9 
The series reported in the above table was started in glass tubes, 
100 grams of soil being used in each instance, but it was found neces- 
sary to transfer the soils to funnels, as there was no percolation at all 
through soil No. 474, and it was extremely slow in Nos. 292, 448, and 
428. The extracts all came through clear for about one month, 
after which they began coming through cloudy, and when the series 
was stopped the percolation was very slow even in the funnels. 
Phosphoric acid being the constituent of phosphates which forms 
insoluble compounds with the bases always present in soils, such as 
iron, aluminum, titanium, lime, and magnesium, it is not very difficult 
to understand the retention of soluble phosphoric acid by soils. In 
the presence of sufficient calcium carbonate the application of soluble 
phosphoric acid will result in a " reversion" of the phosphate, i. e., 
the formation of the less soluble dicalcium phosphate which, however, 
is quite readily available, and hence there results a gain rather than a 
loss. But in case the soil is deficient in lime and contains an excess 
of iron and aluminum hydrates and silicates, similar to Hawaiian 
soils, an entirely different problem is encountered. In this case the 
phosphoric acid will be fixed by the iron and aluminum compounds, 
thus being rendered not only practically insoluble in water, but also 
in weak organic acid solvents. For such conditions various investi- 
gators recommend the application of lime preceding that of the super- 
phosphate, the theory being that the lime will revert the phosphoric 
acid. This theory has been put in practice in the red clay soils of 
the Wahiawa district of Oahu, but has failed to produce any bene- 
ficial results. This is probably due to the excessive amounts of iron 
and aluminum hydrates in these soils. 
As indicated in the preceding tables, there is considerable difference 
in the absorption of the potassium and calcium phosphates. Since 
they were not carried to the saturation point, we can only compare 
the rates of absorption, and here the fixation of calcium phosphate 
is strikingly faster. It will be seen that more phosphoric acid was 
fixed from calcium phosphate in two of the soils and practically the 
same in the other two, even though 1 liter more of the potassium 
phosphate solution was passed through. On the other hand, nearly 
the same weight of the salt has passed through, and the general prop- 
erty of absorption is similar. In both cases soil No. 292 fixed the least 
phosphoric acid, No. 474 next least, No. 428 next, and No. 448 the most. 
Both of the soils that fixed the least phosphoric acid contained a high 
percentage of phosphoric acid, a sufficiency of lime, and a high percent- 
age of organic matter. It is probable that reversion takes place more 
quickly with the calcium salt, which accounts for the higher rate of 
fixation in this case. There appears to be little correlation between 
the rate of fixation and the mechanical composition of the soil in 
cases where the size of the particles is offset by the organic matter, 
48303°— 14— 2* 
