44 
It will be noted that in most cases the agreement between effi- 
ciencies determined at various stages of growth was remarkably 
close. The slight differences that occurred were within the limits of 
experimental error in all cases except that of bone meal. The 
greater efficiency of bone meal at the 51-day than at the 31-day stage 
of the millet crop is probably not due to the fact that bone meal is 
more available to millet during the later stages of growth, but 
rather to the fact that the availability of bone meal is not decreased 
by its remaining in the soil while that of acid phosphate is decreased * 
(see p. 54). The difference in efficiency of bone meal at different 
periods in the growth of millet is, therefore, due to the action of the 
soil rather than to that of the crop. 
So far as assimilation by the crop is concerned, the guanos seem to 
be as efficient during the early growth of the plant as during the 
later growth. Some guanos, during the latter growth of long time 
crops, may show an increased availability relative to acid phosphate, 
due to a smaller loss of availability in the guanos on remaining in the 
soil. 
Citrate solubility as a measure of the available phosphoric acid in bat 
guanos. — Chemical analyses and vegetation tests conducted with 92 
different samples of guano afford data for judging the reliability of 
the citrate method for determining available phosphoric acid in bat 
guanos and leached bird guanos. In Table XIX is shown the per- 
centage of total phosphoric acid in the guano available by the citrate 
method and also the immediate efficiency of the total phosphoric 
acid relative to that of acid phosphate as determined by a vegetation 
test in a sandy soil. As the phosphoric acid of the acid phosphate 
was all available, 2 the figures for efficiencies in the vegetation tests 
also show the percentage of the total phosphoric acid in the guanos 
which was available. 
In Table XIX, the figures for efficiencies by vegetation tests are in 
many cases averages of the values obtained in several different tests 
with the same guano. They all represent tests, however, of the imme- 
diate efficiency of the phosphoric acid in sandy soil with corn or 
millet. The results in Table XIX were compiled from portions of 
Tables IV, VIII, X, XII, and XVIII. 
1 It should be borne in mind that efficiencies are expressed relative to that of acid phosphate, 
s Acid phosphate was used in the tests on the basis of the content of available phosphoric acid, not total 
phosphoric acid. 
