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.
The subject of green manuring may be carried further than is generally supposed. There is scarcely a crop grown, whether fruits, flowers, or vegetables, that cannot be utilized in part as a green manure. Even the weeds and herbage from the banks and waysides can be turned to good account as soil fertilizers, and if utilized will not only pay as a green manure but will also remove one of the chief nesting places for many garden pests.
Taking cultivated crops, the leaves of many of them drop to the ground in autumn and when decayed form an excellent vegetable mould or leaf soil, the value of which is well known to all gardeners who cultivate pot plants of any description. But the leaves and stems of such crops as Potatoes, Carrots, Parsnips, Turnips, Beet, Mangel, Peas, Beans, Jerusalem Artichokes, and the stems of many Cabbage crops, etc, are often available as vegetable refuse, and may be utilized to improve the soil. The quantities of leaves and stems vary according to the different crops, but the following is a fairly approximate estimate per acre of some. Beet, 15 tons; Cabbage crops, 7 tons; Jerusalem Artichokes, 13 tons; Turnips, 12 tons; Potatoes, 6 tons; Parsnips, 10 tons; Apples, Pears, and Plums, 4 tons.
Vegetable refuse of this description, as well as the clippings of hedges, the dead stems and leaves from flower borders, etc, makes an excellent fertilizing material for the soil. It may be utilized in a green or raw state whenever the ground is being trenched, or in a decomposed state as a compost when digging or ploughing. Many market gardeners and farmers are well aware of the value of this material and take advantage of it.
The only danger to be apprehended is in the case of Potato stalks and clubrooted Cabbages. These contain terrible fungoid diseases, and it is generally safer to have them burned than dug into the soil. Although burning will drive off all the organic foods, the ashes left behind will contain valuable fertilizing salts that may be dug in afterwards.