Tillage: how bad is it in organic agriculture? - Organic Agriculture: A Global Perspective - page 297

physical (e.g. structural) damage caused by overweighing machinery, especially when they
have excess soil moisture content and are exposed to traffic.
Another case in which tillage may be deleterious is when the climate is conducive to high
SOM mineralisation and thus to soil fertility depletion (e.g. in the humid tropics). Again, this
effect is not exclusive to organic systems; nevertheless, in such conditions any soil disturbance,
including mechanical weed control, should be avoided since it greatly accelerates microbial
degradation of SOM. For this reason, organic systems management in these environments
should always rely upon the regular application of organic amendments, surface mulching and
conservation tillage practices (Eyhorn et al . 2002). However, in this situation even these prac-
tices would be unlikely to increase SOM content considerably. It is well known that SOM
depletion makes soils frailer and hence more prone to erosion regardless of their innate erodi-
bility, thus any application of tillage under tropical conditions should always be carefully con-
sidered. In the arid Sudano-Sahelian area, however, regular manure application can partly
counteract the negative effects of tillage on SOM content (Mando et al . 2005).
In contrast, organic farmers in cold temperate regions could have the opposite problem
(i.e. insufficient nutrient release due to slow SOM mineralisation), thus some degree of tillage
may be needed to aerate the soil and thus enhance microbial activity. Besides considering the
already mentioned problems (e.g. pathogen and weed population build up), reduced tillage
and/or mulch-based organic systems would probably retard crop emergence and early growth
because of the well-known negative correlation between mulch and crop residue thickness and
the temperature of surface soil layers.
Environmental impact of tillage
In general, the potential environmental impact of organic plough-based systems is assumed to
be higher than those of systems based on non-inversion tillage, since the former are more det-
rimental to populations of beneficial arthropods and earthworms, and may increase nitrate
leaching, pest and disease problems (see When tillage is important in organic agriculture ).
However, appropriate crop rotation planning and timing of soil cultivation can greatly
reduce N leaching in organic systems. In the United Kingdom (UK), Stopes et al . (1996) observed
that about one-third of the total N accumulated by a red clover green manure crop (above-
ground biomass) was lost by leaching following cultivation in September before sowing a winter
wheat crop. In contrast, delayed cultivation until the next spring substantially reduced nitrate
leaching because the soil was protected by clover residues throughout the rainy winter season.
Deep plough-based organic systems may increase energy consumption in agroecosystems
(Kouwenhoven et al . 2002), which is another important indicator of environmental impact. In
contrast, mulch-based systems and systems in which manure is left on the soil surface or is shal-
lowly incorporated would enhance N volatilisation losses to the atmosphere. However, compared
to ploughing, shallow incorporation (rotovation) of an organic ley did not significantly increase
cumulative nitrous oxide (N 2 O) emissions from soil (van der Weerden et al . 2000).
There is little scientific literature available on the effect of different tillage systems on bio-
diversity in organic agriculture. Usually, the literature refers to comparisons between conven-
tional and organic systems in which almost invariably, biodiversity indicators (e.g. f loral,
faunal, habitat, landscape diversity) show more favourable values for organic than for conven-
tional systems (Stolze et al . 2000). Organic systems may also help preserve endangered wild
f flora species which in conventionally managed agroecosystems would otherwise be shifted
towards the brink of extinction (Albrecht and Mattheis 1998). However, these types of studies
often do not consider that the functional value of biodiversity indicators (e.g. field margin