| Soda. | Magnesia. |
|----------------------------------|------------|-----------|---------
|Silica { Oat straw with seeds | 34.00 | 4.00 | 62.00
|Plants. { Wheat straw | 22.50 | 7.20 | 61.50
| { Barley straw with seeds| 19.00 | 25.70 | 55.30
| { Rye straw | 18.65 | 16.52 | 63.89
| { Good hay | 6.00 | 34.00 | 60.00
|Lime { Tobacco | 24.34 | 67.44 | 8.30
|Plants { Pea straw | 27.82 | 63.74 | 7.81
| { Potato plant | 4.20 | 59.40 | 36.40
| { Meadow Clover | 39.20 | 56.00 | 4.90
|Potash { Maize straw | 72.45 | 6.50 | 18.00
|Plants. { Turnips | 81.60 | 18.40 | --
| { Beet root | 88.00 | 12.00 | --
| { Potatoes | 85.81 | 14.19 | --
| { Jerusalem Artichoke | 84.30 | 15.70 | --
----------------------------------------------------------------------

The special application of these facts must be reserved till we come to
treat of the rotation of crops.

It is manifest that, as the crops removed from the soil all contain a
greater or less amount of inorganic matters, they must be continually
undergoing diminution, and at length be completely exhausted unless
their quantity is maintained from some external source. In many cases
the supply of these substances is so large that ages may elapse before
this becomes apparent, but where the quantity is small, a system of
reckless cropping may reduce a soil to a state of absolute sterility. A
remarkable illustration of this fact is found in the virgin soils of
America, from which the early settlers reaped almost unheard-of crops,
but, by injudicious cultivation, they were soon exhausted and abandoned,
new tracts being brought in and cultivated only to be in their turn
abandoned. The knowledge of the composition of the ash of plants assists
us in ascertaining how this exhaustion may be avoided, and indicates the
mode in which such soils may be preserved in a fertile state.

FOOTNOTES:

[Footnote A: Apparently a species of Sinapis.]

[Footnote B: Oxide of Manganese, 0.42.]

[Footnote C: Oxide of Manganese, 0.92.]

[Footnote D: Alumina, 1.02.]

[Footnote E: Alumina, 0.63.]

[Footnote F: