308 
parent materials of Hawaiian soils, basalt and 
trachyte, are relatively free of quartz, so that 
quartz could not accumulate in the A horizon; 
second, there is an accumulation of titanium 
oxide in the A horizon of Hawaiian soils. It was 
their assumption that titanium minerals are 
resistant to weathering and would, therefore, 
accumulate where quartz would normally be 
found in a podsolic profile. 
MEAN ANNUAL RAINFALL C INCHES) 
Fig. 1. The relationship between the kaolinite con- 
tent of the soil colloid and mean annual rainfall. 
Dean (1947) and Tanada (19 44) have 
shown that a high kaolinite content in Hawai- 
ian soils developed in regions receiving 25-35 
inches of rainfall annually and that the kaolinite 
content decreased with increasing rainfall. This 
relationship between rainfall and kaolinite con- 
tent is illustrated in Figure 1. Subsequent work 
has shown that the kaolinite content in certain 
Hawaiian soils (ferruginous humic latosols) 
decreases with age regardless of the amount of 
rainfall under which the soil has developed 
(Sherman et at., 1949). The results of work in 
our laboratories have supported the hypothesis 
that in the weathering of tropical soils two 
weathering processes are ocurring, namely, clay 
mineral formation ( kaolinization ) from the 
decomposition of primary minerals of the soil 
parent material, and clay mineral decomposition 
with free oxide accumulation from the decom- 
position of the kaolinite type of clay minerals. 
The recent publications from this Station ( Sher- 
man et al., 1949, and Fujimoto et al, 1949) 
PACIFIC SCIENCE, Vol. Ill, October, 1949 
have described soils belonging to the ferrugin- 
ous humic latosol in which the kaolinite clay 
minerals have undergone almost complete de- 
composition resulting in the development of 
soil horizons having a high concentration of 
iron and titanium oxides. Hematite, goethite, 
and anatase have been identified as the domi- 
nant minerals occurring in these horizons. 3 The 
decomposition of the kaolinite and the accumu- 
lation of these free oxides of secondary origin 
have occurred under a rainfall of 35 inches per 
annum and thus can be attributed only to age. 
Mohr (1944), in describing the factors 
which influence soil weathering in tropical re- 
gions, has placed considerable emphasis on the 
part played by rainfall distribution, profile drain- 
age, and age or time of exposure to soil form- 
ing processes. He has divided the types of rain- 
fall distribution into five groups, depending on 
the number of months receiving an average 
monthly rainfall of less than 60 mm. (which 
are classified as dry months) and the number 
of months receiving an average rainfall higher 
than 100 mm. (which are classified as wet 
months). His groups range from regions in 
which most of the months are classified as dry, 
to regions which are predominantly wet. Closely 
associated with the distribution of rainfall is 
the type of water movement in the soil. This is 
extremely important in tropical regions since 
the internal drainage soils may be restricted 
due to formation of impervious clay layers. 
Under appreciable rainfall this can produce con- 
ditions which result in the upward movement, 
from the zone of saturation, of a considerable 
portion of the water entering the soil and the 
lateral movement of water through the horizons 
of soils which have developed on slopes. 
Mohr ( 1944), in his discussion of the age of 
soils, recognizes five stages of soil weathering. 
The five stages of soil weathering which he feels 
every soil must pass through are fresh, juvenile, 
virile, senile, and "laterite.” According to his 
hypothesis, the end-product of tropical soil 
Cooperative studies with Dr. M. L. Jackson, Uni- 
versity of Wisconsin. 
