Geoscience Reference
In-Depth Information
5.3
DURATION OF PEDOGENESIS
Presented below are several methods for estimating the duration of
pedogenesis.
Let us assume the following data are available:
Average contents (t m
-3
) of silicon
Si
r
and aluminium
Al
r
of the
parent material of the profile,
Average
Si
s
and
Al
s
of the soil profile (t m
-3
),
Bulk density of the soil and of the parent material to convert
the analytical data from gravimetric percentage to volumetric
percentage values,
Composition of the water that passes through the profile
(lysimetric water) or composition of the drainage water (drawn
from the groundwater) and also the quantity that percolates
annually in mm y
-1
; put simply, this is the difference between
the annual rainfall and the annual potential evapotranspiration
(PET); from these data concerning the water are deduced the
quantities exported
Al
e
and
Si
e
per unit area (say m
2
) per year.
The reasoning used is what had earlier been developed by
Isaac Barshad, then clarified and completed by Lucas. Let
F
be the
annual descent (in m) of the weathering front and
T
the corresponding
thickness of the soil formed, also in m (Fig. 5.6).
Weathering balance (Lucas 1989)
S =
annual
subsidence
Soi
l
F
annual descent
of weathering front
T
= annual
thickness
of soil formed
Rock
Profile in year
n
Prof ile in year
n
+ 1
Fig. 5.6
Weathering model for calculation of duration of pedogenesis (Lucas 1989) placed
in the framework of the theory of progressive penetration.
