20 
included in this work. The general plan of these tests varied some- 
what from the usual plans, embodying, it is believed, some improve- 
ments. A complete description of the plan and its advantages has 
been published in another place. 1 
In brief, the plan was to compare the efficiency of the fertilizing 
element in the guano with the efficiency of the element in a standard 
fertilizer, on the basis of the relative amounts of the elements from 
the two sources required to produce the same increased yields. In 
testing the efficiency of the phosphoric acid, for instance, a series of 
pots received increasing amounts of acid phosphate, while other pots 
received phosphoric acid from different guanos. From the weights 
of the crops grown in the acid-phosphate series a curve was plotted 
showing the amount of phosphoric acid from acid phosphate required 
to produce any increased yield in that particular test. From the 
curve the amount of phosphoric acid from acid phosphate could be 
found which would have been required to produce the same yield as 
that produced by any one of the guanos. The ratio of these two 
quantities of phosphoric acid (from the guano and acid phosphate) 
which produced the same increased yield gave the efficiency of the 
phosphoric acid in the guano relative to that of acid phosphate. 
The efficiency of the citrate-soluble phosphoric acid in acid phos- 
phate in all the following tests was taken as 100 and the other effi- 
ciencies expressed relative to this. Thus, if 2 grams of phosphoric 
acid from a guano gave the same yield as 1 gram of phosphoric acid 
from acid phosphate, the efficiency of the guano phosphoric acid was 
taken as 50. 
Glazed earthenware pots were used as containers. They were kept 
on trucks in a wire inclosure (five meshes to the inch) during fair 
weather, but run into a glass house during rains and violent winds. 
The water content of the soil was kept constant by daily weighings, 
transpired or evaporated water being replaced by rain water caught 
on the glass roof of the plant house. When the plants had attained 
considerable size the weights of the pots plus soil were corrected for 
the added weight of the plants. Plants grown under these condi- 
tions were equal in size to field plants where the maximum fertilizer 
was used. 
Both green and oven-dry weights of the crops from each pot were 
determined, although in most cases it made little difference whether 
efficiencies were calculated from green or dry weights. For the sake 
of conciseness, only oven-dry weights are reported, except in two 
tests (Tables VIII and XX). Determinations of nitrogen or phos- 
phoric acid in the crop were not so essential with the plan employed 
as with the usual method, as has been pointed out. 2 
i Gile, P. L., and Carre ro, J. O. A plan for testing efficiencies of fertilizers. Jour. Amer. Soc. Agron., 
8 (1916), No. 4, pp. 247-255, fig. 1. 
2 Gile, P. L., and Carrero, J. O. Loc. cit. 
