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.