The value of rotation cropping is claimed to be established in two directions - (1) avoiding soil exhaustion, (2) averting insect or fungoid attacks.
1. Given ordinary farm culture, it is likely enough that there is a certain advantage from rotation-cropping, for at all times the food supply is limited; but, given good garden culture, which is a very different thing, there is little or none, because the food supply is abundant. As much manure may be put on the farm as on the garden land, yet the former will not be so fertile as the latter. An attempt may be made to prove, by the ash of a plant, that it abstracts a certain ingredient from the soil, and that if another class of crop, which does not extract the same ingredient, is not put upon the soil in place of the first, exhaustion must follow. The answer to this is
(a) that the ash of a plant is no guide to its requirements; and (b) even if it were, the matter would be of trifling consequence, inasmuch as a thoroughly tilled soil can never be exhausted.
2. Change of ground is of far less value than is supposed in averting the attacks of insects and fungi. The majority approach the plant by means of the air, and not of the soil, and even in the case of the latter a change to a considerable distance is required to be of much use, and that is not possible in small vegetable gardens. The Carrot fly, the Celery fly, the Onion fly, the Cabbage butterfly - these and numerous other enemies come on the wing. Some pests, such as the club-root, certainly make their home in the soil, and attack the root, but shifting the crop which they attack to another part of a garden will not, as a rule, keep them away. Something else besides that is necessary, of which more later.
The small cultivator cannot, as I say, secure the perfect rotation which is possible to the man with many fields. The nearest that he can get is as follows: He can divide his ground into three sections, a half and two quarters. The half he can devote to Potatoes, the two quarters respectively to tap roots (Beet, Carrots, Parsnips, and others) and fibrous crops (Peas, Beans, Cabbages, Cauliflowers, Celery, and the rest). The second year he could plant his Potatoes on the two quarters, and transfer his tap and fibrous rooted plants to the half, thus securing a change.
I should like to add that in some of its aspects succession cropping is well worth studying. If ground is in first-rate mechanical condition it can be cropped up very closely and made to yield an enormous amount of produce. I herewith indicate a few examples.
1. A piece of ground, planted with early Potatoes and cleared of them in June, may be sown with an early variety of Pea for a late crop, and this in turn followed by a sowing of Turnips. When cleared of the Turnips it will have a short fallow, and be ready for Carrots and Parsnips, without manuring, the following spring.
2. A piece of ground under Onions, and cleared of them in August or September, may be planted with summer sown Cabbages in October. These, giving hearts in spring, may be cut and cleared off, to be succeeded by late sown Carrots, which, sown even as late as July, give delicious little roots. Or the Cabbages may be left to yield greens until November.
3. Ground under early Peas may be cleared of them in June, and planted with late Celery and Leeks.
4. Ground sown with autumn Onions, or planted with Shallots in late winter, may be cleared of them in June and sown with Rosette Colewort.
5. Ground sown with Turnips in February or March may be cleared of them by June, and Celery planted, to be followed by Peas.
Intercropping is frequently practised. The intercropping of Potatoes and winter Greens is a very common proceeding, but it often throws out elaborate schemes of rotations, as the Greens remain on the ground longer in spring than it is calculated that they will do. I am of opinion that the intercropping of Potatoes and Greens is best confined to dwarf varieties of the one and hardy kinds of the other. I do not approve of putting Broccoli between late Potatoes. The shade and coolness of the Potato rows, no doubt, help to give the Broccoli a good start, but it is very apt to become drawn, in which state it will not be in good condition for enduring the rigours of a hard winter.
Sowing Turnips and Spinach between Peas, and planting Lettuces on Celery ridges, are thoroughly legitimate examples of intercropping.