I have, by Experience and Observation, found it to be a Rule, That long Tap-rooted Plants, as Clover and St. Foin, will not succeed immediately after those of their own or any other Species of long Tap-roots, so well as after horizontal-rooted Plants; but, on the contrary, horizontal will succeed those Tap-roots as well or better than they will succeed horizontal.
I confess, this Observation did, for a great while, cheat me into the common Belief, That different Species of Plants feed on different Food; till I was delivered from that Error, by taking Notice, that those Tap-roots would thrive exceedingly well after Turneps, which have also pretty long Tap-roots, though Turneps never thrive well immediately after Clover[210], or St. Foin: I found the true Cause of this Exception to that Rule to be chiefly the different Tillage[211].
[210]But when Clover has been fed by Cattle, the Ground being good, and well tilled, Turneps may thrive immediately after Clover: Therefore this is an Exception to the general Rule.
[211]Very mellow rich Land is so full of vegetable Food, that ’tis an Exception to most Rules; and therefore I speak not of that.
Land must be well tilled for Turneps, which also are commonly hoed; they stand scarce ever above Three-quarters of a Year, and are then fed on the Ground; and then the succeeding Crop of Corn has, by that means, the Benefit of twice as much Tillage from the Hoe, as otherwise would be given to it; and the Broad Clover, or St. Foin, sown with the Corn (if the Corn be not so big as to kill it), will enjoy, in its Turn, a Proportion of the extraordinary Tillage, and of the Dung of Cattle, which feed the Turneps, and thrive accordingly: But Broad Clover and St. Foin, being perennial Plants, stand on the Ground so long, that it lies several Years untilled; so that Turneps, sown immediately after these, do fail, for want of their due Tillage, for which there is not sufficient time, by plowing often enough; because, by the common Ploughs, it requires Two or Three Years to make it fine enough for Turneps, or for a Repetition of Clover, or St. Foin, in strong or swerdy Land.
Another Reason why any Crop succeeds well after Turneps (and besides their being spent on the Ground where they grow) is their cold Constitution, by which they are maintained with less Food than another Plant of the same Bulk.
The Parenchyma, or fleshy Part of a Turnep, consisting of a watry Substance, which cools the Vessels, whereby the Sap’s Motion is very slow, in proportion to the very low Degree of Heat it has, and sends off its Recrements in the same Proportion likewise; and therefore requires the less of the terrene Nourishment to supply those Recrements.
This is seen, when a Bushel of Turneps, mixed with a Quantity of Wheaten Flour, is made into Bread, and well baked: This Bushel of Turneps gives but few Ounces Increase in Weight, more than the same Quantity of Wheaten Flour made into Bread, and baked without any Turneps. This shews there is in a Turnep very little Earth (which is the most permanent Substance of a Plant); the Oven discharges in Vapour near all but the largest Vessels: Its earthly Substance being so small, is a Proof ’tis maintained by a small Quantity of Earth: and, upon that Account also, of less Damage to the next Crop than another Plant would be, which required more of the solid Nourishment to constitute its firmer Body, as a Charlock does; for when a Charlock comes up, contiguous to, and at the same time with a Turnep, it does so rob the Turnep, that it attains not to be of the Weight of Five Ounces; when a single Turnep, having no more Scope of Ground, and, in all respects (but the Vicinity of the Charlock), equal, weighs Five Pounds, yet that Charlock does not weigh One Pound.
And where Three Turneps coming up, and growing thus contiguous, will weigh Four Pounds; a Charlock joined with Two or Three Turneps, all together, will be less than one Pound, upon no less Space of Ground.
This Observation cannot be made, except where Turneps are drilled in Rows; and there ’tis easy to demonstrate, that a Charlock, during the time of its short Life, draws much more Earth than a Turnep of equal Bulk, from an equal Quantity of Ground[212].