16 
to the ferric condition on being heated to higher temperatures, thus 
becoming less soluble, then the figures show a marked increase in 
solubility of the iron with increase in temperature. It will be noticed 
that the three soils which it is proposed to disregard in drawing con- 
clusions show a markedly regular decrease in solubility of the iron 
from the air-dried to the ignition state. A qualitative test of the 
water extract from these three soils showed a very high concentra- 
tion of ferrous iron from the wet and air-dry samples. In several of 
the samples in the series there is a close correlation between the effects 
of the heat on iron and alumina, but it is by no means general. 
Iron, alumina, and silica are apparently the constituents least solu- 
ble in water. The greater solubility of iron in the air-dried soil may 
be explained by the fact that the normal mechanical condition of 
Hawaiian soils is conducive to reducing conditions which result in 
the formation of small quantities of ferrous compounds. Hawaiian 
soils, although characteristically basic, normally give an acid reac- 
tion, due indirectly to the high clay content and its accompanying 
poor aeration. Magnification of this condition is to be found in the 
rice and taro soils, as will be shown in a later table (p. 24), in which 
soluble iron is found in comparatively large amounts. In such cases 
it is to be expected that the direct effect of heat would be to oxidize 
the iron and thus render it less soluble. Further confirmation of 
this theory is found in the cultivated and uncultivated soils (Nos. 
416 and 417, respectively), in which the iron content of the latter is 
shown to be the more soluble. After heating at 100° the solubility 
in many instances is greater than in the air-dry samples and is prob- 
ably due to physico-chemical effects upon the soil films and hydrated 
silicates. These latter effects are also responsible for the increased 
solubility of iron in dilute nitric acid as a result of heat. 
MANGANESE. 
The results obtained from the manganese (Mn 3 4 ) determinations 
are shown in the following table : 
Solubility of manganese in water and fifth-normal nitric acid. 
[Calculated on basis of dry soil.] 
Soluble in water (parts per 
million). 
Soluble in fifth-normal nitric 
cent). 
acid (per 
Soil No. 
Air dry. 
Dried at 
100° C. 
Dried at 
250° C. 
Ignited. 
Air dry. 
Dried at 
100° C. 
Dried at 
250° C. 
Ignited. 
74 
26.5 
9.0 
23.5 
5.2 
29.4 
5.9 
2.2 
15. 7 
9.1 
l.i. 9 
6.9 
2.8 
30.4 
11.0 
26.5 
4.1 
20.4 
10.3 
4.5 
£8.7 
4.4 
161.1 
9.0 
13.4 
30.4 
10.0 
25.5 
41.4 
40.8 
14.9 
14.4 
180.5 
6.0 
32.8 
90.3 
108.7 
30.4 
9.0 
20.1 
14.5 
45.1 
18.4 
14.8 
219.2 
9.6 
6.3 
33.8 
119.6 
0.063 
.003 
.494 
.070 
.049 
.062 
.349 
.385 
.035 
.094 
.004 
.028 
0.041 
.008 
. 669 
.098 
.076 
.062 
.529 
.580 
.030 
.102 
.012 
.032 
0.217 
.013 
1.692 
.106 
.115 
.047 
1.314 
1.052 
.048 
.225 
040 
.126 
0.071 
1C4 
.006 
9 
1. 129 
292 
.067 
290 
.118 
405 
.041 
416 
.739 
417 
.748 
406 
.040 
428 
.135 
426 
.023 
448 
.050 
