1st. _Plants have different systems of roots, stems and leaves, and adapt themselves accordingly to different kinds of soils:_ the Tussilago prefers clay, the Spergula sand; Asparagus will not flourish on a bed of granite nor Musus Islandicus on one of alluvion. It is obvious, that _fibrous rooted_ plants, which occupy only the surface of the earth, can subsist on comparatively stiff and compact soils in which those of the leguminous and cruciform families would perish, from inability to penetrate and divide.
2d. _Plans of the same, or of a similar kind, do not follow each other advantageously in the same soil._ Every careful observer must have seen how grasses alternate in meadows or pastures, where nature is left to herself. At one time, timothy, at another clover, at a third red-top, and at a fourth blue grass prevails. The same remark applies to forest trees; the original growth of wood, is rarely succeeded by a second of the same kind; pine is followed by oak, oak by chesnut, chesnut by hickory. A young apple tree will not live in a place where an old one has died; even the pear tree does not thrive in succession to an apple tree, but stone fruit will follow either with advantage. "In the Gautinois (says Bosc,) saffron is not resumed but after a lapse of twenty years; and in the Netherlands, flax and colzat require an interval of six years. Peas, when they follow beans, give a lighter crop than when they succeed plants of another family."[4]
[4] The ill effect of a succession of crops of the same kind was not unknown to the Romans. We have proof of this in the following passage of Festus: "Resistilibus ager fit qui continuo biennio seseritur farreo spico id est aristato, quad ne fiat _solent, qui pradia locant, excipere_."
3d. _Vegetables, whether of the name family or not, having a similar structure of roots, should not succeed each other._ It has been observed, that trees suffer considerably by the neighbourhood of sainfoin and lucern, on account of the great depth to which the roots of these plants penetrate--whereas culmiferous grasses do them no harm.
4th. _Annual or biennial trefoils, prevent the escape of moisture by evaporation, or filtration, from sandy and arid soils_, and should constantly cover them in the absence of other plants;[5] while _drying and dividing crops_, as beans, cabbages, chickory, &c. &c. _are best fitted to correct the faults of stiff and wet clays_.