351 
% 
It has been contended, that the benefits derived by a rotation of crops 
was owing to root excretions, that plants exuded from their roots certain 
matters which were injurious to plants of the same hind, but beneficial 
or useful to others ; this notion is now considered to be untenable. All 
our crops are liable to be attacked by animal or vegetable parasites, each 
crop having its peculiar parasites ; and it is probable that the growth 
and increase of these pests will be favored to some extent by frequently 
growing the same kind of crop on the same soil; and a given crop, when 
frequently repeated, may be more liable to disease and less productive in 
consequence. 
C ft* / 
But the chief benefit derived from a judicious rotation of crops is 
owing to the plants which succeed each other requiring different kinds 
or proportions of mineral matters for their growth. The following table 
of Prof. Solly, contains a list of the inorganic substances usually found 
in plants, and exhibits the relative proportion of each contained in the 
ashes of several kinds of plants commonly cultivated by the farmer: 
+1 
d 
cy 
\ 
o 
■4^ 
c3 
<D 
ra 
o 
r T H 
5 
0> 
M 
*3 
£ 
o 
P 
M 
cz 
«5 
P 
C3 
<D 
,6 
aJ 
O 
&J0 
C3 
x> 
d 
« 
« 
Ph 
W 
Ph 
O 
1 Potash. 
Soda. 
20 
29 
ISO 
48 
189 
4 
332 
62 
j 510 
1656 
50 
138 
0? 
2370 
1154 
Lime.. 
240 
554 
652 
704 
230 
624 
2928 
1747 
Magnesia. 
32 
76 
236 
1292 
480 
209 
488 
22 
Alumina. 
90 
146 
6 
26 
2 
10 
52 
17 
Silica.. 
2870 
3856 
2708 
140 
20 
220 
801 
210 
Oxide of Iron... 
14 
4 
15 
10 
7 
58 
8 
Oxide of Manganese.... 
20 
20 
32 
44 
Sulphuric Acid. 
37 
118 
106 
217 
66 
34 
245 
959 
Phosphoric Acid. 
170 
160 
54 
288 
118 
226 
32 
785 
Chlorine. 
30 
70 
6 
95 
20 
80 
0? 
274 
100,000 parts dry yield ? 
of Ashes.) 
3518 
5242 
3985 
3203 
1456 
3116 
4786 
7546 
By this table we learn that silica is an important constituent of wheat, 
barley and corn ; that buckwheat requires large quantities of magnesia ; 
the potato lime ; the cabbage pot-ash, besides sulphuric and phosphoric 
acids and chlorine. Now none of these substances exist in the generality 
of soils in any considerable quantity excepting alumina, silica and oxide 
of iron. If this be so, it is easy to understand, by studying this table. 
