Geoscience Reference
In-Depth Information
itself has been associated with the active surfaces of clay minerals.
12
Besides
surfaces, much in soils depends on the organic constituents. The rhizosphere, the
soil region in direct contact with plant roots, is a zone not only of increased
microbiological activity, but
uence
nutrient uptake and thus, the perceived fertility of the soil. SOM content is decisive
for water uptake and storage ability, in
its own chemical characteristics. These in
uences pore structure and microbial activity
and hence is crucial for the role of soils as sinks or sources of greenhouse gases.
Cultivation lowers SOM content. Agricultural techniques such as manuring or
plowing in stubble are geared at restoring SOM in cultivated soils.
3.2.1 Soils and Their Fertility
Agriculture intervenes into the biodiversity of ecosystems. It transforms them in a
planned way by management of the agro-ecosystem, e.g. by crop selection. It also
in
uences associated biodiversity, made up from organisms which colonize the
agro-ecosystem after it has been set up by the farmer. The combination of both is
responsible for ecosystem functions in an agro-ecosystem.
13
Much of this associ-
ated biodiversity is that of the soil, which only came to be recognized with the
development of soil microbiology in the second half of the 19th century.
14
One
cubic centimeter of soil can contain more than 1,000,000 bacteria. A hectare of
pasture land in a humid mid-latitude climate can contain more than a million
earthworms and several million insects.
15
Biological and chemical activity is
concentrated in the uppermost 10
15 cm of soil,
16
but there is more to soils than the
uppermost layer. Pedogenesis does not create uniform mixtures of particles. Most
soils are multi-layered,
-
over depth serve as the main discriminator
between soil types. Most existing overviews for a general readership give details
about soil types by pro
'
soil pro
les
'
le.
17
Questions of soil fertility are more important for the historian, as it is the pro-
ductive relation with the soil that is decisive in human history. Patzel et al. have
shown that the concept of soil fertility itself is not historically stable.
18
Nowadays
productive soils are conceptualized as systems governed by both natural and
anthropogenic factors. In Fig.
3.1
factors in
uencing yield in an agro-ecosystem are
shown. The natural fertility depends on the factors identi
ed by Jenny, of which all
but time are depicted, minerals and morphology being combined into one factor.
12
Orgel (
1998
).
13
Altieri (
1999
).
14
Eldor (
2007
), Berthelin et al. (
2006
).
15
Pidwirny and Heimsath (
2008
).
16
Coleman et al. (
2004
).
17
For example, Beach et al. (
2006
), Pidwirny and Heimsath (
2008
) and Nortcliff (
2009
).
18
Patzel et al. (
2000
).
