One true Cause of a Crop’s failing, is want of a Quantity of Food to maintain the Quantity of Vegetables, which the Food should nourish.
When the Quantity of Food which is sufficient for another Species (that requires less), but not for that which last grew, to grow again the next Year, then that other is beneficial to be planted after it.
The Second true Cause is from the Constitution of Plants; some require more Food than others, and some are of a stronger Make, and better able to penetrate the Earth, and forage for themselves.
Therefore Oats may succeed a Crop of Wheat on strong Land, with once plowing, when Barley will not; because Barley is not so well able to penetrate as Oats, or Beans, or Pease, are.
So a Pear-tree may succeed a Plum-tree, when another Plum-tree cannot; because a Pear is a much stronger Tree, and grows to a much greater Bulk; so inclined to be a Giant, that ’tis hard to make it a Dwarf; and will penetrate and force its Way thro’ the untill’d Earth, where the other cannot; being of a weaker and less robust Constitution, not so well able to shift for itself.
The Pear could penetrate Pores, that the other could not. Mr. Evelyn says, in his Discourse of Forest-trees, ‘That a Pear will strike Root thro’ the roughest and most impenetrable Rocks and Clifts of Stone itself.’ He says likewise, in his Pomona, ‘That Pears will thrive where neither Apple or other Fruit could in Appearance be expected.’
I can scarce think, that a large Plant takes in larger Particles than a small one, for its Nourishment: If it did, I can’t believe, that the Thyme could have starv’d the Apricot-tree; it must have left the larger Particles of Food for that Tree, which probably would have sufficed to keep it alive: I rather think, that great and small Plants are sustain’d by the same minute Particles; for, as the fine Particles of Oats will nourish an Ox, so they will nourish a Tom-tit, or a Mite.
Some Plants are of an hotter Constitution, and have a quicker Digestion, like Cormorants or Pigeons, devouring more greedily, and a greater Quantity of Food, than those of a colder Temperature, of equal Bulk, whose Sap, having a more languid Motion, in proportion to the less Degree of Heat in it, sends off fewer Recrements; and therefore a less Supply of Food is required in their room. This may make some Difference in the one’s succeeding the other; because the hot-constitution’d leaves not enough for its own Species to succeed again, but leaves enough for a Species of a colder Constitution to succeed it.
But the Third and chiefest Cause of the Benefit of changing Sorts is Quantity of Tillage, in proportion to which the Food will be produced.
The true Cause why Wheat is not (especially on any strong Soil) to be sown immediately after Wheat, is, That the first Wheat standing almost a Year on the Ground, by which it must grow harder; and Wheat Seed-time being soon after Harvest in England, there is not Space of Time to till the Land so much as a second Crop of Wheat requires.