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.
Whether the soil is to be ploughed, dug, or trenched in spring or autumn will depend largely upon its nature. Generally speaking it is better to turn up heavy soils in the autumn and light soils in the spring. As heavy soils contain a larger amount of dormant food than light soils, and as it takes longer to transform them into a soluble condition, it is better to have them ploughed, dug, or trenched during the autumn and winter months. Fresh surfaces are thus exposed to the action of the weather; clods are crumbled down into powdery masses by the action of frost, rain, snow, wind, &c; the air enters more freely between the particles, and sourness and acidity are driven out by the sweetening action of the atmospheric oxygen. The soil thus becomes "sweeter"; it also becomes warmer, because better drained, and owing to the action of the carbonic acid in the air, and arising from decomposing manure, supplies of potash, phosphoric acid, nitrates, and other valuable foods become available by spring. At this period also the over-harsh clods are easily broken down by the rake or harrow, and can be rendered sufficiently fine for the reception of seeds of various crops.
If "light" land is ploughed, dug, or trenched in autumn precisely the same beneficial results would follow, but much more quickly. This would be a distinct disadvantage to the farmer and gardener at this season. Having no growing crops on the land to take up the freshly liberated food, there is a danger that this would be washed down into the lower layers out of reach of the roots. Thus, when sowing and planting time arrived in spring, although the soil would be easily worked, it is possible that the upper layer would be much poorer in plant food than it was before the autumn breaking up.
The cultivator therefore must always pay attention to the physical or mechanical condition of his soil rather than to its chemical composition, and he must regulate his operations accordingly. It may be stated as a good general rule, that whenever a crop is ready to be placed on the soil, it is a good plan to have it dug in advance whether it be spring, summer, or autumn.