Hence the practice of green manuring has been in use from very early periods. The second or third crop of _lucerne_ was ploughed in by the ancient Romans—as it still is by the modern Italians. In Tuscany, the _white lupin_ is ploughed in, in preference—in Germany, _borage_. In French Flanders, two crops of _clover_ are cut, and the third is ploughed in. In Sussex, _turnip_ seed has been sown at the end of harvest, and after two months again ploughed in, with great benefit to the land. Turnip leaves and potato tops decay more readily, and more perfectly, and are more enriching when buried in the green state. It is a prudent economy, therefore, where circumstances admit of it, to bury the potato tops on the spot from which the potatoes are raised. Since the time of the Romans, it has been the custom to bury the cuttings of the vine stocks at the roots of the vines themselves; and many vineyards flourish for a succession of years without any other manuring. Buckwheat, winter tares, clover, and rape, are all occasionally sown for the purpose of being ploughed in. This should be done _when the flower has just begun to open_, and if possible at a season when the warmth of the air and the dryness of the soil are such as to facilitate decomposition. That the soil should become richer in vegetable matter by this burial of a crop than it was before the seed of that crop was sown, and should also be otherwise benefitted, will be understood by recollecting (see page 42) that perhaps three-fourths of the whole organic matter we bury has been derived from the air—that by this process of ploughing in, the vegetable matter is more equally diffused through the whole soil than it could ever be by any merely mechanical means;—and that by the natural decay of this vegetable matter, ammonia and nitric acid are, to a greater extent (pages 33 and 34), produced in the soil, and its agricultural capabilities in consequence materially increased. These considerations, while they explain the effect and illustrate the value of green manuring, will also satisfy the intelligent agriculturist that there are methods of improving his land without the aid either of town or of foreign manures—and that he overlooks an important natural means of wealth who neglects the green sods and crops of weeds that flourish by his hedgerows and ditches. Left to themselves, they ripen their seeds and sow them annually in his fields—collected in compost heaps they would materially add to his yearly crops of corn.