39 
table, due to a difference in method of extraction. It may bo safely 
said that the' method of percolation more nearly represents soil 
conditions. The motive in using several solvents was to determine 
the combinations formed in the soil by the phosphates. The data 
show sodium and potassium phosphates to be readily converted into 
iron and aluminum phosphates. These salts also show a high solubility 
in fifth-normal nitric acid, indicating either a residue unconverted 
as shown in column 3, or a partial conversion to the soluble calcium 
salt, which would be the only calcium salt possible of formation in 
the absence of an excess of lime. The iron phosphates proved to be 
quite soluble in fifth-normal nitric acid, in fact, to a greater degree 
than the calcium salts, acid phosphate, and phosphate rock. Tri- 
potassium phosphate is, in all solvents, less soluble than disodium 
phosphate. 
It may be said, then, that soluble phosphates, when added to 
Hawaiian soils, combine with iron and aluminum to a greater degree 
than with calcium, even in soils containing a high percentage of the 
last. But this chemical combination in itself does not explain the 
unavailable nature of phosphates in the soils, as is shown in the 
results obtained from both the pot experiments and the solubility 
experiments noted in the latter half of this bulletin. The unavail- 
able condition is brought about through physico-chemical activities, 
and is more rapid when a sodium or potassium phosphate is added, 
because of the rapid deflocculation of the clay, which causes more 
complete dissemination of the salts. 
As a means of measuring solubility, all solvents used are of more or 
less value. Solubility can hardly be considered a measure of availa- 
bility except in so far as a comparison of the solubility in several of the 
solvents will indicate the form in which the phosphoric acid is com- 
bined in the soil. As the above data show, the results obtained by 
the use of the different solvents are quite conflicting On certain soils. 
At the beginning of this work the most plausible theory suggested 
to explain the unavailability of the phosphoric acid in Hawaiian soils 
seemed to be its possible combination with titanium. While it is 
undoubtedly true that the phosphoric acid may be present to a 
certain extent in this form, which is highly insoluble, this fact is of 
minor importance in explaining the low availability, although it is 
perhaps the most serious chemical factor. Titanium is widely dis- 
tributed in Hawaiian soils, which contain, on the average, from 
5 to 10 per cent of titanium oxid, and as much as 34 per cent has been 
found. If this constituent were to bo considered a prime factor in 
the availability of the phosphoric acid, it would bo expected that 
the titanium soils would have a high phosphate content, due to reten- 
tion of the phosphoric acid as titanium phosphate. The analysis of 
