MANURE, This is a term applied to a great variety of substances, mineral as well 213 organic, which have been used for the purpose of increasing the produce of those plants. that man selects for cultivation. Lime, and the ashes of vegetables, have been applied o the land to increase its fertility from time immemorial; so also have all kinds of organic substances, whether vegetable or animal. The rationale of such applications to growing plants was but little understood till chemistry revealed to us the nature of the materials which entered into the composition of all plants. At the present day, much definite knowledge has been acquired of the true nature and action of the various substances that are found to increase the growth of our cultivated crops. It was long supposed that the food of such a varied class of plants as the globe presents must neces sarily be very different, almost as much so as the difference in their forms and proper ties of their products. Chemistry, however, has shown that the food of all plants is very rnuch alike, though some classes miist be supplied with certain substances greater abundance than others. The great mass of all vegetables is resolved into car bonic acid, water, and ammonia, on being subjected to heat or burned in a fire. It is these same substances which constitute the chief food of all plants. The light of the sun enables plants to decompose and assimilate carbonic acid and ammonia, and to manufacture out of them the various products they contain. All organic substance:4 yield these by slow decomposition, as well as by combustion. It is for this reason that such substances increase the fertility of land when added to it. Water is so common an article, that nature provides all that plants require. Carbonic acid, too, is contained in considerable proportion in the atmosphere, and is readily taken from it by the leaves: still, it is of great use when applied to the soil as vegetable matter, and the decom position rendered accessible to the roots of plants. Ammonia exists in exceedingly sparing quantities in the atmosphere, as well as in rain and river water. so that artificial applications to the soil are generally needed to produce full crops. The nitrogen which enters into the composition of plants is generally supposed to be capable of being only assimilated either in the form of ammonia or nitric acid; it is for this reason that the salts of ammonia and nitric acid are all very powerful fertilizers. They generally
produce a dark-green color in the leaves, such is associated with healthy growth and luxuriance.
But besides carbonic. acid, water, and ammonia, plants feed upon certain mineral or earthy substances, which seem to impart the power of condensing and digesting the other organic elements. On plants being burned, they leave lime, potash, soda, mag nesia, silica, sulphates, and phosphates, as ash. These substances are all found to exist in certain kinds of plants in proportions which are confined within rather narrow limits. The earthy substances, it must be remeinbered, enter into combinations in definite pro portions with the other constituents. and are thus linked together in the vegetable organisms as part and parcel of their structure.
Lime acts n's mantiring substance directly by supplying Otie of "Ole constituents of plants; so also does magnesia. But lime is often added as an agent to assist in digest ing and preparins the organic materials existing in the soil. See Liam. Magnesia Is. seldom applied singly to the soil; it is usually associated with limestone, and is generally contained in the soil in quantities sufficient for the wants of plants.
Potash is a substance most essential for all our cultivated plants; its market-price, however, is so high, that farmers seldom apply it directly to the soil. They employ cer tain crops, such as clover and turnips, to gather it up for them in the soil. These are consumed on the farm by cattle and sheep, and as little potash enters into animal tissues. 'as a permanent constituent, it is mostly returned to the dunghill in the excrernentitions matters. Farm-yard dung thus possesses a value of its own, by supplying this con- , stituent, which cannot be bought economically in the market. Soda can be easily obtained in the form of common salt, but as this substance is usually associated with potash, the one is found in the dung-heap as well as the other. Common salt is applied ' to corn-crops that are growing too radpidly. The salt has the effect of stiffening the straw, and rendering it less liable to lo ge. Salt is also used with great success in grow ing mangold-wurzel, as this is a plant which was originally taken from the sea-shore.