Agriculture Reference
In-Depth Information
FeS
2
. When exposed to air, the pyrite oxidizes to produce sulfuric acid that reacts
with the chalk, and makes Fe more available to the vines (section 5.6.2). Also,
mineral matter in the lignite improves the soil texture. In more recent years, com-
posted organic wastes from the villages of Champagne have been added as mulch
to build up the soils.
Artificial drainage also modifies soil formation. Soils with hydromorphic fea-
tures form wherever water collects in natural depressions or where soils remain
wet for long periods as a result of high rainfall and low evaporation rates. Char-
acteristic hydromorphic features are (1) an accumulation of organic residues be-
cause of a very slow rate of decomposition in wet or cold conditions or both,
which eventually gives rise to
peat
, and (2) the chemical reduction of Fe
3
to more
soluble Fe
2
compounds, which produces a
gleyed
soil (typically blue-grey in color,
as in the clay subsoils of the Pomerol and parts of the Médoc, Bordeaux; see sec-
tion 9.3.2).
On removal of excess water by drainage, Fe
2
iron is reoxidized to the more
stable Fe
3
form, which precipitates as localized, bright orange and red hydrated
oxides, to give a characteristically
mottled
profile. Changes in soil due to drainage
are discussed further in section 1.3.4.
1.3.4
Relief
1.3.4.1
Catenas
The term
catena
(Latin for a chain) describes a suite of contiguous soil types ex-
tending from hilltop to valley bottom. The parent material of a soil catena may
be the same, with the soil profile differences due entirely to variations in drainage.
Other catenas may be on slopes carved out of two or more superimposed rock
formations. The soils on the upper slopes and escarpment are usually old and de-
nuded, whereas those on the lower slopes are younger, reflecting the rejuvenation
of the in situ soil by materials derived from upslope.
Where the parent material is uniform and permeable, the soils at the top of
the slope are freely drained with the water table at considerable depth; whereas in
the valley bottom the soils are poorly drained, with the water table near or at the
soil surface (fig. 1.11a). As drainage conditions deteriorate, the oxidized soil pro-
file at the top, with its uniform, warm orange-red colors, is progressively trans-
formed into an imperfectly drained mottled soil, and finally into the peaty, gleyed
waterlogged soil at the bottom (fig. 1.11b).
1.3.4.2
Slope and Aspect
Subtle changes in local climate and vegetation are associated with the slope and
aspect of valley sides and escarpments. At higher latitudes, slopes facing the sun
dry out more quickly than the reverse slopes. This affects the type and density of
vegetative cover, which in turn affects runoff. The steepness of the slope also af-
fects drainage and runoff. Soil moves on slopes, so a midslope site is continually
receiving material from sites immediately upslope by wash and creep, and con-
tinually losing material to sites below. In this case, the form of the slope is im-
portant, whether it is smooth or uneven, convex or concave, or broken by old
river terraces. Solifluction deposits under escarpments and on slopes are common
in Europe, as in the Rhine, Saône, and Rhone Valleys.
