Oceanic Deposits and Guanos

Guano

This valuable natural manure is produced from the excrement of sea-birds, and occasionally of other animals, which has been chemically altered by exposure. Peruvian guano has been mined for many centuries. That from the Chincha Islands was used by the ancient Peruvians according to von Humboldt, 1804.

Guano contains from 11 to 17 per cent, of phosphoric oxide, 11 to 19 per cent, of lime, up to 1 per cent, of magnesia, 3 to 15 per cent, of nitrogen, about 2.5 per cent, of potash and 13 to 30 per cent, of water. Hence it is almost a complete plant food.

Phospho-guanos

These deposits occur chiefly in oceanic islands, situated in tropical latitudes, and have probably the same origin as guano, but the changes have been more far-reaching, so that their composition is transitional between the guanos and the phosphatic rocks. By intense bacterial action the nitrogenous compounds have been converted quickly into ammonia and nitric acid, and the soluble nitrates and ammonium salts have been washed out by heavy rain, or the breaking of high seas over low-lying atolls. The resulting deposit contains usually less than 1 per cent, of nitrogen. The phosphate of lime ranges from 60 to 77 per cent. When it is 50 per cent, or less the deficit is usually composed of organic matter (loss on ignition) or calcium carbonate, or both.

The Pacific Island deposits belong to this class and are found chiefly on those islands which lie between Australia and Japan. The Barker Island deposit, now exhausted, contained about 78 per cent, of calcium phosphate and 6 per cent, of magnesium phosphate. Rich deposits were also found on the Fanning Island and Makatea Islands in the Paumotu Group, Angaur Island in the Pelew Group, etc.

The Australasian Dominions, Australia and New Zealand, largely obtain their supplies from Nauru and Ocean Islands, the produce of which was, after 1920, divided in definite ratios between the United Kingdom, Australia and New Zealand. Nauru is a coral island over 200 feet high and covering an area of about 5,000 acres, a considerable part of which consists of a deep deposit containing 80 to 87 (usually 86 to 87) per cent, of calcium phosphate, which probably has been formed by the leaching of guano deposits into the coral limestone. The phosphate is quarried out, leaving pinnacles of the harder limestone. The reserves on the island are estimated at 80 to 100 million tons. From 1913 onwards the island has yielded nearly 100,000 tons per annum. The Ocean Island deposits are of about the same quality, and about 50 feet thick on the central table-land. The reserves are estimated at 50 million tons, and the output for several years was between 100,000 and 200,000 tons per annum.

Other well-known deposits are those of Christmas Island, also Redonda and Sombrero (West Indies).

The phosphates of the British Empire are described in an official pamphlet. Analyses of all the different types of phosphate minerals have been collected by Fritsch.