of— 
Salts of lime, 
Salts of potash ; cis 
magnesia, and Silica. 
and soda. peroxide of iron. 
Ibs. Ibs. Ibs. lbs. lbs. 
Wheaten tare aeo0h 13051 Bo} ert 260-05 
Bease (astra, ae 198-42 T6638} 371-46 46°60 
Rye { ta eel 42-08} 82-78 2182} 57:82 139 77 
Beetroot, without leaves, 361:00 37 84 
Helianthus tuberosus 55600 104/00 
The same surface is deprived by these crops of 
the following quantity of phosphates :— 
: Helianthus 
Pease. Wheat. Rye. Tuberosus, Turnips. 
117: 11248 77-05 122 87-84 
* * * Ina soil rich in alkaline silicates, but con- 
taining only a limited supply of phosphates, the 
period of its exhaustion for these salts will be 
delayed if we alternate with the wheat plants 
which we cut before they have come to seed; or, 
what is the same thing, with plants that remove 
from the soil only a small quantity of phosphates. 
If we cultivate on this soil pease or beans, these 
plants will leave. after the removal of the crop, - 
a quantity of silica in a soluble state sufficient 
for a succeeding crop of wheat; but they will 
exhaust the soil of phosphates quite as much as 
wheat itself, because the seeds of both require 
for their maturity nearly an equal quantity of 
these salts. We are enabled to delay the period 
of exhaustion of a soil of phosphates by adopting 
a rotation, in which potatoes, tobacco, or clover, 
are made to alternate with a white crop. The 
seeds of the plants now named are small, and 
contain proportionally only minute quantities of 
phosphates ; their roots and leaves, also, do not 
require much of these salts for their maturity. 
But it must be remembered, at the same time, 
that each of these have rendered the soil poorer 
by a certain quantity of phosphates. By the ro- 
tation adopted, we have deferred the period of 
exhaustion, and have obtained in the crops a 
greater weight of sugar, starch, &c., but we have 
not acquired any larger quantity of the con- 
stituents of the blood, or of the only substances 
