GENERAL MANURES ANIMAL AND VEGETABLE 297 



10 to 90 per cent., and pc/tash to 5 per cent. The 

 richest deposits of nitrogenous guano are all long since 

 exhausted, but fairly good samples can still be obtained 

 containing from 6 to 12 per cent, of nitrogen, 10 to 20 

 per cent, of phosphates, and 1 to 4 per cent, of potash. 



Manurial Value. - - Guanos are highly concentrated 

 manures and have little or no effect on the mechanical 

 properties of the soil. The manurial value depends 

 entirely upon the chemical composition and is generally 

 estimated simply by the proportions of plant foods they 

 contain. In view of the peculiar and varied states in 

 which the plant foods exist in guanos, it is not improbable 

 that they have a somewhat higher fertilising value than 

 those of compound manures. .Guanos should therefore 

 be purchased subject to analysis, and under a guarantee 

 that they are pure and genuine. 



Guanos are general manures, but the proportions of 

 the different constituents are often not very suitable for 

 general agricultural purposes. In order to remedy this, 

 quantities of potash, phosphates or nitrogenous matter 

 are sometimes added to thenij and the mixtures are called 

 " rectified " or " equalised " guanos. The only objection 

 to this, from the farmer's point of view, is that no dis- 

 tinction is made between the original and the added con- 

 stituents, and the latter are often charged at guano 

 prices. It is therefore better for the farmer, knowing 

 the composition of the guano, to buy the constituents 

 in which it is deficient and apply them, to the soil 

 separately. As the major part of the phosphates and 

 nitrogenous matter are present in an insoluble form, 

 guanos are occasionally treated with acid and sold 

 as "dissolved guano." This treatment no doubt renders 

 the plant food more readily available, and it is claimed, 

 greatly improves their manurial value. It completely 

 alters the peculiar condition in which the fertilising in- 



