So that the quantity of inorganic food required by different vegetables is greater or less according to their nature; and if a soil be of such a kind that it can yield only a small quantity of this inorganic food, then only those plants will grow well upon it which require the least. Hence, trees may often grow where arable crops fail to thrive, because many of them require and contain very little inorganic matter. Thus while 1000 lbs. of elm wood leave 19 lbs. and of poplar 20 lbs. of ash, the same weight of the willow leaves only 4½ lbs., of the beech 4 lbs., of the birch 3½ lbs., of different pines less than 3 lbs., and of the oak only 2 lbs. of ash when burned. 2. The quantity of inorganic matter varies in _different parts of the same plant_. Thus while 1000 lbs. of the turnip root sliced and dried in the air leave 70 lbs. of ash, the dried leaves give 130 lbs.; and while the grain of wheat yields only 12 lbs., wheat straw will yield 60 lbs. of earthy matter. So, though the willow and other _woods_ leave little ash, as above stated, yet the willow leaf leaves 82 lbs., the beech leaf 42 lbs., the birch 50 lbs., the different pine leaves 20 lbs. to 30 lbs., and the leaves of the elm as much as 120 lbs. of incombustible matter when burned in the air. Most of the inorganic matter, therefore, which is withdrawn from the soil in a crop of corn is returned to it again, by the skilful husbandman, in the fermented straw,—in the same way as nature, in causing the trees periodically to shed their leaves, returns with them to the soil a very large portion of the soluble inorganic substances which had been drawn from it by the roots during the season of growth. Thus an annual top-dressing is given to the land where forests grow; and that which the roots from spring to autumn are continually sucking up, and carefully collecting from considerable depths, winter strews again on the surface, so as, in the lapse of time, to form a soil which cannot fail to prove fertile,—because it is made up of those very materials of which the inorganic substance of former races of vegetables has been entirely composed. 2. The quantity of inorganic matter often differs in _different specimens of the same plant_. Thus, 1000 lbs. of wheat straw, grown at different places, gave to four different experimenters 43, 44, 35, and 155 lbs. of ash respectively. Wheat straw, therefore, does not always leave the same quantity of ash.