Geoscience Reference
In-Depth Information
The additions to the system correspond to receipts. If there is no
fertilization, these are partly
atmospheric fallout
(for example, dusts
containing calcium) and partly provided by
weathering of the parent
material
and coarse elements present in the soil. The outflows correspond
partly to
leaching of bases
by water passing through the soil and partly
to
cutting and export of wood
, if man intervenes.
Atmospheric
fallouts
Fixation
Vegetation
Cutting
and export
Restitution,
excretion
excretion
Mineralization
Withdrawals
Humus
Root
extraction
Soil
Weathering
Parent material
Fig. 2.23
Cycle of mineral elements in a soil under forest.
The balance may thus be written as
Balance
=
fallout
+
weathering
-
fixation
-
leaching
Annual values are considered, so that it is necessary to introduce
fixation in this balance, but not cutting and export, which occur more
rarely. This fixation is evaluated as
Fixation
=
withdrawals
-
additions
of litter
-
excretions
Table 2.8 indicates the order of magnitude of stocks and flows for
three major elements (Ranger
et al
. 1995; Marques 1996; Ezzaim 1997).
This kind of balance sheet is essentially established on the basis
of classic chemical determinations. But isotopic methods also can be
employed for estimating the contribution from the rock (weathering) at
least when it has a particular
87
Sr/
86
Sr ratio.
In Table 2.8 the total reserves in the soil (line 4), which correspond to
the primary minerals contained in the silts, sands, pebbles, stones and
substratum in the process of being weathered, are at least a thousand
times larger than the amounts fixed annually in the trunks (line 6).
Dynamics of bases
