Dutch ashes are the ashes of peat burned for the purpose of being applied to the land. They vary in constitution with the kind of peat from which they have been prepared. They often contain traces of potash and soda, and generally a quantity of gypsum and carbonate of lime, a trace of phosphate of lime, and much siliceous matter. In almost every country where peat abounds, the value of peat ashes as a manure has been more or less generally recognised.
SECTION VI.—USE OF LIME, SHELL-SAND,
AND MARL.
The use of lime is of the greatest importance in practical agriculture. It has been employed, in Europe at least, in one or other of its forms of shells, shell-sand, marl, chalk, limestone, and quicklime, from the most remote periods.
Native limestone, and all the unburned varieties of chalk, shells, &c. consist of carbonate of lime ([p. 51]), more or less pure. When burned in the kiln, the carbonic acid is driven off, and lime, burned lime, or quicklime remains.
Quicklime, when exposed to the air, gradually falls into the state of an exceedingly fine white powder. It will do so more rapidly if water be thrown upon it, when it also heats much, swells, and becomes about one-third heavier than before. After being exposed to the air for some time in this white powdery state, it is found to have again absorbed from the air a portion of carbonic acid, though a very long period generally elapses before it is all reconverted into carbonate. In compost heaps, where much carbonic acid is formed during the fermentation, the conversion of any quicklime that may be mixed with them into carbonate of lime, is much more rapid and complete than in the open air.
Lime, therefore, is laid on the land in two states.
1st, In the mild state—that of carbonate—in marls, in chalk, in shell-sand, &c.
2d, In the caustic, or quick state, as it comes hot from the kiln, or after it is simply slaked.
Limes are laid on also in a more or less pure form. Marl contains only from 5 to 20 per cent. of carbonate of lime, generally in the state of a very fine powder. Shell-sand consists of a mixture of minute fragments of shells with from 20 to 50 per cent. of siliceous sand. The limestones which are burned are also more or less impure, though, when the impurity is very great, they do not burn well, and are therefore usually rejected.
Some limestones contain much magnesia, by which their agricultural qualities are materially affected. These are known by the name of magnesian limestones. There are few limestones in which a small quantity of magnesia may not be detected, and this minute proportion is likely to be beneficial rather than otherwise; but when it is present to the amount of 10 per cent. or upwards, it appears to have for some time a poisonous influence upon vegetation, if added in the same large doses in which other lime may be safely spread upon the land.