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.