57 
Whether it will pay to use a given bat guano to supplement other 
fertilizing materials depends, of course, on the relative cost of the 
materials at the place where they are to be used. The valuation of 
the guanos given in Tables III and IV shows their worth relative to 
standard fertilizers. It is thus possible to calculate the relative 
cheapness of guanos and other fertilizers at different places by com- 
paring values and costs. 
Guanos are what may be termed neutral fertilizers. There is no 
danger of an accumulative acid or alkaline effect from their con- 
tinued or excessive use. However, low-grade guanos containing 
much carbonate of lime should not be applied in large quantities to 
pineapples on very sandy soils. 
Before transporting guanos from the caves the material should, 
in many cases, be screened to remove stones and large concretions 
not easily pulverized, as these have little fertilizing value. If the 
material is to be stored in bags, the bag should be treated with a 
dilute solution of copper sulphate to prevent decay, as guano not 
thoroughly 4 dry will rot through a bag in a few days. This is com- 
monly and erroneously attributed to a caustic action of the guano. 
From the estimated quantities of guanos given in the appendix and 
the valuations assigned in this report, it is evident that bat guano will 
never form an appreciable part of the fertilizer consumed on the 
island. Most of the deposits are too valuable, however, to be neg- 
lected, and even after these deposits have been extracted, fresh 
material will be formed having an annual value of several thousand 
dollars. The fresh material should be removed from the cave fre- 
quently, as it is fairly uniform in composition and in nearly all caves 
(on account of leaching) is worth more when fresh than when partially 
decomposed. 
THE USE OF BAT GUANOS. 
For the intelligent use of any fertilizing material it is obviously 
necessary to know its composition and the availability of its fertilizing 
components. This information concerning most Porto Bican bat 
guanos is given in the first part of the report. It is also necessary to 
know the f ertilizer requirement of the particular crop on the particular 
soil, that is, the best fertilizing formula for the conditions. This 
information is not given here, but the following remarks on the use of 
the bat guanos apply to any fertilizer formula. 
USE OF DATA IN COMPOUNDING A FORMULA. 
The following examples will show how the data contained in this 
report should be used in making a fertilizer mixture of required 
formula: Suppose a formula of 3 per cent nitrogen, 12 per cent phos- 
phoric acid, and 12 per cent potash is required and that guano No. 
