“Several of the South American guano beds are now exhausted, but new varieties are constantly being introduced; and although the qualities are continually varying, guanos, on the whole, may be divided into two classes, the one characterised by the abundance of ammonia, the other by that of phosphates, the Peruvian and Angamos being characteristic of the former, and the Saldanha Bay and Bolivian of the latter. In selecting a guano, the following points (Anderson) ought to be attended to by the farmer:—
“1st. The guano should be light coloured and dry, colouring very slightly when squeezed together, and not gritty.
“2nd. It should not have too powerful an ammoniacal smell, and should contain lumps which, when broken, appear of a paler colour than the powder.
“3rd. A bushel should not weigh more than from 56 to 60 pounds.
“These characters are, however, imitated with great skill, so that they cannot be implicitly relied upon, and they are applicable to Peruvian guano only.”[340]
[340] The above particulars are from an elaborate paper by Dr H. M. Noad, in the ‘Chemist and Druggist,’ vol. ii.
Purity, Adulteration. Guano, owing to its high price, is very commonly adulterated, or is in an advanced stage of decomposition when sold. Much of what is vended under the name is altogether a fictitious article. These artificial mixtures are made to look so like genuine guano, that the mere practical man, who goes only by their appearance, is very often deceived by them, and, owing to the failure of his crops in consequence, is led to distrust the efficacy of guano as a manure. A sample of pretended guano examined by Johnstone was found to contain, in the state in which it was sold, more than half its weight of gypsum, the rest being peat or coal ashes, with a little common salt, crude sulphate of ammonia, and either dried urine or the refuse of the glue manufactories, to give it a smell. “I could not satisfy myself that it
contained a particle of real guano.”[341] Vessels which sail hence for the guano stations are now very commonly ballasted with rough gypsum or plaster of Paris. This substance is mixed with the guano as it is loaded, and enables the importers to deliver from the vessel a “nice-looking, light-coloured article.” Purchasers of guano are very desirous of having it delivered from the vessel, as they believe they thus obtain it pure. The favourite material for the adulteration of guano, at the present moment, is a variety of umber, which is brought from Anglesea in large quantities. The rate of admixture is said to be about 15 cwt. of umber to about 5 cwt. of Peruvian guano, from which an excellent-looking article is manufactured, which is sold under the name of ‘African guano.’
[341] ‘Elem. of Agric. Chem.’
Pure guano has a pale-brown colour, a more or less offensive odour, and the average sp. gr. of 1·63 to 1·64. If the sp. gr. exceed 1·75, it is either damaged or adulterated; and if it is less than 1·62, it contains an undue quantity of moisture. The best is neutral to test-paper, and sometimes has even an acid reaction; but that of commerce has generally an alkaline reaction, owing to the presence of free ammonia, and, in consequence, turns turmeric paper brown, and gives white fumes when a glass rod dipped in hydrochloric acid is held over it. Triturated with quicksilver or caustic potassa, good guano evolves a powerful odour of ammonia; digested in water, fully one half of it is dissolved; dried by the heat of boiling water, it does not lose more than from 7 to 9% in weight; and burned upon a red-hot shovel, it leaves a white ash, not a red or dark-coloured one. (See directions for selecting guano given above, also below.)