294 SOILS AND MANUKES 



given to manures which can be sold on their merits, i.e., 

 on the strength of the proportions of potash, phosphates 

 and nitrogen which they contain. That, of course, is 

 the ultimate test of all concentrated manures and must 

 be applied to genuine guanos as well as to artificial pro- 

 ducts of ali kinds. 



Origin and Occurrence. Guanos are found in large 

 natural deposits in islands and on the coasts of the main- 

 land in various parts Of the world, but chiefly in tropical 

 regions. They have been formed mainly of the excre- 

 ment of sea-birds which congregate together in colonies, 

 especially at certain seasons of the year. Feathers and 

 the whole bodies of birds, more or less decayed, are 

 usually present in considerable quantities, and the re- 

 mains of seals and other large marine animals are also oc 

 casionally found. The bulk of the deposits is, however, 

 made up of excrement. The word guano is simply the 

 Spanish for dung. 



Preservation of the Deposits. Some of the deposits are 

 upwards of 200 feet thick and are covered over with 

 sand, etc., to a considerable depth. Owing to the com- 

 pression thus produced and the dryness of the climates 

 in which they are found, comparatively little fermenta- 

 tion has taken place. Otherwise .they could not have 

 been preserved for they are by no means of recent 

 formation and the material of which they consist is 

 subject, under ordinary conditions, to very rapid decom- 

 position. Fermentation has not, however, been entirely 

 arrested, and the materials have undergone slow but con- 

 tinuous change since they were first deposited. Uric 

 acid, which is the principal nitrogenous compound in 

 the excrement of birds, has been largely converted into 

 ammonium carbonate and intermediate products, and in 

 most cases, a certain amount of nitrification has also 

 taken place. A part of the nitrogen and other constituents 



