This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This operation can only be carried out where there is a good depth of soil. Hence in hilly or mountainous districts, where only a few inches of soil rest on hard rock beneath, trenching and even double digging is often out of the question.
When trenching ground, the soil is marked out in strips about 3 ft. wide, and the first trench is taken out to a similar depth. Before the soil adjacent is thrown into the trench in front it is a good plan to place a good layer of weeds, green vegetable refuse, twigs, etc. - in fact all coarse and untidy vegetation at hand - in the bottom. The only refuse to avoid putting in the trenches is Potato haulms and the clubbed roots of cabbage crops. These should be always burned to destroy the spores of the terrible diseases which often afflict them. Having prepared the first trench in the way indicated, the next piece of ground is marked off 3 ft. wide, and the soil from this, spit by spit and layer by layer, is placed in the trench, until this is of course filled up, and a new trench is along-side. Where plenty of refuse and manure are available it will pay to place a layer between the spits, keeping the best and most rotted manure for placing beneath the top spit. A kind of sandwich of soil and manure will thus be formed, as shown in the diagram (fig. 86).
Fig. 86. - Diagram showing how ground may be trenched 3 ft. deep, bringing the bottom layer to the top to be fertilized by the weather, and to allow the free passage of air, rain, and roots downwards. Shaded portions indicate layers of manure. Note how trenched soil A is raised higher than the un-trenched B. C shows how soil has been dug out and placed at A. In B the figures 1 to 9 show how the soil is to fill the trench 0 in the same way as at A.
Very few, if any, commercial gardeners adopt this system of cultivation, partly because of the cost of labour and manure, and partly because they fear that it would be one of the greatest mistakes possible to bring up the subsoil from a depth of 3 ft., and place it on the surface, especially if it happens to be of a clayey, sticky nature, or of gravel. But it would be well to remember the words of Virgil:
"Well must the ground be digged and better dressed New soil to make, and meliorate the rest".
One can appreciate the argument against trenching on the score of expense; but if it is going to be done there will be no more, or very little more, expense or labour attached to bringing up the bottom spit and exposing it to all the fertilizing influences of the weather. As a rule this is the case. There is only one possible danger, and that is if the subsoil should contain a large proportion of ferrous oxide or protoxide of iron. This is distinctly unfavourable to plant growth, and is often met with in yellow clay soil; but, as already stated at p. 97, it would be foolish to choose a soil of this nature in the first place. This poisonous ferrous oxide must not be confused with ferric oxide or peroxide of iron, which is a valuable constituent of the soil. It promotes vegetation and the development of the green colouring matter in leaves, and performs other useful functions.
Even if one is so unfortunate as to have a soil containing much poisonous ferrous oxide, the best way to remedy this defect is by bringing up the bottom soil and exposing it to the action of the weather. One of the most important changes that takes place is the absorption of oxygen from the air by the ferrous oxide, which in the course of time becomes converted into the useful and fertilizing ferric oxide.
The cost of trenching soil to a depth of 3 ft. will vary from £8 to £12 per acre, an appalling item apparently to a man with limited capital; and then manuring, hoeing, etc, must be added, so that the cost of deep cultivation may well average £9 or £10 per annum per acre if vegetable crops only are to be grown. Against this great expense, however, must be placed the following advantages: (1) An abundance of available plant food; (2) earlier, heavier, and more remunerative crops; (3) abundance of warmth and moisture at the roots in the hottest of summers; (4) lack of insect pests and fungoid diseases; (5) saving in insecticides and fungicides; and (6) an absence of weedy vegetation and a consequent saving in plant food.
If it is intended to grow fruit trees and bushes, it would be even more wise to trench the soil to a good depth at first before planting, because once fruit trees are planted it will be afterwards almost impossible to rectify any troubles in the soil without incurring great expense. To bring soil into a proper condition for fruit culture, it may be advisable to crop it with Potatoes, Cabbage crops, Jerusalem Artichokes, Celery, Parsnips, etc, the roots of which would penetrate the soil deeply and break it into finer particles.
When digging, double digging, or trenching, it will be found convenient to divide the ground into convenient portions, as shown at a, b, c, d (fig. 87). By dividing each portion, as shown at ef. into two sections, a good deal of labour and wheeling will be saved. The soil from the first trench, b to f, when taken out, may be placed in front of the section efcd at fd When the work has reached ae, the trench there is to be filled with soil taken from e to c. The work then proceeds to fd, and the soil which has been wheeled there from bf is used to fill the last trench.
Fig. 87. - Diagram showing how the Ground may be marked out for Digging or Trenching.