Agriculture Reference
In-Depth Information
ship between earthworm numbers and cover crops in vineyards is discussed in
chapter 7.
Earthworms are also important in soil formation in warmer regions with moist
conditions. But generally in warm regions, the activities of termites, ants, and
dung-eating beetles are of greater significance, particularly in the semiarid regions
of Africa, Australia, and Asia.
Human Influence
Humans have influenced soil formation through agriculture and settlement for
several thousand years. For example, continual cultivation to 20-25 cm transforms
the humus-enriched A horizon (Ah) of an undisturbed soil to a featureless “plow
layer” (Ap horizon) in which organic and mineral matter is well mixed. The nat-
ural soil structure is also greatly modified. Clearing the native vegetation changes
the course of soil formation. Cultivation for crops and grazing of grasslands pre-
disposes the soil to erosion. In some cases, the A horizon may be completely re-
moved, leaving the less fertile B horizon exposed, as in the case of many duplex
soils in Australia. Soils on limestone and chalk are particularly vulnerable because
on such parent materials, the soil profile is generally shallow and the rate of soil
accumulation very slow. Figure 1.10 shows vines growing on limestone fragments
with no soil cover in Languedoc-Roussillon, France.
Even under intensive cultivation, a soil profile can be enriched through the
input of organic residues and sometimes fine mineral materials. For example, the
thin soils on Cretaceous Chalk in the Champagne region of France have been aug-
mented for centuries with lignite from local Tertiary clays and silts. The lignite
(called
cendres noires
or black ash) is a soft, low-grade coal that contains iron pyrite,
1.3.3.2
Vines growing on limestone fragments near St Jean de Minervois, Languedoc-Roussillon.
Photograph by the author. See color insert.
Figure 1.10
