Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

There are two purposes which vegetable manure is generally supposed to serve when added to the soil. It loosens the land, opens its pores, and makes it lighter; and it also serves to supply organic food to the roots of the growing plant. It serves, however, a third purpose: it yields to the roots those saline and earthy matters which it is their duty to find in the soil, and which exist in decaying plants in a state more peculiarly fitted to enter readily into the circulating system of new races.

Decayed vegetable matters, therefore, are in reality mixed manures, and their value in enriching the land must vary considerably with the kind of plants and with the parts of those plants of which they are chiefly made up. This depends upon the remarkable difference which exists in the quantity and kind of inorganic matter present in different vegetable substances, as indicated by the ash they leave ([see pages 52 to 62]). Thus if 1000 lbs. of the saw-dust of the willow be fermented, and added to the soil, they will enrich it by the addition of only 4½ lb. of saline and earthy matter, while 1000 lbs. of the dry leaves of the same tree fermented, and laid on, will add 82 lbs. of inorganic matter. Thus, independent of the effect of the vegetable matter in each, the one will produce a very much greater effect upon the soil than the other.[17]

There are three states in which vegetable matter is collected by the husbandman for the purpose of being applied to the land—the green state; the dry state; and that state of imperfect decay in which it forms peat.

1. Green Manuring.—When grass is mown in the field, and laid in heaps, it speedily heats, ferments, and rots. But, if turned over frequently and dried into hay, it may be kept for a great length of time without undergoing any material alteration. The same is true of all other vegetable substances—they all rot more readily in the green state. The reason of this is, that the sap or juice of the green plant begins very soon to ferment in the interior of the stem and leaves, and speedily communicates the same condition to the moist fibre of the plant itself. When once it has been dried, the vegetable matter of the sap loses this easy tendency to decay, and thus admits of long preservation.

The same rapid decay of green vegetable matter takes place when it is buried in the soil. Thus the cleanings and scourings of the ditches and hedge-sides form a compost of mixed earth and fresh vegetable matter, which soon becomes capable of enriching the ground. When a green crop is ploughed into a field, the whole of its surface is converted into such a compost—the vegetable matter in a short time decays into a light, black mould, and enriches in a remarkable degree and fertilizes the soil.

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.

Sea-weeds.—Among green manures, the use of fresh sea-ware deserves especial mention, from the remarkably fertilizing properties it is known to possess, as well as from the great extent to which it is employed on all our coasts. The produce of the isle of Thanet in Kent is said to have been doubled or tripled by the use of this manure; the farms on the Lothian coasts are said to be let for 20s. or 30s. more rent when they have a right of way to the sea, where the weed is thrown on shore; and in the Western Isles the sea-ware, the shell-marl, and the peat-ash, are the three great natural fertilizers to which the agriculture of the district is indebted for the comparative prosperity to which it has in some of the islands already attained.