Geoscience Reference
In-Depth Information
cultural practices that leave the soil bare for too long, intervene with
adapted techniques (deep ploughing across the slope, beds separated
by vegetated protective strips). Overall, Luvisols are productive because
they are deep, rich in fine particles and have good water reserves.
But they are also fragile as they are susceptible to compaction and
to erosion.
8.5.2 Excess of Water
In the plains region under humid climate, presence of a poorly permeable
B horizon at shallow depth provokes formation of a perched water table
in the rainy season. The lands are then impenetrable to machines and
the autumn sowings could be flooded. In Western Europe, traditionally,
till the mid-twentieth century, one used to adjust to the problem by
cultivating in
ados
. With suitable ploughing, the lands were shaped
into ridges and furrows (Fig. 8.14). One part of the land was above the
perched water table, and the other part, sacrificed, was below it and
formed, lengthwise, runoff channels.
Portion of the land reserved for seedbed
seedbed
Length of
the field
Portion of land sacrificed
20-30cm
10 to 20 metres
Water table in winter
Evacuation of water
Fig. 8.14
Traditional layout of the ridges and furrows in soils with clayey subsoil.
But this system has gradually been abandoned because it restricts
the useful area and considerably hampers the movement of machinery.
Hence it has been replaced by drainage through buried pipes. Logic
would demand that the drains should be laid on the impermeable
(actually
slightly
permeable) layer, that is, the top of the B horizon. But
this is impossible because this layer is generally located at 50-cm depth.
Observations and calculations show that the rate of flow through the
drains becomes zero when the relative height of the water table midway
between the drains drops to less than 30 or 35 cm above them (Zimmer
et al.
1991). In other words, drains at 50 cm depth enable amelioration of a
layer of soil just 15-20 cm thick, which is insufficient. In these conditions,
it is necessary to place the drains
within
the poorly permeable layer itself.
Experience shows that they function well in these conditions because
the soil in the drainage trench would have been dug up when the pipes
