Chapter XVII: Section III: Of the Quality of the Ash of Plants

But much also depends upon the _quality_ as well as upon the _quantity_ of the ash. Plants may leave the same weight of ash when burned, and yet the nature of the two specimens of ash, the kind of matter of which they respectively consist, may be very different. The ash of one may contain much lime, of another much potash, of a third much soda, while in a fourth much silica may be present. Thus 100 lbs. of the ash of _bean_ straw contain 53½ lbs. of potash, while that of _barley_ straw contains only 3½ lbs. in the hundred; and, on the other hand, the same weight of the ash of the latter contains 73½ lbs. of silica, while in that of the former there are only 7½ lbs.

The quality of the ash seems to vary with the same conditions by which its quantity is affected. Thus—

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 Barley Oat
Straw. Straw. Straw.
Potash, ½ 3½ 15
Soda, ¾ 1 trace.
Lime, 7 10½ 2¾
Magnesia, 1 1½ ½
Alumina, 2¾ 3 trace.

Oxide of Iron, }
Oxide of manganese, } 0 ½ trace.

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]

[8] And occasionally _do_ give; for a plump grain, and even a well-filled ear, are not unfrequently found where the straw is unusually deficient.

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.