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PACIFIC SCIENCE, Vol. VI, April, 1952 
characteristics, including a decrease in the 
percolation rate. Kudrin and Rozanov (1940) 
reported a magnesium . sierozem which was 
derived from both high magnesium parent 
materials and high magnesium ground waters. 
Such soils possessed a poor physical struc- 
ture, low porosity, and a generally poor 
physical condition. Von Kreybig (1935), 
while studying Hungarian soils, found that 
20 to 25 per cent magnesium saturation pro- 
duced soil properties which were similar to 
those of soils having a high sodium satura- 
tion. 
Several workers have reported evidence of 
magnesium salinization in Hawaiian soils. 
The Hawaiian soils have developed on basaltic 
rocks which are very rich in olivine, a 
magnesium-iron silicate. This would provide 
a source of magnesium for ground waters 
which would ultimately find their way to the 
areas of restricted drainage. McGeorge (1930) 
pointed out that exchangeable magnesium 
was increasing in certain soils where the irri- 
gation water contained appreciable amounts 
of magnesium salts. Hance and Stewart (1927) 
reported a low calcium-magnesium ratio in 
certain poorly drained Maui soils which have 
a high base saturation. This would indicate 
the presence of magnesium salts. Sherman 
et al. (1947) showed that dolomitization oc- 
curs in certain calcareous semi-arid Hawaiian 
soils where the ground water is rich in mag- 
nesium salts. Results of the soil research work 
at the University of Hawaii have established 
that, when a soil has more than 30 per cent 
of its exchange capacity occupied by mag- 
nesium, and the sum of the exchangeable cal- 
cium plus exchangeable magnesium amounts 
to more than 90 per cent of the exchange 
capacity, the soil will exist in a dispersed con- 
dition. All soils possessing this relationship 
are black, sticky plastic clays. The soils of the 
gray hydromorphic group are of this type. 
The purpose of this investigation is to 
study the factors which give rise to the plastic 
properties of the gray hydromorphic soils. 
The influence of such factors as cation ex- 
change capacities, per cent magnesium satura- 
tion, calcium-magnesium ratios, organic 
matter content, amount of clay-size particles, 
and the nature of the clay minerals on the 
plasticity of the soil were studied in this in- 
vestigation. 
METHOD OF ANALYSIS 
Soil samples were collected from typical 
soils of the three soil families belonging to 
the gray hydromorphic group. All of the soils 
used in this investigation were collected on 
the island of Oahu. Most of the samples were 
collected from Ewa, areas adjacent to Pearl 
Harbor, Wailupe, Koko Head, and the wind- 
ward areas on the north side of Oahu, 
The chemical analysis of the soils was made 
by standard methods. In the determination of 
exchange capacity and exchangeable cations, 
neutral normal ammonium acetate solution 
was used as the replacing agent. The water- 
soluble salts were removed by leaching the 
soil with a 40 per cent ethyl alcohol solution 
until the leachate was free of chlorides and 
sulfates prior to the ammonium acetate ex- 
traction. Alcoholic solutions of ammonium 
acetate were used on soil samples containing 
carbonates, according to the procedure de- 
scribed by Magistad and Burgess (1928). 
The plastic index or plastic number — the 
range of soil moisture over which the soil 
exhibits plastic properties— was determined 
by the method proposed by Baver (1932). The 
upper and lower plastic limits were deter- 
mined by this procedure, and the difference 
is referred to as the plastic number. 
Organic matter was determined in these 
soils by the method developed by Walkley 
(1935). 
CHEMICAL AND PHYSICAL PROPERTIES OF THE 
GRAY HYDROMORPHIC SOILS 
The cation exchange capacity, exchange- 
able cations, per cent magnesium saturation, 
organic matter content, and plastic number of 
typical soils of the three soil families of the 
gray hydromorphic group are given in Table 
