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time there was a decrease in availability of phosphate as measured by 
the growth of millet. 
In the types of Hawaiian soils that contain, abnormally high per- 
centages of lime, the phosphate is always present in an available form. 
This indicates that the lime, when present in such large quantities, 
is able to perform its function in spite of the excessive quantities of 
iron and aluminum, but in order that it may exercise this function, it 
must be present in amounts far in excess of that indicated by the 
determination of the lime requirement. 
The conclusion is evident that the unavailability of the phos- 
phates in the clay soils of Hawaii is not due entirely to their chem- 
ical combination with iron and aluminum as phosphates but to 
other causes of a far more complex nature. The experiments re- 
ported in this bulletin, as well as those of several other investigators, 
have shown the power of certain plants to assimilate the precipitated 
phosphates of iron and aluminum both when applied in sand cul- 
tures and when applied to soils. On the other hand, it will be 
shown later in this bulletin that the major part of the phosphates of 
Hawaiian soils does exist in the form of iron and aluminum phos- 
phates and that the addition of soluble phosphates results in a rapid 
combination with these elements. The answer to the question why 
plants can not assimilate the phosphates of Hawaiian soils is probably 
to be found in the realm of soil physics as indicated above. 
SUMMARY. 
(1) Hawaiian soils are uniformly higher in phosphate than main- 
land soils, but this is less available, especially in the heavy clay 
types. 
(2) The unavailability of the phosphoric acid in the ferruginous-clay 
soils is not due entirely to chemical combination but partly to physical 
occlusion . 
(3) Phosphoric acid should be applied to this type of soil in the form 
of soluble phosphates and in light applications at frequent intervals 
if rapid returns are anticipated. 
(4) In most locations it is poor economy to add bone meal or other 
difficultly soluble phosphates to Hawaiian soils because they already 
contain enough insoluble phosphate to grow crops for an indefinite 
number of years provided the plants had the power to assimilate it. 
(5) In wet districts (uplands) phosphate rock, bone meal, basic 
slag, or reverted phosphate should be very effective, more especially 
so if applied to highly organic soils or used in systems of diversified 
agriculture where they may be incorporated with green manure crops. 
(6) The availability of all the phosphate fertilizers varies with the 
fineness, and for this reason all the samples used in the preceding 
