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.
Besides the overground stems and leaves of crops, one must not forget the roots. Although many crops are said to be cleared off the ground, the fact remains that a very large quantity of fibrous roots of all crops are left behind in the soil; and the more rudely the plants are taken up the larger the quantity of roots left behind. This may be easily seen by pulling up a cabbage or a lettuce by hand, and comparing the roots attached with those on similar plants that have been carefully lifted with a fork. As the roots decay they become humus and have all the fertilizing value of that organic material. It has been estimated that in an acre of grass land at Rothamsted there were over 4 1/2 tons (10,400 lb.) of roots in the soil at 9 in. deep; and these roots contained 78 lb. of nitrogen to the acre.
It will thus be seen that, even if stable manure and artificial fertilizers are excluded altogether, very large supplies of plant foods may still be secured from the waste leaves, stems, and roots of the crops themselves. It is therefore wise to take a leaf out of the book of the Continental, as well as the Chinese and Japanese cultivators, and avoid wasting the vegetable remains of any crop. If they are burned or thrown away, it is equivalent to wasting valuable supplies of nitrates, potash, phosphoric acid, lime, sulphur, soda, magnesia, and other plant foods, for which high prices will have to be paid.