This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is a valuable manure, consisting chiefly of the dried excrements and waste of sea birds, which have accumulated for centuries on the coasts and rainless districts of Chili and Peru. The famous traveller Humboldt first brought samples of guano to Europe in 1804, but it was not till 1840 that the first cargo reached Britain. Five years later nearly 300,000 tons were imported, and enormous quantities arrived annually, until soon after 1870 the supplies began to get exhausted. The Peruvian guanos are now completely worked out, and supplies have to be obtained from other sources, such as the coasts of Bolivia, Colombia, and Patagonia, Australia, South-west Africa, and certain islands in the Pacific. The importations now are small in comparison with those of earlier times. In 1901 only 13,000 tons were imported, and in 1907, 31,278 tons of all kinds of guano. The original Peruvian guanos were very rich in plant foods, containing 14 to 16 per cent of nitrogen, 12 to 14 per cent of phosphoric acid, and 2 to 3 per cent of potash. They were thus "complete" fertilizers. Modern guanos, however, seldom contain more than 10 per cent of nitrogen, and may contain as little as 2 1/2 per cent. Purchasers should always insist on obtaining a warranty when buying guano, and samples should be analysed from time to time to test the manurial value.
"Guanos are commonly divided into nitrogenous and phosphatic. Nitrogenous guanos are those which contain a considerable percentage of nitrogen, generally over 4 per cent. They may also contain a large percentage of phosphate. A recent sample, for instance, contained 6 3 per cent of nitrogen and 32 per cent of phosphate. Phosphatic guanos, on the other hand, contain little nitrogen, generally from 1 to 3 per cent, but they should contain a considerable percentage of phosphate. Usually the phosphate is from 30 to 50 per cent, but samples containing as much as 70 per cent are sometimes on the market" (The Standard Cyclopedia of Modern Agriculture).