Tillage: how bad is it in organic agriculture? - Organic Agriculture: A Global Perspective - page 298

complexity and landscape diversification) may be different between conventional and organic
systems because the aims of these systems differ. In organically grown wheat, Petersen (2002)
showed that tillage technique (mouldboard ploughing v. chiselling) significantly affected the
distribution of Collembola populations over soil depth, but the functional implications of
these differences were unclear. In studies comparing the effect of alternative tillage systems on
biodiversity in organic systems it would be desirable to include parameters expressing the
quantity beside the quality of indicators and weigh their positive and negative functions
accordingly. For example, the value of a biodiversity indicator such as 'spontaneous vegetation'
(i.e. potential weeds) should be based on both quality (e.g. species richness, evenness) and
quantity (e.g. density, biomass) parameters, to also express the negative functions (weediness,
competition) potentially associated with them (Bàrberi 2002).
Conceptualising tillage management in organic agriculture
Tillage can have either a positive or negative connotation in organic agriculture depending on
the specific context, and especially on the climatic region where agriculture is conducted.
Socioeconomic and cultural factors may also inf luence organic farmers' attitude towards
tillage; here however, only some agronomically based concepts will be discussed. To date, sur-
prisingly few scientific investigations have tried to address the tillage issue with such a global
approach.
Since SOM build up or conservation is crucial to the success of organic systems, an apparent
SOM budget (inputs minus outputs) may be proposed as a reference indicator to guide tillage
choice and management in different environments (Table 1). If the SOM budget is negative,
then increased biomass supply (e.g. through cover crop use, cautious crop residue manage-
ment) or recourse to mixed farming systems should be coupled with minimum soil distur-
bance to reduce SOM mineralisation. If the SOM budget is neutral or slightly positive, then no
specific restrictions should be applied to tillage, besides those based on agronomic common
sense (e.g. tillage avoidance close to expected heavy rainfall to reduce the risk of nitrate leaching
and wherever possible, abandonment of heavy machinery use to protect soil structure). If the
SOM budget is decidedly positive (e.g. in some farming systems of cold temperate regions with
large amounts of soil-applied farmyard manure or compost) (Asdal and Bakken 1999), cautious
tillage management is again required to prevent groundwater nitrate pollution. This problem,
frequent in Northern European organic farming systems (Eriksen et al . 2004), has driven
research towards the development and application of reduced tillage in organic systems. Much
of this work has been done in Denmark. Henriksen et al . (2000) studied the application of the
Kemink exact soil tillage system to sugar beet production. The Kemink system is based on use
of non-inversion soil tillage, subsoiling, ridges, controlled traffic and low manure input. The
top 35-40 cm of the soil is loosened by a winged subsoiler before and/or during and after the
growing season. Kemink subsoiling prior to sowing stimulated sugar beet growth and increased
root yield by 7%, probably because of better soil tilth formation and increased N mineralisa-
tion. In contrast, root yield diminished when the Kemink subsoiler was passed post-emer-
gence, because of the likely indirect and/or direct damage caused to growing beet roots.
However, the effect of subsoiling on soil tilth is controversial even in the same environment
(i.e. Denmark). Munkholm et al . (2001b) showed that, compared to mouldboard ploughing at
22 cm depth (MP), subsoiling at 35 cm (S) + rotovation (S + R) resulted in a poorer soil tilth in
the surface layer (i.e. higher soil strength and a lower ease of fragmentation and friability
index), although it loosened the plough pan (the soil penetration resistance measured at field
capacity was about 1800 kPa in MP and <1000 kPa in S + R). Cropping system structure,
incorporating the inclusion of a grass ley in the rotation or differences in manure application