Agriculture Reference
In-Depth Information
organic farming systems. Favourable rooting conditions as a result of microbial activity and a
good exchange of water and air ensure improved spatial and chemical availability of nutrients
(Stolze et al . 2000).
Stockdale et al . (2001) reported evidence of increased aggregate stability under organic
farming (Jordahl et al . 1993, Gerhardt 1997, Siegrist et al . 1998). Reganold (1995) showed sig-
nificant differences in soil structure when 16 fields of biodynamic or conventional commercial
farms were compared in a paired study in New Zealand. There were also highly significant dif-
ferences in total topsoil C and a range of physical parameters (e.g. reduced bulk density and
penetration resistance and increased topsoil depth under organic and/or biodynamic farming).
Reganold (1988) undertook a similar paired study on a conventional and an organic farm in
the USA and again found improved physical properties under the organic system. In contrast,
Stolze et al . (2000) found that in most relevant long-term trials in Europe no significant differ-
ences in soil physical parameters, like macropore volume, bulk density and soil stability, could
be detected between organic and conventional farming systems (Meuser 1989, Alföldi et al .
1993, Niggli et al . 1995). Again, a long time of organic management and longer term changes
may be more important than short-term effects.
Erosion
Soil erosion by wind and water is assumed to be the main cause of soil degradation worldwide
(Oldeman 1994, Pimentel et al . 1995). The loss of fertile topsoil by erosion results in a lower
yield capacity, and in an undesired transfer of nutrients, pesticides and sediments in surface
water. Although erosion partly depends on site-specific risk factors, such as topography and
climate, the extent of damage by soil erosion can be limited by farm management practices.
The C-factor (tillage and coverage factor) describes soil losses on a slope relative to soil losses
at full fallow (Schwertmann et al . 1990) as figured in the Universal Soil Loss Equation (USLE).
Organic farming systems are usually characterised by a lower C-factor and a reduced erosion
risk because of the wider crop rotations (Stolze et al . 2000). In addition, soil management tech-
niques like organic fertilisation, mulching and cover cropping improve soil structure and,
therefore, increase the water infiltration and retention capacity, and thus reducing the erosion
risk substantially. These management techniques are particularly relevant on the porous ferra-
litic soils of the tropics and subtropics to reduce the soil erosion risk after heavy rainfall (Alföldi
et al . 2002).
However, highly effective soil erosion minimising measures like direct drilling and mulch
drilling can be found more often on conventional farms than on organic farms as these
measures require a herbicide-based management (Dabbert and Piorr 1999, Stolze et al . 2000).
Comparative data for erosion under organic and conventional systems are rare (Unwin et al .
1995). There are only a few studies and the most cited is that of Reganold (1988), who compared
the long-term effects (over 40 years) of organic and conventional farming on selected proper-
ties of the same soil on farms near Spokane in Washington, USA. The organically farmed soil
did not only have a thicker topsoil but also had a significantly higher organic matter content
and less soil erosion than the conventionally farmed soil. In the long-term DOK-trial carried
out by the Swiss FiBL (Fließbach et al . 2001) relevant soil parameters of conventionally and
organically farmed soils were compared. One of the results was that organic soil management
improved soil structure by increasing soil activity, thus reducing the risk of erosion (Alföldi et
al . 2002).
In conventional farming in the tropics, even f flat soil gets eroded as a result of the use of
herbicides and the lack of soil cover; however, organic farming can counter erosion success-
fully. In trials on a Cuban citrus plantation, the Cuban Citrus Institute and Swiss FiBL used
locally adapted leguminous crops and were able to restore degraded soils very quickly, to suc-
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