PACIFIC SCIENCE, Vol. VI, April, 1952 
curves of the gray hydromorphic soils were 
•similar to those of montmorillonite clays. 
DISCUSSION 
The results of this investigation have 
brought out the fact that there are several 
factors which influence the plastic properties 
of a soil. In general the type of clay, the or- 
ganic matter content, and adsorbed cations 
are the most important. The exact details of 
the mechanism of magnesium-induced im- 
permeability and plasticity are not known. It 
is suspected that hydration of the exchange- 
able magnesium ion is greater in the presence 
of certain humates and that this hydration re- 
sults in a dispersion of the clay and organic 
matter. However, this dispersion is dependent 
upon a type of clay which perhaps must pos- 
sess certain external and internal surface 
characteristics. The results of this study would 
indicate that only a montmorillonite clay can 
develop these plastic properties with these 
factors. 
Humus influences the plastic properties of 
a magnesium-saturated clay by forming a 
complex or by establishing conditions which 
enable magnesium to become hydrated. 
Humate attached to the clay indirectly 
through the magnesium could widen the 
Helmholtz layer and cause an increase in the 
TABLE 2 
The Chemical Composition of the Clay Fraction of Typical Soils of the Gray Hydromorphic Group 
Soil family 
Si02 
AI 2 O 3 
Fe203 
Ti02 
CaO 
MgO 
Honouliuli 
per cent 
per cent 
per cent 
per cent 
per cent 
per cent 
A Horizon 
38.10 
23.02 
14.08 
1.80 
1.02 
1.26 
Kalihi 
A Horizon 
41.07 
25.12 
11.80 
1.52 
0.50 
0.99 
Kalihi 
B Horizon 
41.20 
25.30 
10.65 
1.34 
0.66 
0.91 
Kaloko 
A Horizon 
39.81 
25.20 
14.70 
1.85 
0.44 
0.80 
Kaloko 
B Horizon 
39.02 
26.27 
13.90 
1.52 
0.29 
0.81 
142 
Fig. 4. The influences of organic matter content and 
exchangeable magnesium level on the rate of percolat- 
ing water. 
tion has been identified by differential ther- 
mal methods developed by Norton (1939). 
The thermal curves show that the dominant 
clay mineral in the Kalihi and Kaloko soils is 
of the montmorillonite type. The soils of the 
Honouliuli family have appreciable amounts 
of kaolinite. This would account for their 
lower cation exchange capacity. Matsusaka 
and Sherman (1950) found that the titration 
