Environmental impacts of organic farming - Organic Agriculture: A Global Perspective - page 269

found that an improvement of microbial activity correlated with the period soils were farmed
organically. In comparison to conventional plots, several scientists found higher microbial
biomass (Mäder et al . 1996, Fließbach et al . 2001), higher microbial activity and diversity (Beck
1991, Fließbach and Mäder 1997, Fließbach 1998) as well as a higher efficiency in organic
carbon turnover in organic plots (Mäder et al . 1995). Hole et al . (2004) reviewed 14 studies that
investigated microbial communities under organic and conventional systems and found only
limited differences (Yeates et al . 1997, Shannon et al . 2002, Girvan et al . 2003). However, they
detected a general trend towards elevated bacterial (Bossio et al . 1998) and fungal (Yeates et al .
1997, Shannon et al . 2002) abundance/activity under organic systems.
According to Raupp (1995), who reviewed the results of several experiments in Germany,
Sweden, Denmark and Finland, the parameters of biological activity were inf luenced to differ-
ent degrees by types of organic fertilisers dependent upon type and quality of the applied
manure and agronomic techniques (crop rotation, soil tillage). Scow et al . (1994) found in the
Sustainable Agriculture Farming Systems (SAFS) Project (USA) that after 4 years, microbial
biomass levels were consistently higher in organic and low input systems than in conventional
systems, while plant parasitic nematode numbers were also consistently lower.
Concerning soil fungi, Elmholt (1996) found a larger number and abundance of sapro-
phytic soil fungi with a higher potential of decomposition of organic material. Furthermore,
the degree of mycorrhizal root colonisation was found to be significantly greater in organic
plots than in conventional plots (Mäder et al . 2002).
Shannon et al . (2002) reported recent work under United Kingdom (UK) conditions and
concluded that differences in the size, activity and diversity of the soil microbial biomass were
subtle, rather than dramatic. They found no consistent differences between organic and con-
ventional farming. A possible explanation for the differing results could be that the level of
biological activity changes very slowly in response to altering fertilisation levels and cultiva-
tion techniques. This could explain why no differences in microbial activity between organic
and conventional plots were observed in several on-farm investigations (Maidl et al . 1988,
König et al . 1989, Necker et al . 1992). Several authors state that any experiment trying to assess
these changes requires 8-10 years of post-conversion farming (Peeters and van Bol 1993, Rinne
et al . 1993).
Since 2002, the International Federation of Organic Agriculture Movements (IFOAM) has
regulated total copper input on organic farms to a maximum of 8 kg ha -1 y -1 . These restric-
tions acknowledge the potential for copper levels in orchard topsoil to accumulate with
repeated application. Van Zwieten et al . (2004) have demonstrated that copper residues origi-
nating from fungicide application reduce soil microbial biomass while stressing the microor-
ganisms. Therefore, it is necessary to better evaluate the potential impacts of copper
contamination in agricultural land. In order to reduce the bioavailability of existing residues,
management strategies and technologies need to be developed.
On ferralitic soils of the tropics and subtropics, mineralisation occurs much faster than on
soils typical of temperate and continental zones. Consequently, a high organic matter content
and high biological activity are essential for sustainable soil fertility. The beneficial impacts of
organic farming on biological activity, micro-organisms and soil organic matter content are,
therefore, particularly important for soils in the tropics and subtropics (Alföldi et al . 2002).
Soil structure
The environmental importance of a favourable soil structure lies in an improved resistance to
structural soil damage, such as compaction and erosion. Soil structure can be measured by
several physical parameters, such as the stability of aggregates, coarse pores, air capacity and
water holding capacity. The maintenance of a favourable soil structure is a major concern in