42 
PACIFIC SCIENCE, January, 1947 
any soil profile. In this profile, magnesium 
carbonate was found in the greatest quan¬ 
tities in the soils above the zone of gypsum 
accumulation. The portion of carbonates in 
the form of dolomite was found to be great¬ 
est in the 12- to 18-inch layer of soil, which 
corresponded to the zone of weakest effer¬ 
vescence with cold dilute hydrochloric acid. 
The molecular ratio of calcium carbonate to 
magnesium carbonate was 1.12 for this layer. 
The carbonates were considered to be in the 
form of dolomite, since if they were a mix¬ 
ture of calcium and magnesium carbonates 
the quantity of calcium carbonate present 
would have been sufficient to give a very vio¬ 
lent effervescence with cold dilute acid. Sher¬ 
man and Thiel (1939) found that when car¬ 
bonates of the soil were 87 per cent or more 
dolomite they gave a characteristic efferves¬ 
cence with cold dilute acid which was similar 
to that given by pure dolomite. Approxi¬ 
mately 94 per cent of the carbonates in this 
soil layer are in the form of dolomite. In the 
soil from the zone of gypsum accumulation, 
calcium carbonate constituted approximately 
80 per cent of the total carbonates. 
DISCUSSION 
This study has presented an instance of 
the dolomitization of a calcareous soil by the 
action of magnesium salts. The occurrence 
of dolomitization in soil, a process by which 
the double carbonate of calcium and magne¬ 
sium is formed, has been established by 
chemical analysis of the soil carbonates, by 
weakness of effervescence of the carbonates 
when treated with cold dilute hydrochloric 
acid, and by the presence of one of the two 
end products of the process in the soil, gyp¬ 
sum. The formation of dolomite in these 
soils is associated with the soil-forming pro¬ 
cesses, since the materials which make up the 
alluvial parent material do not contain dolo¬ 
mite as a mineral. The soil is rich in mag¬ 
nesium silicate minerals and thus provides a 
source of magnesium as soil weathering pro¬ 
gresses. 
The mechanism by which the dolomite is 
formed may need some explanation, since 
calcium exceeds magnesium in both the 
water-soluble salts and exchangeable cations. 
The soils of this area receive most of their 
precipitation during very short intervals of 
TABLE 4 
The Composition of the Soil Carbonates of a Lualualei Valley Soil Which Contains 
Dolomitic Carbonates 
DEPTH OF 
SOIL 
SAMPLES 
SO* 
PERCENT¬ 
AGE 
SATURA¬ 
TION 
WITH 
Ca-fMg 
CARBONATE ANALYSIS 
Efferves¬ 
cence in 
cold dilute 
HC1 
Total 
carbonates 
MgCOs 
Carbo¬ 
nates as 
dolomite 
Ratio 
Mols 
CaCOs 
to 
Mols 
MgCOs 
inches 
m.e./lOO gin. 
per cent 
gm. /100 gm. 
gm. /100 gm. 
per cent of 
soil 
soil 
total 
0-6 . . 
0.52 
96.9 
strong 
6.06 
1.50 
54.1 
2.56 
6-12 . . 
0.36 
96.6 
weak 
4.62 
1.35 
63.9 
2.03 
12-18 . . 
0.40 
97.5 
very weak 
3.44 
1.47 
93.9 
1.12 
18-24 . . 
0.41 
97.0 
weak 
0.87 
0.21 
51.7 
2.64 
24-30 . . 
10.10 
97.1 
strong 
0.84 
0.09 
23.8 
6.82 
30-36 . . 
30.12 
97.9 
very strong 
2.55 
0.23 
19.6 
8.60 
36-42 . . 
31.08 
98.0 
very strong 
3.67 
0.28 
16.6 
10.27 
