Tillage: how bad is it in organic agriculture? - Organic Agriculture: A Global Perspective - page 296

the difficulty of meeting timely crop nutrient requirements, especially in environments char-
acterised by high climatic variation. For example in Wales, Adams and Jan (1999) observed
that the optimum date (August-September) of incorporation of a clover ley differed between
years due to different rainfall patterns, which affected nitrogen (N) leaching during the germi-
nation and early growth phases of the subsequent ryegrass crop. In temperate climates, some
degree of biomass incorporation by tillage is usually required to trigger SOM mineralisation
and consequent nutrient release, especially in stockless organic arable crop rotations (Schmidt
et al . 1999). Drinkwater et al . (2000) showed that N release from vetch residues was quicker
when incorporated by chisel or mouldboard ploughing as compared to mow-killed vetch with
residues left on soil surface. In addition to crop growth and yield, biomass incorporation in the
soil facilitated weed control and reduced soilborne pest and pathogen loads in several cases
(Liebman and Davis 2000). However, this effect is controversial since in south-western Japan,
Hidaka (1997) observed that the abundance of Sogatella furcifera (Horvath) (a rice insect pest)
was much reduced while its most important predators (sedentary lycosids) increased in lower-
input organic farming systems based upon no tillage and winter legume surface mulch
compared to traditionally managed organic paddy rice. Similarly, van Bruggen and Termor-
shuizen (2003) claimed that SOM conservation through reduced tillage application is a key
element of integrated disease management because it favours the establishment of more equili-
brated soil microbial communities.
However, the main reason why some soil tillage is needed in organic systems is to facilitate
weed management. Most studies that have analysed weed community dynamics following
conversion from conventional to organic agriculture have shown increased weed problems
(Belde et al . 2000), especially when reduced or no-till systems were adopted compared to
organic plough-based systems (Gruber et al . 2000). In particular, no-till increases the abun-
dance of grasses, perennial weeds and anomochorous (wind-dispersed) species (Zanin et al .
1997). These species are particularly favoured by reduced soil disturbance because of a lack of
primary dormancy, vegetative propagation and possession of ecological traits favourable to
site recolonisation, respectively. The same effect is usually observed in reduced or no-till con-
ventional systems, but direct (e.g. mechanical, thermal) weed control methods applied in
organic systems are usually less effective than chemical herbicides. This reduces the selection
pressure against weeds and hence the risk of undesired evolution in weed community compo-
sition (Bàrberi 2002), but accelerates the build up of weed seedbank densities that can more
often turn into competitive weed stands. Also, minimum tillage coupled with organic fertili-
sation can increase the abundance of troublesome arable weeds like Galium aparine (McClos-
key et al . 1996). It can then be assumed that, at least in temperate regions, management of
organic systems without returning to tillage from time to time would soon become unsustain-
able due to intolerable weed pressure, although this negative effect could be retarded by the
adoption of an appropriately diversified crop rotation (Rasmussen et al . 1999).
When oer-reliance on tillage can be troublesome
Whereas tillage may be necessary in organic systems, there are other cases in which the negative
effects can overcome the benefits. This is the case in soils highly susceptive to erosion (e.g.
sloping soils) and compaction (e.g. silty or heavy clayey soils). Pulleman et al . (2003) observed
that soil compaction, although lower than in a conventionally managed arable crop rotation,
was all but negligible in an organically managed one. It is then incorrect to assume that amel-
ioration of soil fertility parameters, as typical of organic management (see Conceptualising
tillage management in organic agriculture ), would always prevent these soils from undergoing