30 
subjected to dense tropical plant growth, resulting in the accumula- 
tion of high percentages of humus, which has been shown in the 
previous tables to affect materially the absorbing power. Further- 
more, the data indicate that the concentration of the soil solution 
does not depend primarily upon the solubility of the mineral con- 
stituents, nor the amount of fertilizer added, but upon the absorbing 
power of the soil. 
As was expected, the fixation of phosphoric acid was much higher 
than the other elements. This is due to the highly basic character of 
the soils, and especially to the large amounts of iron, aluminum, 
and titanium present. It has been found in recent pot experiments 
with this type of soil that crops respond most readily to soluble 
phosphates — namely, sodium phosphate and acid phosphate. There 
was considerable difference in the physical action of calcium and po- 
tassium phosphates, the latter having a decided deflocculating action 
upon the clay, while the calcium salt filtered through the soil column 
perfectly clear. This, coupled with the results of the pot experi- 
ments cited above, indicates that absorbed sodium and potassium 
phosphates are not insoluble, but diffuse more readily and are more 
easily available for the growing plants. This indicates that phos- 
phate should be applied to Hawaiian soils in the soluble form, and 
the best time for application is just before planting, not on account 
of any danger of loss through drainage, but through the danger of a 
slight decrease in availability, due to reversion. 
Apparently the controlling factors in the fixation of potash are the 
amounts of lime and magnesia present. This is very clearly shown 
in the above tables, and the soils used in the experiments were good 
examples with which to illustrate this point. The fixing power for 
this element, while not so strong as for the phosphoric acid, is quite 
marked. However, it should not be applied in too large quantities, 
nor too often, as it is quite readily leached from the soil by rains and 
irrigation. 
The fixation of ammonium nitrogen, as already mentioned, is con- 
trolled by the same general factors which govern the absorption of 
potash. But the point of saturation is in most cases above that of the 
potash. However, it is not so strongly fixed and is leached out quite 
readily by the rains and drainage water. Some investigators claim 
that ammonia replaces the bases combined with the complex "hu- 
mates," and, if so, this accounts for the soils in the first series having 
such a high fixing power both for potash and ammonium nitrogen, 
while the red clay soil was strikingly lower. 
The power of the soil for fixing nitrate nitrogen is almost negligible, 
except in case of the highly organic soils. Apparently the organic 
