PHYSICAL PROPERTIES OF HAWAII SOILS 7 
The classification of soils on the basis of color is significant because 
it gives information on the nature and degree of weathering and fur- 
nishes a clue to the probable chemical composition. 
It is a well-known fact that in the soil-formation processes increase 
in temperature and decrease in rainfall act in a similar manner in the 
evolution of various soil types. According to Lang (22, p. 336), 
with increasing temperature or decreasing rainfall, the other weather- 
ing factors remaining constant, the soil types developing are black 
(raw humus), brown, yellow, and red, and laterites, in the order 
named. Usually dark soils of humid regions indicate imperfect 
drainage, slow oxidation of organic material, and consequently an 
accumulation of humus ; whereas red soils and laterites are indicative 
of conditions favoring good oxidation. However, the original color 
of the parent material must not be lost sight of in these considerations. 
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Figure 4. — Type of vegetation responsible for the accumulation of organic matter in 
certain high elevations 
Robinson and Holmes (31, p. 25) show that the chemical compo- 
sition of the soil colloids is often indicated by their color and note 
that the soil and the corresponding colloid have the same color, 
differing only in intensity. Hence, there is a certain relationship 
between the color and the chemical composition of a soil. This 
has been observed in most instances with Hawaii soils also — gray, 
dark-brown, or black soils and colloids having a higher, and yellow 
and red soils and colloids a lower, silica-sesquioxide ratio. 
For instance, in Table 9, giving the silica-sesquioxide ratio for the 
different fractions of some important Hawaii soil types, samples 
Xos. 291, 292, 428, and 474 are dark or gray organic soils, whereas 
Xos. 392, 448, and 547 are red, yellow, and brown, respectively. 
Organic-matter content ranges from almost none in certain barren 
infertile soils to as much as 20 per cent in some mountain soils where 
on account of high rainfall, poor drainage, and dense native vegeta- 
tion existing prior to cultivation, there is an accumulation of organic 
material. Figure 4 shows the type of native vegetation that causes 
an accumulation of humus in certain mountain soils where high rain- 
fall and slow drainage do not permit rapid oxidation of organic 
