PHYSICAL PROPERTIES OF HAWAII SOILS 
21 
This fact suggests the existence of large disparity between the ad- 
sorptivity of organic colloids and inorganic colloids. It further sug- 
gests the possibility that organic matter, or at least a part of it, may 
exist in the coarser-than-colloid fractions, such as silt, fine silt, and 
very fine silt fractions, and that the organic matter in these fractions 
shows power of adsorptivity. It matters little whether or not the 
organic matter in these coarser-than-colloid fractions is in ultimate 
particles or exists simply as colloidal aggregates or as coating around 
inorganic particles which resist successfully all efforts of dispersion, 
so long as they show power of adsorptivity. That large disparity 
exists between the hygroscopicity and water-holding capacity of the 
inorganic and organic soil constituents can be seen from Table 6. 
Table 6. — W^ter-holdbig capacity and hygroscopicity of certain soil 
constituents x 
Soil constituent 
Water-holding capacity according 
to— 
Hygro- 
Schubler 
Trommer Wollny 
Quartz sand... .. . 
Per cent 
25 
85 
87 
181 
Per cent 
26-32 
80 
70 
180 
Per cent 
19. 4-29. 1 
Per cent 

Limestone . . . 
1 
Clay (kaolin) . 
45. 9-55. 9 
106. 5-253. 6 
5-25 
Humus 
25-50 
i Mitscherlich (27, p. 138). 
Table 6 indicates that the moisture-adsorbing power of humus is 
on the average about two and one-half times as great as that of pure 
clay; the average water-holding capacity of humus about three and 
one r third times that of clay. This explains the fact that certain soils 
high in organic matter, such as many of the Hawaii soils, show 
high adsorptivity despite the fact that their clay or colloidal fraction 
is relatively low. On the other hand, it is easily possible for a soil 
with a high colloidal fraction to show a relatively low adsorptivity. 
As the organic-matter content of the soil increases the hygroscopicity 
rises rapidly. 
That this disparity in hygroscopicity between the various soil con- 
stituents may easily obscure the parallelism between amount of clay 
or colloids as determined in a mechanical analysis and the hygro- 
scopicity of the soil is illustrated by Lyon and Buckman (23) in the 
case of mainland soils. Table 7 shows the hygroscopic coefficient com- 
pared with other soil factors. 
Table 7. — The hygroscopic coefficient compared with certain other soil factors 1 
Soil 
Hygro- 
scopic 
coeffi- 
cient 
Clay 
Loss on 
ignition 
Humus 
Dunkirk silty clay loam (surface) 
Per cent 
3.80 
5.77 
18.90 
17.40 
Per cent 
12.9 
20.0 
20.1 
74.5 
Per cent 
5.08 
3.05 
14.54 
Per cent 
1.26 
Dunkirk silty clay loam (subsoil) . .- 
.20 
Clyde clay loam (surface) 
4.34 
5. 79 . 49 
• Lyon and Buckman (23). 
