22 
peratures, due either to a chemical change in its combination to a form 
less soluble in water or an increase in the absorbing power of the soil. 
The increase effected at 100° and 250° C. is undoubtedly partly due to 
destruction of organic matter and to the breaking up of the colloidal 
film. The action of dilute nitric acid is somewhat different, in that 
an increase in solubility upon ignition accompanies that of iron, 
alumina, silica, and titanium. Iron and aluminum in Hawaiian soils 
occur in the form of hydrates to a certain extent and are more or less 
impregnated with the phosphoric acid and titanium oxid, not only 
holding them in chemical combination, but also mechanically. The 
effect of heat would directly increase the solubility of these con- 
stituents in dilute nitric acid gradually up to the point of ignition, 
at which point the decomposition of the hydrates would be at a 
maximum. Changes due to the destruction of organic matter would 
cause an increase in the solubility of this element. Another factor 
of some importance in this connection is that of precipitation subse- 
quent to solution. The increased solubility of aluminum and man- 
ganese would probably produce some precipitation of phosphoric 
acid, particularly in the water extract. 
SULPHATES. 
The following table shows the solubility of sulphates (S0 3 ) as 
affected by heat: 
Solubility of sulphuric acid in water and fifth-normal nitric 
[Calculated on basis of dry soil.| 
acid. 
Soil No. 
Soluble 
n water (parts per million). 
Soluble in fifth-normal nitric acid (per 
cent). 
Air dry. 
Dried at 
100° C. 
Dried at 
250° C. 
Ignited. 
Aix dry. 
Dried at 
100° C. 
Dried at 
250° C. 
Ignited. 
74 
172.4 
72.3 
111.4 
164.7 
100. 2 
129. 6 
59.4 
98.9 
2C.0. 3 
494.8 
46.2 
110.0 
130.5 
66.2 
146.2 
159.5 
176.2 
149.3 
100.6 
103.1 
326.4 
2, 123. 7 
54.2 
107.4 
1,961.6 
206.7 
1,339.7 
722.5 
1,128.3 
942.0 
702.3 
872.1 
1,555.9 
2, 598. 
119.7 
1,621.3 
1,309.2 
168.6 
1,294.4 
532.3 
799.5 
583.6 
664.3 
926.0 
1, 479. 9 
680.1 
241.6 
1,575.7 
0.027 
.037 
.017 
.067 
.022 
.018 
.028 
.032 
.061 
.089 
.018 
.028 
0.019 
.031 
.019 
.018 
.026 
.016 
.035 
.032 
.086 
.073 
.034 
.029 
0.106 
.027 
.053 
.019 
.062 
.067 
.038 
.037 
.163 
.101 
.019 
.044 
0.067 
164 
.025 
9 
.034 
292 
.055 
290 
.063 
405 
.048 
416 
.041 
417 
.052 
406 
.187 
428 
.177 
426 
.024 
448 
.122 
It will be seen from this table that the effect of heat upon the solu- 
bility of the sulphates is quite marked, more so in the water extracts. 
In this series the air-dried soil is the least soluble, that dried at 100° C* 
next, while the maximum solubility is reached at about 250° C, de- 
creasing upon ignition. On the other hand, the solubility in dilute 
nitric acid is slightly different in that the average shows the maximum 
solubility to be obtained from the ignited samples, the least soluble 
being in the oven-dried (100° C.) soil. The surprising feature of these 
results is the markedly greater solubility of sulphates in water than in 
