Agriculture Reference
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biomass C, earthworm numbers, loss of tensile strength in buried cloth) (Penfold et al. 1995).
Research is needed to determine the effect on the sustainability of organic farming systems in
semi-arid areas where it is difficult to increase soil organic matter contents or biological
activity. It is still unclear whether species diversity is, as is often proposed, critical to the integ-
rity and long-term sustainability of soil ecosystems (Welbaum et al. 2004).
Further research is needed to evaluate the claim that organic farming systems alter the
function of the soil biological community to compensate for the absence of soluble fertilisers
(Oberson et al. 1993, Penfold et al. 1995, AQIS 1998, Ryan 1999). In the SAFS trial, decompo-
sition rates in a laboratory comparison of organic and conventional soils were similar, but
cover crop decomposition was more consistent over time in organically managed soil
(Gunapala et al. 1998). Results from the DOK trial indicate that microbial cycling of P con-
tributed more to plant-available P in biodynamic systems compared to conventional systems
that receive synthetic fertilisers (Oehl et al. 2001 2004b). However, organic P mineralisation
contributed only 10% of the inorganic P released into the soil solution and the higher P min-
eralisation rate in the biodynamic soils only partly compensated for its lower P availability
(Oehl et al. 2004a). Other results from the DOK trial compared the biodynamic treatment
with the conventional treatment that also received manure. Despite both treatments receiv-
ing the same quantity of manure, the biodynamic treatment had larger functional diversity
(Fließbach et al. 2001), microbial C, incorporation of 14 C-labelled plant material into the
microbial biomass and it mineralised 58% of the added C, compared to 50% in the conven-
tional soil and unfertilised control (Fließbach et al. 2000, Mäder et al. 2002). However, differ-
ences between the treatments may have been as a result of the quality of the manure inputs or
herbicide application in the conventional treatment decreasing microbial measurements by
reducing weed populations (Fließbach et al. 2000). Previous studies have not supported the
hypothesis that organic management alters soil function (Dann et al. 1996, Ryan and Ash
1999, Cookson et al. 2005a).
Results of field trials (Table 2.2) suggest that the effect of organic farming systems on
physical fertility depend on the particular organic matter and tillage practices employed. The
Washington State apple trial (Glover et al. 2000, Reganold et al. 2001) attributed lower bulk
density in the organic treatment to the addition of compost and the use of tillage for weed
control. However, tillage may have also reduced soil physical fertility in the organic treatment
since aggregate stability was higher in the integrated treatment where compost was used
without tillage. In the Roseworthy trial, the reduced physical fertility of the integrated treat-
ment was attributed to tillage and stock trampling (Penfold et al. 1997). These differences
among trials illustrate the importance of practices being matched to climate, soil types and
production systems. In the DOK trial, the organic and conventional soils both received manure
applications, had the same rotation and almost identical tillage practices. The treatments
showed no difference in bulk density or the volume of total and large pores. The higher aggre-
gate stability in organic treatment was assumed to correspond to lower soil erodibility, although
measurements of splash erosion showed no difference (Siegrist et al. 1998). Lower rates of
erosion were found on organic farms in paired comparisons (Lockeretz et al. 1981, Reganold et
al. 1987) due to fewer tillage operations and the rotations used. In the Apelsvoll trial, soil
erosion was reduced by presence of perennial grasses in the pasture phases of the forage
systems, rather than by organic or conventional management (Eltun et al. 2002).
There were no measurable changes in plant available nutrients in the soil of the Apelsvoll
trial, but N, P and K budgets were increasingly negative in the order of conventional > inte-
grated > organic (Eltun et al. 2002). In this case, budgets may provide an early indicator of
potential sustainability of the farming system. The two organic treatments had the lowest N
leaching. The organic and biodynamic treatments of the Roseworthy trial showed declining
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