1. It varies with the kind of plant. 100 lbs. of the ash of wheat, barley, and oats, for example, contain, respectively,
| Wheat. | Barley. | Oats. | |
| Potash, | 19 | 12 | 6 |
| Soda, | 20½ | 12 | 5 |
| Lime, | 8 | 4½ | 3 |
| Magnesia, | 8 | 8 | 2½ |
| Alumina, | 2 | 1 | ½ |
| Oxide of Iron, | 0 | trace. | 1½ |
| Silica, | 34 | 50 | 76½ |
| Sulphuric acid, | 4 | 2½ | 1½ |
| Phosphoric acid, | 3½ | 9 | 3 |
| Chlorine, | 1 | 1 | ½ |
| 100 | 100 | 100 |
A comparison of the several numbers opposite to each other in these three columns, shews how unlike the quantities of the different substances are, which are contained in an equal weight of the ash of these three varieties of grain. The ash of wheat contains 19 lbs. of potash in the 100 lbs., while that of oats contains only 6 lbs. In wheat are 20½ per cent. of soda, in oats only 5 per cent. Wheat also contains more sulphuric acid than either of the other grains, while barley contains a still greater predominance of phosphoric acid.
It is thus evident that a crop of wheat will carry off from the soil—even suppose the whole quantity of ash left by each the same in weight—very different quantities of potash, soda, &c. from a crop of oats. It will take more of these, of sulphuric acid, and of certain other substances, from the soil. It will, therefore, exhaust the soil more of these substances—as barley and oats will of others—hence one reason why a piece of land may suit one of these crops and not suit the others. That which cannot grow wheat may yet grow oats. Hence, also, two successive crops of different kinds of grain may grow where it would greatly injure the soil to take two in succession of the same kind, especially of either wheat or barley; and hence we likewise deduce one natural reason for a rotation of crops. The surface soil may be so far exhausted of one inorganic substance, that it cannot afford it in sufficient quantity during the present season to bring a given crop to healthy maturity, and yet may, by natural processes, be so far supplied again, during the intermediate growth of certain other crops, as to be prepared in a future season fully to supply all the wants of the same crop, and to yield a plentiful harvest.
2. The kind of inorganic matter varies with the part of the plant. Thus the grain and the straw of the corn plants contain very unlike quantities of the several inorganic constituents, as will appear by comparing the following with the preceding table:—
| Wheat Straw. | Barley Straw. | Oat Straw. | ||
| Potash, | ½ | 3½ | 15 | |
| Soda, | ¾ | 1 | trace. | |
| Lime, | 7 | 10½ | 2¾ | |
| Magnesia, | 1 | 1½ | ½ | |
| Alumina, | 2¾ | 3 | trace. | |
| Oxide of Iron, |  | 0 | ½ | trace. |
| Oxide of manganese, | ||||
| Silica, | 81 | 73½ | 80 | |
| Sulphuric acid, | 1 | 2 | 1½ | |
| Phosphoric acid, | 5 | 3 | ¼ | |
| Chlorine, | 1 | 1½ | trace. | |
| 100 | 100 | 100 |
Not only are the quantities of the several inorganic substances kinds of straw very unlike—especially the proportions of potash, lime, and phosphoric acid in each—but these quantities are also very different from those exhibited by the numbers in the preceding table as contained in the three varieties of grain. In this difference we see, further, one reason why the same soil which may be favourable to the growth of straw may not be equally propitious to the growth of the ear. Wheat straw contains little either of potash or of soda; the ash of the grain contains a large proportion; while the ash of the oat-straw, on the other hand, contains a much larger proportion of potash than that of its own ear does. It is clear, therefore, that the roots may, in certain plants and in certain soils, succeed in fully nourishing the straw while they cannot fully ripen the ear; or contrariwise, where they feed but a scanty straw, may yet be able to give ample sustenance to the filling ear.[8]
3. The quality of the ash varies also with the soil in which it grows. This will be understood from what is stated above. Where the soil is favourable, the roots can send up into the straw every thing which the healthy plant requires; when it is poorly supplied with some of those inorganic constituents which the plant desires, life may be prolonged, a stunted or unhealthy crop may be raised, in which the kind, and perhaps the quantity, of ash left in burning will necessarily be different from that left by the same species of plant grown under more favouring circumstances. Of this fact there can be no doubt, though the extent to which such variations may take place without absolutely killing the plant, has not yet been by any means made out.
4. It varies also with the period of a plant’s growth, or the season at which it is reaped. Thus, in the young leaf of the turnip and potato, a greater proportion of the inorganic matter they contain consists of potash than in the old leaf. The same is true of the stalk of wheat; and similar differences prevail in almost every kind of plant at different stages of its growth.
The enlightened agriculturist will perceive that all the facts above stated have a perceptible connection with the ordinary processes of practical agriculture, and tend to throw considerable light on some of the principles by which they ought to be regulated. One illustration of this is exhibited in the following section.