10 
BULLETIN 52, HAWAII EXPERIMENT STATION. 
based on the fact that the usual manganese ore is the dioxid (pyro- 
lusite), and that solutions of manganese in the carbonate form" in 
which form it is probable that the manganese is leached out of the 
original lava, 3 soon precipitate manganese dioxid because of their 
strong hydrolysis and oxidation by the air. 
IRON IN THE MANGANIFEROUS SOILS. 
Kelley (32) reported the presence of 18.24 to 26.85 per cent of 
iron as ferric oxid, while the writer found a variation of 10.42 to 15.5 
per cent in the soil samples he analyzed. Hawaiian soils 4 contain 
an abundance of iron, having several times the quantity found in 
ordinary soils of the mainland or pineapple soils of other countries. 
Kelley (28) determined the solubility of the manganese and iron 
with a 1 per cent solution of citric acid. In this determination he 
gives the average amount of iron soluble as 0.243 per cent ferric 
oxid, or about 8,500 pounds per acre-foot. It is a striking peculiarity 
that, notwithstanding the presence in these manganiferous soils of 
an immense quantity of total iron and of citric acid soluble iron, the 
pineapple plants seemed unable to assimilate the iron but showed a 
pronounced change after they had been sprayed with 30 to 40 pounds 
of iron sulphate per acre. 
The failure of the plants to absorb iron, notwithstanding the large 
amount soluble in citric acid, seems to constitute a serious criticism 
of the general applicabilit} r of the citric-acid method for determining 
the available constituents of the soil. 
REACTION OF THE MANGANIFEROUS SOILS. 
The manganiferous soils when tested with litmus show an acid 
reaction. Kelley (28) examined a large number of these black soils 
and found most of them slightly acid and few neutral. 
In order to determine more exactly the acidity of these manganese 
soils, the hydrogen-ion concentrations were determined electrically. 
The hydrogen-ion concentrations, expressed in pH values, are given 
in Table 4. 
Table 4. — Hydrogen-ion concentrations (expressed in pH values) of the mangani- 
ferous soils. 
Laboratory soil number. 
Manganese 
oxid 
(M113O4). 
pH value. ; 
i 
Laboratory soil number. 
Manganese 
oxid pH value. 
(Mn 3 4 ). 
9 
Per cent. 
9.74 
4.80 
4.01 
4.14 
4.32 
t 
6.5 
6.4 
7.0 | 
5.7 j 
5.9 
636 __ 
Per cent. 
4. 80 6. 1 
11 
638 
5. 12 6. 3 
15 
639 . 
2. 51 6. 
27 
641 
2.85 6.0 
51 
The table indicates that the manganese soils in nearly every instance 
are fairly acid, since soils having a pH value lower than 7, the neutral 
point of pure water, are acid. That these soils are lacking in car- 
bonate of lime is proved by the fact that calcareous soils would have 
* Lava is the original material from which nearly all the upland soils of the island are derived. 
4 Iron is one of the most abundant elements of Hawaiian soils. 
