26 
the former. Basic slag meal is a waste product in the manufacture 
of steel, and in addition to 15-20 per cent phosphoric acid, also 
contains about 30 per cent lime. It has thus far found little appli- 
cation as a fertilizer in Hawaii. Acid phosphate and double 
superphosphate ai»e [manufactured from phosphate rock. The acid 
phosphate is the simpler product and contains about 20 per cent 
phosphoric acid as monocalcium phosphate. The double super- 
phosphate, through a modification in the process of manufacture, 
contains about 45 per cent phosphoric acid as monocalcium phos- 
phate. This class of phosphate fertilizers, more especially the former, 
is widely used in Hawaii. It may be used with advantage on practi- 
cally all types of soil which show a deficiency of phosphoric acid. 
Phosphate rock, known as floats when finely ground, is the natural 
tricalcium phosphate found in large deposits in various parts of the 
world, but it is little used in Hawaii. Bone meal is quite widely 
used as a source of phosphoric acid in Hawaii, but is effective only 
where the soil receives plenty of water. It is especially effective if 
applied in connection with green manure. Reverted phosphate, 
which is primarily dicalcium phosphate, is being used to an increas- 
ing extent in the islands. It is prepared by adding lime to acid 
phosphate. 
The results of the experiments indicate that the soluble phosphates 
are the most effective on Hawaiian soils, especially those of the red- 
clay type. As already indicated, this is contrary to the conclusions 
reached by others in regard to the application of soluble phosphates 
to soils high in iron and aluminum oxids. 
Iron and aluminum phosphates are readily available sources of 
phosphoric acid in Hawaiian soils, the former more so in the first 
crop in the absence of added lime. In sand cultures the trivalent 
phosphates surpass the calcium phosphates. Hence, since the pre- 
cipitated phosphates of iron, aluminum, and even titanium are 
available to plants, factors other than chemical combination must 
be considered in order to explain the apparent insolubility of phos- 
phoric acid in Hawaiian soils. 
That the phosphoric acid of the red-clay soils of Hawaii exists in 
some form extremely unavailable to plants is proved by the data 
obtained in Experiments I and II (pp. 10, 15). The soluble phos- 
phates were the most effective on the first crop. Their effectiveness 
decreased somewhat in the following crops. The phosphate rock was 
least effective at the outset, and its effectiveness increased and 
then decreased as compared with other phosphates, if the weight of 
succeeding crops may be used as a criterion. Through fermenta- 
tion changes and chemical action the availability of the phosphate 
rock was increased to such an extent that the plants of the second 
millet crop to which this fertilizer was applied had a larger reserve 
