FACTORS OF SOIL FORMATION - Midlands State University

depth of 10 in. under pine-heath associations and to 25 in. under a
mixed forest rich in mosses. In another locality, the sand deposit had
become covered with a mattress of raw humus, and enough
podsolization had taken place to permit a photographic recording of a
thin bleached A 2 horizon and a dark orterde zone (B horizon). Tamm
estimates that a normal podsol with 4 in. of raw humus, 4 in. of A 2
horizon, and from 10 to 20 in. of B horizon requires from 1,000 to
1,500 years to develop. Older soils, presumably 3,000 to even 6,000
or 7,000 years of age, do not exhibit horizons of greater magnitudes.
Evidently in these localities profile formation has come to a standstill.
We owe to Tamm a series of chemical data on podsols from
alluvial sand terraces the ages of which are known with considerable
certainty. In Fig. 20, an attempt is made to present the analyses in the
form of time graphs. The leaching factor β of the older profiles (β =
0.947) has values that are characteristic for podsols in general (Table
23, page 120). The behavior of the silica-alumina ratio (sa) of the soil
also is instructive. Compared with the C horizon, the sa value of the
bleached layer A 2 passes through a minimum at 100 years and then
tends toward a maximum that corresponds to a relative accumulation
of silica in the A 2 horizon. Tamm also determined the amounts of
limonitic iron that may be taken as an index of the translocation of
colloidal iron hydroxide. Bleaching signifies removal of Fe(OH) 3 ;
darkening or reddening of the soil layers results from accumulation of
FIG. 20.—Time functions for Swedish podsols.
this compound. To bring out the contrast more forcibly, the ratio
limonitic Fe 2 O 3 / total SiO 2 of the B horizon was divided by the same
ratio for the A 2 layer.
As may be seen from the lower solid curve in Fig. 20, there is in
the initial phase of podsolization relatively more limonite in A 2 than in
B, but at later stages the relationship is reversed. The magnitudes are
indicative of substantial shifting of iron compounds.
A notable feature, common to all three curves, is the declining
change of slope with increasing age. After drastic changes during the
initial phases of profile formation, the soil characteristics are tending
toward a more or less steady state, the equilibrium state or mature
profile.
In this connection, two recent papers by Aaltonen (1) and by
Mattson (13) are pertinent. Aaltonen studied the formation of the
illuvial horizon in sandy soils of Finland. He concludes that it grows
from the bottom up. In young soils, the colloidal particles are
flocculated at greater depth than in old soils. Consequently, during the
process of podsol formation, the portion of maximum colloid
accumulation moves upward (Fig. 21). The behavior of the A horizon
is not yet fully clarified. Aaltonen tentatively assumes that the