Agriculture Reference
In-Depth Information
potential to use excess N though practices such as catch crops (Thorup-Kristensen 2001) and
by intercropping (Hauggaard-Nielson
et al.
2003).
Rotations
Rotations facilitate processes that alleviate some of the fertility constraints to production that
are addressed in conventional farming systems by use of synthetic inputs (Lampkin 1990,
Reganold
et al.
1990). Organic rotations may include greater use of green manures and cover
crops (Reganold
et al.
1987, Drinkwater
et al.
1995), emphasise different regional crops (Lock-
eretz
et al
. 1981), and have longer pasture phases (Murata and Goh 1997, Derrick and Dumaresq
1999, Kirchmann and Bergström 2001, Deria
et al.
2003) than on conventional farms, leading
to higher plant diversity in space and time (Stockdale
et al.
2001). Rotations are also used to
manage soil physical fertility by emphasising inputs of organic matter from pasture phases,
green manures or cover crops.
Organic matter management
Organic matter incorporation into soil in organic farming systems is pivotal to increasing
chemical fertility and improving soil structure and this has been extensively investigated (Ryan
1999, Goulding
et al.
2001, Watson
et al.
2002a, Stockdale and Cookson 2003). Organic farming
systems emphasise frequent additions of diverse sources of organic matter from catch crops,
crop residues, manures, some forms of organic fertiliser and perennial crops (Reganold
et al.
1990, Drinkwater
et al.
1998). Numerous paired comparisons of organic and conventional
farms demonstrated that under organic farming practices, soil organic carbon (C) content
increased by up to 30% (Lockeretz
et al.
1981, Lytton-Hitchins
et al.
1994, Drinkwater
et al.
1995, Nguyen
et al.
1995, Droogers
et al.
1996) and total N content also increased (Bolton
et al.
1985, Reganold
et al.
1993, Drinkwater
et al.
1995, Muratah and Goh 1997, Liebig and Doran
1999). Examples of more substantial increases in soil organic C content (up to 200%) are
probably due to the comparisons being limited to a single organic and conventional farm
(Gerhardt
et al
. 1997, Jordahl and Karlen 1993) or to assessment immediately following large
inputs of organic matter (Wells
et al.
2000).
Apart from benefits to chemical and physical fertility, whether or not organic farming
systems increase organic matter inputs relative to conventional farming systems determines
their ability to increase soil biological activity (Ryan 1999, Stockdale and Cookson 2003).
Increases in the abundance, activity and diversity of soil organisms under organic manage-
ment are primarily caused by increases in the amount and quality of organic matter inputs
(Robertson and Morgan 1996, Yeates
et al.
1997, Ryan 1999, Stockdale and Cookson 2003,
Cookson
et al.
2005a,b, Hole
et al.
2005). In some situations, the higher soil organic matter
contents under organic farming increased microbial biomass and activity by increasing soil
water content (Robertson and Morgan 1996, Fraser
et al.
1998).
The difficulty in increasing soil organic matter content in some environments may threaten
the sustainability of organic farming systems. In semi-arid environments with high tempera-
tures and low precipitation, C and N inputs to soil may be low and organic matter content
unrelated to soil texture (Hassink 1997, Ryan 1999). Also, the degradation of organic inputs
may be very rapid in these environments if the soils are sandy, because they offer little protec-
tion to organic matter. Several Australian studies showed that compared to conventional
farms, organic management did not increase soil organic C in dryland grain-livestock pro-
duction in southern Australia (Penfold
et al.
1995, Derrick and Dumaresq 1999, Deria
et al.
2003). In contrast, soil organic C did increase in irrigated organic farming systems (Lytton-
Hitchins
et al.
1994, Wells
et al.
2000). The reasons organic farming may not always increase
soil organic C contents include (Gosling and Shepherd 2005):
