Gray Hydromorphic Soils — Gill and Sherman 
cant relationship between these two factors. 
Baver (1928) has shown that the plastic 
number of a soil increases as the organic 
matter content of the soil increases. The 
plasticity of these soils and their imperme- 
ability is much greater than those of other 
soils of corresponding clay and organic matter 
content. 
In order to study the influence of organic 
matter on the plastic number, the organic 
matter was removed from a soil of the Kalihi 
family by treatment with hydrogen peroxide. 
Samples of the treated and untreated soil were 
placed in percolation tubes, and the percola- 
tion rate of distilled water through the samples 
was measured. Since there is an observed 
effect of per cent magnesium saturation on 
the physical condition, the level of magne- 
sium was altered to determine the role of 
interrelationship between level of exchange- 
able magnesium and organic matter content 
on the impermeability of the soil. The data 
are presented in Figure 4. The removal of the 
organic matter increased the rate of percola- 
tion of water through the soil. Percolation of 
water through the sample possessing the or- 
ganic matter progressed at a steady rate, 
while in the untreated soil the rate of perco- 
lation was fast at the beginning and then 
became progressively slower. The soil with 
Fig. 2. The relationship between per cent mag- 
nesium saturation and plastic number of gray hydro- 
morphic soils. 
141 
Fig. 3. The relationship between content of organic 
matter in the soil and plastic number of gray hydro- 
morphic soils. 
low magnesium had the fastest percolation 
rate and the soil with the high magnesium 
level percolated in the same fashion as the 
untreated soil. The progressively slower per- 
colation of the untreated sample and of the 
soil having a high magnesium level can be 
attributed to an increase in the dispersion of 
the colloidal system. Thus, the effect of both 
organic content and high magnesium level 
appears to be on the state of dispersion of the 
colloidal system, which can be increased by 
the degree of hydration. 
NATURE OF THE CLAY FRACTION 
The chemical composition of the clay 
fraction of the gray hydromorphic soils is 
given in Table 2. There is a great similarity in 
the analysis of the clays of these soils. The 
silica content ranges from 38.1 to 41.2 per 
cent. Likewise, the aluminum oxide and iron 
oxide content of the clays of the three soils 
show very little variation. The low content of 
magnesium oxide in the crystal lattice rules 
out the possibility of magnesium substitution 
into lattice position being the cause of the 
plastic properties. 
The type of clay minerals in the clay frac- 
