- 25 
After about two weeks the plants were growing most vigorously in 
the pots containing sodium and acid phosphates. Ferrous phosphate 
was very toxic at this stage, but the plants were able to survive and 
partly overcome the toxicity. After two months' growth phosphate 
rock and ferrous phosphate had produced the smallest plants. Ferric 
and aluminum phosphate had produced the most vigorous plants, 
the former being slightly better than the latter. Sodium, acid, and 
titanium phosphates, and slag were slightly less favorable than 
ferric and aluminum phosphate. 
DISCUSSION. 
The availability of a phosphate will vary with the type of soil, the 
climatic conditions, and the character of the crop to which it is applied. 
In view of this, due consideration must be given to the fact that the 
preceding data have been obtained with only three types of soils and 
a limited number of crops grown under modified conditions. 
The type of Hawaiian soil on which phosphate fertilization is most 
effective is that known as the red ferruginous clay. According to 
Hilgard, 1 it would be expected that when water-soluble phosphates 
are applied to tins type of soil, the phosphoric acid would be with- 
drawn from useful action. Hence any excess that the plant is not 
able to immediately utilize becomes inert and useless, that is, it com- 
bines with the oxids and hydroxids of the trivalent metals, iron and 
aluminum, and in this form it is for all practical purposes insoluble 
and inaccessible to the crop. For this reason he advised the use of 
diflicultly-soluble phosphates, such as bone meal and basic slag, 
which react less readily with iron and aluminum. Until very recent 
years this has been the generally accepted theory among soil chemists. 
The results of more recent investigations, however, indicate that 
iron and aluminum phosphates are readily available to plants, in 
many cases more so than the insoluble forms of calcium phosphate, 
such as bone meal, slags, and floats. Recent work at the Wisconsin 
Experiment Station, 2 for example, has shown that 9 out of 10 plants 
tested made better growth when fertilized with aluminum phosphate 
than with calcium phosphate, while 6 of the 10 made better growth 
with ferric phosphate. 
In view, therefore, of the uncertainty on the subject, the peculiar 
character of Hawaiian soils, and the practical importance of the matter, 
it was deemed necessary to study carefully the behavior of various 
phosphates on typical Hawaiian soils. 
The phosphates used as the basis of the preceding experiment arc 
all of commercial importance. Other phosphates were added to the 
series in order to obtain information relative to the availability of 
i Soils. New York and London, 1906, p. 9S7. » V'Lsconsin Sta. Bui. 240 (1914), p. 22. 
