30 
The efficiencies of the phosphates as found by vegetation tests are 
expressed relative to that of acid phosphate taken as 100. Thus, if 
the percentage of total phosphoric acid in a guano be multiplied by 
its efficiency value and divided by 100, the result will be the per- 
centage of phosphoric acid in the guano which is as available as that in 
acid phosphate. For example, No. 321, containing 14.47 per cent 
total phosphoric acid with an efficiency of 27, contains 3.91 per cent 
of phosphoric acid which is as available as that in acid phosphate, 
14.47 per cent X 27 M 
inn =3.91 per cent. 
It will be noted that certain guanos were tested several times. 
Some of these duplications were made to gain an idea of the accuracy 
of the work, others are due to the same guano's being used in several 
tests of the effect of different conditions on efficiencies. 1 
The efficiency of the phosphoric acid in the different samples tested 
varied between and 108. In respect to availability in a sandy soil, 
some of the guanos are, therefore, as good as the best fertilizers carry- 
ing phosphoric acid, while others are practically worthless. In about 
half the samples tested, the phosphoric acid had an efficiency of 20 or 
more, which compares well with bone meal under the same conditions. 
While a large part of the guanos must be considered as phosphatic 
fertilizers of low availability, nearly all were more effective than 
finely ground phosphate rock or floats. 
Six samples of fresh or only slightly decomposed bat manure were 
tested, Nos. 472, 503, 751, 780, 881, and 977, The phosphoric acid 
was of high efficiency in all, ranging from 59 to 108, the average being 
84. In respect to efficiency (not quantity) ol its phosphoric acid, the 
fresh bat manure, therefore, ranks with the best phosphatic fertil- 
izers. 
Efficiency of the phosphoric acid as affected by the crop. — As differ- 
ent crops are supposed to vary in their ability to utilize the slightly 
soluble phosphates, the relative efficiency of different phosphates de- 
pends somewhat on the crop used as a test. In some cases different 
efficiencies of phosphates for different crops are due to distinct second- « 
ary effects of the phosphates, as acidity or basicity. 2 This, however, 
is really a question of the interaction between soil and phosphate 
rather than between crop and phosphate. 
When a quick-growing crop requires considerable phosphoric acid, 
one phosphate may be more effective than another because it is more 
» Determinations of efficiency of the same material repeated in different tests agree very closely for the 
most part. In some cases lack of agreement was partially due to experimental errors. Differences due to 
experimental errors, however, were probably small in most cases, as duplicate determinations made in the 
same test agreed very closely. The larger variations, such as occurred with bone meal, were doubtless due 
to real differences in the efficiencies of the materials in the different lots of soil. While the same type of soil 
was used in all the tests reported in Table VIII, different lots were secured for different experiments, and 
these lots of course varied somewhat in character. 
a The good effect of basic slag on clover has often been noted. 
