46 
It is evident that for most guanos the citrate solubility is a fair 
test of the availability of the phosphoric acid, but with many guanos 
the citrate method gives far too high results. In some cases a serious 
error would be made in relying on the citrate method. This is not 
surprising in view of the very variable composition of the guanos. 
By consulting the complete analyses of guanos in Table III it is 
apparent that the inaccuracy of the citrate method for certain guanos 
can not be correlated with the content of any constituents, as iron, 
alumina, lime, volatile matter, silica, or sulphates. 
In judging these results it should be considered that the availa- 
bility or efficiency of a phosphate is not an unalterable property, but 
is affected by many conditions already mentioned. On the other 
hand, solubility of a phosphate in neutral ammonium citrate is a 
fixed property if the method is rigidly followed. Consequently the 
citrate solubility can not be a true measure of the efficiency of any 
phosphate under all conditions. In a comparison of the citrate and 
vegetation methods for availability, it is, therefore, essential to qualify 
the conditions of the vegetation tests. Under certain vegetation 
conditions the availability of the phosphate should be the same as 
the solubility in ammonium citrate if the chemical method is appli- 
cable to the material. 
It is reasonable to hold that the citrate method, if it is applicable 
to a given material, should show the amount of phosphoric acid 
immediately available to a crop under conditions where phosphatic 
efficiency is not particularly enhanced or depressed. It is believed 
that in the tests of immediate availability in river sand there were 
no special conditions enhancing or depressing the efficiency of the 
phosphates and that the tests therefore fairly show the applicability 
of the citrate method to guanos. The fact that in these tests citrate- 
solubility was a fairly to exceedingly accurate measure of availability 
with 72 per cent of the samples, but a very inaccurate measure with 
28 per cent of the samples, confirms this idea. The agreement of 
some samples and the nonagreement of others show that the citrate 
method is not applicable to all kinds of guanos. 
Had the vegetation tests all been conducted in the acid clay soil, 
doubtless nearly all guanos would have shown a greater vegetation 
than chemical availability. Such a soil, however, would not be a 
fair medium for determining the applicability of the citrate method. 
Summary of results on efficiency of the phosphoric acid in guanos. — 
Vegetation tests showed that the immediate efficiency of the phos- 
phoric acid in bat and bird guanos varied between and 108, com- 
pared with 100 for the efficiency of phosphoric acid in acid phos- 
phate. Samples of fresh bat manure had an average efficiency of 84, 
showing that they are almost as available as acid phosphate. About 
half the samples examined had an efficiency of 20 or better, practically 
