16 BULLETIN 1311, U. S. DEPARTMENT OF AGRICULTURE 
combined H 2 0, 9.94 to 11.94 per cent. With two exceptions, these 10 
colloids came from Maryland and North Carolina. Compared with 
the wide geographical range which all the soil colloids analyzed rep- 
resent, this is considered a restricted area. This small variation in the 
composition of colloidal matter from a restricted area is the same as 
has been noted before in the analyses given in Table 1 by Puchner, 
by Hall and Russell, and by Robinson. 
Soil colloids vary relatively much more in the quantities of cal- 
cium, magnesium, potassium, sodium, phosphorus, chlorine, and 
organic matter they contain than in the content of the main con- 
stituents. The sum of the CaO, MgO, K 2 0, Na 2 0, S0 3 , and CI 
varies from 1.T8 per cent in the Norfolk fine sandy loam colloid to 
11.66 per cent in the colloidal matter extracted from the Yolo clay. 
It seems probable that the quantity of these constituents present 
bears an inverse relation to the degree of leaching to which the col- 
loids have been subjected. 
Soda is present in soil colloids in comparatively small amount, 
being under 1 per cent in all but two cases. Potash, however, is 
relatively high. This is in accordance with the idea that potash is 
adsorbed by soils to a greater extent than soda and the fact that 
drainage waters contain much more soda than potash, while the 
quantities of these constituents present in the earth's crust are much 
the same. 7 It is further noted that the soil colloids analyzed are, 
with only one exception, higher in magnesia than lime. The differ- 
ence is considerable. This also is in accordance with the composi- 
tion of drainage waters, which contain about four times as much 
lime as magnesia, the lithosphere containing 4.86 per cent CaO and 
3.74 per cent MgO. 
In considering relations between the constituents of soil colloids it 
may be said that in general the alumina and silica vary inversely, 
the silica being high when the alumina is low. The iron oxide shows 
no consistent variation with either the silica or alumina, although 
high alumina and high iron oxide are frequently associated. In 
general, the sum of the more soluble constituents of soil colloids are 
high when the silica is high and low when the silica is low. The sum 
of the more soluble constituents and also the silica generally bear an 
inverse relation to the combined water. Sulphur is present in 
largest quantities when the organic matter is high. 
Owing to these more or less regular variations, the colloids from 
different soils can be arranged into certain groups. The colloids 
high in silica and the more soluble constituents and low in alumina 
and combined water may be taken as one group ; the colloids low in 
silica and soluble bases and high in alumina and combined water 
may be taken as another group; and the colloids intermediate be- 
tween these two extremes may be taken as constituting a third group. 
There is no sharp distinction between these groups, and one group 
gradually merges into another. There are, of course, exceptions to 
this classification. For instance, some colloids low in silica are not 
especially high in alumina, and one colloid is high in silica without 
being especially high in soluble bases. 
7 The percentages of these elements in the lithosphere are as follows : KoO, 2.98 per 
cent; and Na 2 G, 3.75 per cent (Clarke, 8). 
