Agriculture Reference
In-Depth Information
• thehistoricalperceptionof'SOMbene-
fits' by the scientific community, mainly
between the 18th century (at the time
when a distinction was drawn between
organic and mineral components) and
today;
• someexamplesofhowvarioussoci-
eties and religions perceive both soil
and SOM; and
• howfarmerspractisingvarioustypesof
agricultural interventions refer to SOM
and to its attributed properties.
of humus in the form of soluble carbon was
assimilated directly by plants (carbon hetero-
trophy) and was the almost unique source for
plant carbon nutrition. At the same time as
cited by Bourde (1967), several authors, for
example Priestley (1777), Ingen-Housz (1779),
Senebier (1782) and de Saussure (1804), par-
tially refuted these assumptions by demon-
strating both the gaseous origin of carbon and
the role of light in photosynthesis. None the
less, de Saussure (1804) still considered that
a small part of plant material was possibly
derived from soluble humus. Contradictory
debates arose on the topic, but many agricul-
tural scientists shared an intermediate point
of view and assigned functions to both SOM
and the air in plant nutrition. In particular,
this was the case for the famous German agro-
nomist, Albrecht Daniel Thaer (1752-1828),
known for the 'theory of humus' developed
in his seminal book,
Principles of Rational
Agriculture
(1809).
Thaer's principles contained some un-
verified theoretical concepts on plant nutri-
tion that served as a basis for the first rational
and systematic approach to fertilization
within the context of sustainable cropping
practices (de Wit, 1974; Feller
et al
., 2003).
Thaer's 'theory of humus' incorporated an
analysis of the management of soil fertility
as well as the concept of sustainability that
deserve particular attention.
Thaer's book was published during the
controversy about whether soil or atmos-
phere was the actual source of carbon used
by plants. Thaer did not deny that atmos-
pheric CO
2
could be a carbon source for
plants, but since this source seemed unlim-
ited, he considered soil humus and its man-
agement as the main limiting factor of plant
carbon nutrition. According to Thaer: (i) most
plant dry matter derives from the 'soil nutri-
tive juices' contained in the portion of soil
humus that is soluble in hot water; and
(ii) plant demand for 'juices' is selective and
varies according to the species cultivated.
Management of soil fertility must therefore
be based both on the management of soil
humic balance and on crop succession.
Although erroneous, these assertions
encompassed the whole soil-plant system
and were used to support the first quantified,
Historical Perception of
'Soil OM Benefits'
1
One of the first questions to be clarified in
relation to likely 'soil OM benefits' is the as-
sessment of the origin of plant nutrients that
have the greatest effects on soil fertility.
This discussion has been the subject of heated
scientific controversy since the end of the
18th century, up to 1840. The importance of
SOM in sustaining other soil functions be-
yond the provision of nutritive elements to
the plant was identified progressively there-
after. The role of SOM regarding the cap-
acity of the world's soil resources to deliver
agricultural and environmental services and
to sustain human societies at both local (e.g.
fertility maintenance) and global (e.g. mitiga-
tion of atmospheric C emissions) scales has
only more recently been established (Dewar
and Cannell, 1992).
Humus as a source of plant carbon
nutrition (1770-1840)
From ancient times until the 18th century,
many hypotheses were formulated about
the source of nutrients for plants, i.e. air,
water or soil. It was only after a distinction
had been made between mineral and or-
ganic components in the second half of 18th
century that 'humus' (or SOM) was con-
sidered a plant nutrient, in addition to other
sources.
Hassenfratz (1792a,b) - without referring
to experimental facts - argued that a fraction
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