Until 1874 most of the so-called Peruvian gua no was obtained from the Chincha Islands, situ ated about 12 miles off the coast of Peru, between latitude 13° and 14° S. Each of these islands is from 5 to 6 miles in circumference, and is of granite, formerly covered with guano, in places to a height of 200 feet, in successive horizontal strata, varying in thickness from three inches to a foot, and in color from a light to a dark brown. Sometimes, however, a vertical surface of upward of 100 feet of a perfectly uniform appearance was found.
As good Peruvian guano always commanded a high price, and there was a strong inducement to adulterate it, umber, powdered stones, vari ous earths, partially decomposed sawdust, and other substances were used for this purpose, and specimens have been sold containing mere traces of the genuine article. Moreover, as the deposits become exhausted the quality deteriorates. For instance, certain of the Peruvian guanos origi nally contained as high as 14 per cent. of nitro gen, but declined to 3 to 4 per cent. as the deposits became worked out. To make good this deficiency, it has been a common practice to 'equalize' or `rectify' the guano by the addition of ammonium sulphate. In view of this wide variation in composition and quality, large purchases of guano should be made only on guaranty of composition. As the chemical composition of natural guano is known, an artificial guano may be readily com pounded by the admixture, in due proportions, of its constituents. The following mixture forms a good imitation of guano, pounds of it being considered equal in power to 1 cwt. of good Peruvian guano: 78% pounds of bone-dust, 25 pounds of sulphate of ammonia, pounds of pearlash, 25 pounds of common salt, and 2% pounds of dry sulphate of soda.
Guano formerly was largely used for all the cultivated crops on the farm, and is now when it can be procured. It was the first artificial fertil izer to be used in large quantities, and the re markable benefits derived from its application led to a rapid extension of its employment and pre pared the way for the general use of commer cial manures and intensive culture. Guano is
therefore of great importance historically, be cause of its influence on agricultural practice, although, as Aikinan states, it is now largely a manure of the past. It is essentially a nitro genous and phosphatic fertilizer, although it usually contains a small percentage of potash. When used continuously, therefore, and not sup plemented by potash fertilizers, it may result in an undue exhaustion of the soil as regards potash, and a consequent refusal of the soil to respond to further applications of the guano. The practice, formerly common, of applying salt with guano tended to hasten this exhaustion of potash in the soil. Guano is especially valuable as a nitrogenous fertilizer. The nitrogen, being in a variety of forms, is supplied to the plant gradually and continuously throughout its period of growth without danger of serious loss by leaching.
At the beginning of the Civil War there were annually imported into the United States about 80,000 tons of guano. used chiefly in the South. The importation since has not been so large. In 1856 Congress passed an act giving to any Ameri can citizen who discovered a guano island the ex clusive right to sell the guano. This stimulated the exploitation of deposits for a time to a marked extent; but the business has declined in recent years, due both to the exhaustion of deposits and to the utilization of cheaper sources of commer cial fertilizers, such as the immense phosphate beds of the South. The total annual import into Great Britain in 1893 is estimated by Aik man as under 30,000 tons, while in 1855 it was over 200,000 tons.
The term guano is commonly applied to vari ous other manures, such as fish-guano and bat guano. The latter is the more or less altered accumulation of bat-manure found in certain caves. It sometimes contains as high as 9 per cent. of nitrogen and 25 per cent. of phosphoric acid, but is found in too limited quantities to be of much commercial importance. See MA NURES and MAxtuaNn.