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.
Soon after 1870, when the supply of Peruvian guano began to fail, it was thought that fish refuse might be utilized for the production of guano - especially as the latter manure came from birds that fed largely on fish. Although the methods of manufacture were at first very crude, and a good deal of oil was incorporated with the manure, great improvements have been effected in late years. Fish guano is chiefly valuable as a manure for its nitrogen and phosphates, the quantities of which vary according to the kind of fish. The supply of nitrogen will be larger in fish having plenty of flesh and little bone, while the phosphates will be greater in fish having much bone and little flesh. There is also a small quantity of potash and lime. The nitrogenous value varies from 7 to 16 per cent, according to the kind of fish and the process of manufacture. The phosphates vary from 3 to 20 per cent. What is known as "white fish" guano is made from the heads, bones, and waste of haddocks, cod, ling, and other non-oily fish, and is superior to the guano obtained from herrings.