310 
PACIFIC SCIENCE, Vol. Ill, October, 1949 
REDDISH 
BROWN 
LOW HUMIC 
LATOSOL (A) 
LOW HUMIC 
LATOSOL (B) 
FERRUGINOUS 
HUMIC LATOSOL 
LATERITE 
CRUST 
PRIMARY 
MINERALS 
CLAY 
FORMATION 
CLAY MINERALS 
DECOMPOSING 
FREE OXIDE 
ACCUMULATION 
FREE OXIDE 
SOIL 
YOUTHFUL 
VIRILE 
VIRILE 
SENILE 
LATERITE 
Fig. 2. The influence of the lengthening of the wet season of an alternating wet and dry season climate on 
the chemical composition of the A horizon of the soil. The wet season increases in this series of soils from 
left to right. (A) and (B) profiles are two different soils belonging to the low humic latosol. 
fact that the soils differ greatly in their chemi- 
cal composition and physical properties would 
indicate that the distribution of rainfall must 
play an important role in the type of weathering 
which has occurred in the soil. 
CHEMICAL COMPOSITION OF SOILS 
DEVELOPED UNDER A 
DEFINITE DRY SEASON 
The chemical analysis of several soil profiles 
belonging to the low humic latosol and fer- 
ruginous humic latosol to determine their "ma- 
jor” oxide content was made by methods de- 
scribed by Piper (1944). The soils were 
selected to represent the following successive 
stages of weathering: (a) the youthful soil 
containing most of its primary minerals; 
(b) the peak of clay formation — kaolinization; 
(c) the stages showing the cessation of clay 
formation and the increasing oxide formation or 
clay mineral decomposition; and (d) the final 
end-product of weathering— -the free oxide soil. 
With the exception of the end-product stage, 
these stages of soil weathering are found under 
increasing rainfall. The end-product stage— the 
oxide soil— exhibits evidence of lateral move- 
ment of water through the soil solum. Like- 
wise, the stages of soil weathering exhibit the 
progressive effect of the shortening of the 
drought season of these soils. 
The results obtained from the analysis of soil 
samples from the A horizons of the selected 
soil profiles presenting successive weathering 
stages are shown graphically in Figure 2. The 
data show a steady decrease in the silica content 
of the A horizons with the advancement in the 
weathering of the soil Alumina content of the 
soil increases with the clay formation and de- 
creases rapidly as the clay minerals decompose. 
The iron and titanium oxides accumulate as the 
