Agriculture Reference
In-Depth Information
Outside of temperate climates, it may be more difficult to maintain levels of soil nutrients
because manure is less readily available (Condron
et al.
2000). Several Australian studies have
reported that P availability limits production on organic livestock-crop farms and threatens
the sustainability of organic farming systems in Australia (Dann
et al.
1996, Derrick and
Dumaresq 1999, Ryan and Ash 1999, Deria
et al.
2003). Deria
et al
. (2003) noted that several of
the organic grain-livestock farms in their study did not use fertilisers and that those that did
used only small amounts of poultry manure (40 kg ha
-1
). In a comparison of organic and con-
ventional mixed livestock-cereal farms in New Zealand, the N budgets were balanced by the
use of legumes (Nguyen
et al.
1995), while P and S budgets were balanced on the farms that
used compost, rock phosphate and elemental S and negative on the farms that did not. Clearly,
this is an area that requires further research and highlights the potentially vital role of soil
microorganisms in organic farming systems if they can be managed to more effectively cycle
nutrients.
Considerable commercial attention is being given to development of microbial products
that can be added to soil to stimulate release of nutrients and benefit plant growth in organic
farming systems (Welbaum
et al.
2004). Many other forms of organic (plant, animal and
microbial) fertilisers are permitted, but most have not been scientifically investigated under
field conditions. Further research is required to validate the capacity of these materials to
enhance soil fertility in the long term and in combination with other management practices,
using well-replicated scientific studies, otherwise claimed effects will remain anecdotal.
Long-term effects
Several published long-term field trials have assessed aspects of soil fertility for a range of
organic and conventional treatments (Table 2.1). Although these studies cannot compare
management at a whole farm level, this has been addressed in studies using paired compari-
sons (e.g. adjacent farms). Examples of changes in physical, chemical and biological aspects of
soil fertility from some of the field trials are listed in Table 2.2. Drinkwater
et al.
(1995) showed
that differences in soil fertility were time dependent and were more pronounced in fields with
greater than four years of organic management compared to those with less than three. Gosling
and Shepherd (2005) suggested that ten years was insufficient to observe changes in soil levels
of nutrients.
Many of the proposed benefits of organic farming systems depend on their ability to
increase soil organic C (Table 2.1), but not all field trials have confirmed that organic farming
systems increase soil organic C content (e.g. the Roseworthy trial). The Californian apple trial
had stable soil organic C content despite C inputs of 2.1 t ha
-1
greater than the conventional
treatment over three years. In this case, measurements were taken in only the second and third
years of organic management (Werner 1997) and total organic C content can take many years
to change.
In the SAFS trial (Table 2.2), potentially mineralisable N was higher in the organic treat-
ment than the conventional treatment only after the incorporation of a green manure crop
(Gunapala and Scow 1998). Differences in organic matter quality between the organic treat-
ments were evident in the Rodale trial where the use of manure in the organic crop-livestock
treatment increased mineralisable C and N compared to the organic crop rotation (Wander
et
al.
1995 ). In the Rodale trial differences in organic matter quality between the organic treat-
ments were evident where the use of manure in the organic crop-livestock treatment increased
mineralisable C and N compared to the organic crop rotation (Wander
et al.
1995).
In the Apelsvoll trial (Table 2.2), biological activity increased during the pasture phases
(Breland and Eltun 1999). However, the Roseworthy trial showed very few differences between
systems in soil biological fertility (measured as root colonisation by AM fungi, microbial
