Gray Hydromorphic Soils — Gill and Sherman 
143 
zeta potential. This would call for an inherent 
characteristic of magnesium alone or in com- 
bination with unique surface phenomena of 
the type of clay mineral in the soil. 
Both the nature and amount of clay present 
in a soil play an important role in the plastic 
properties of the soil. The clay particles of 
the montmorillonite type of clay, which are 
plate-like structures, secure their plasticity 
from a small film between two particles 
which easily permits sliding along the axis 
of the longitudinal plane but requires a con- 
siderable force to pull the particles apart. An 
equilibrium of the component factors is set 
up in the clay fraction of the soil. The follow- 
ing are the component factors concerned: 
(1) Clay. The clay has its maximum plastic 
properties only when the clay is completely 
dispersed and when water is present in suf- 
ficient quantities to fill the space between 
all of the particles to a uniform degree of 
thickness. The thickness will determine the 
viscosity of the plastic state. The ability of 
the clay to attract water will be determined 
by the type of clay and its charge (cation 
exchange capacity) which in turn will be 
determined by the characteristics of its 
crystal lattice and internal and external sur- 
face area. 
(2) Exchangeable magnesium. The amount of 
exchangeable magnesium, which has the 
maximum observed potential dispersing 
power, is limited in its dispersing power by 
the amount of exchangeable calcium (a 
flocculating agent) present or by the 
amount of organic matter, which increases 
dispersive properties of magnesium by in- 
creasing its hydration. 
(3) Organic matter. The amount and nature 
of the organic matter present in the soil 
have a maximum potential as a co-dispers- 
ing agent. The dispersing power of the 
magnesium is modified by the amount of 
organic matter present in the soil. 
One of the observed characteristics of the 
gray hydromorphic soils is that exchangeable 
magnesium occupies 30 or more per cent of 
the cation exchange capacity. It has been 
observed that soils having this level of mag- 
nesium saturation always possess a high 
degree of plasticity when the calcium-mag- 
nesium ratio approaches equivalence. Since 
neither the per cent magnesium saturation 
nor the calcium-magnesium ratio is corre- 
lated with the plastic number of the soil, it is 
believed that neither of these factors is 
limiting. The per cent organic matter and 
the cation exchange capacity of the soil are 
correlated to a high degree to the plastic 
number. This would indicate that the two 
main factors involved in the development of 
the plastic properties are the amount of 
organic matter present and the inherent 
properties of the clay which is a montmoril- 
lonite type of clay mineral. The clay fraction 
of the gray hydromorphic soils does not 
possess an excessive electrical charge which 
can cause dispersion. The dispersion of the 
gray hydromorphic soils appears and disap- 
pears as the exchangeable magnesium, the 
organic matter, and the water hulls of hydra- 
tion are added or removed. 
SUMMARY 
The gray hydromorphic soils of the Ha- 
waiian Islands are capable of becoming dis- 
persed and plastic when the per cent mag- 
nesium saturation exceeds 30 per cent in the 
presence of organic matter, providing the 
ratio of exchangeable calcium to exchange- 
able magnesium is approximately unity. 
The calcium-magnesium ratio and the per 
cent magnesium saturation are not correlated 
with the plasticity of the soil. The content of 
organic matter and the cation exchange 
capacity of the soil are significantly related to 
the development of the plastic properties. 
A decrease in the amount of exchangeable 
magnesium present or the removal of the 
organic matter from these soils increases the 
rate of percolation of water through these 
soils, which indicates a decrease in its degree 
of dispersion. Since all soils do not respond 
