Agriculture Reference
In-Depth Information
Guillou and Scharpé 2000, IFOAM 2002). To achieve the long-term goal of sustainability,
organic farming systems aim, as far as possible, to be self-sufficient for nutrients and organic
matter by producing and reusing materials on-farm. Therefore, practices that facilitate the
efficient re-use of nutrients and organic matter within the farm are stressed. Also, organic pro-
duction standards only permit non-synthetic fertilisers that are poorly soluble in the soil
solution. Nutrient management in organic farming systems is not simply replacement of
soluble fertilisers with insoluble fertilisers (Lampkin 1990, IFOAM 2002). To build and
maintain adequate soil fertility, organic farming systems must integrate management prac-
tices such as those discussed below.
The unscientific nature of the ban on manufactured inputs in certified organic farming
systems has been criticised (Kirchmann 1994, Kirchmann and Thorvaldsson 2000, Kirch-
mann and Ryan 2004), but the contrary practice of seeking to maximise crop yield using syn-
thetic inputs regardless of the environmental consequences is not scientifically based either
(Doran
et al.
1996, Leonardos
et al.
2000). The design of treatments in Wells
et al.
(2000)
shows how a farmer's bias towards achieving certain objectives from production inf luences
management choices. However, it is not the purpose of this review to justify the underlying
assumptions of organic farming systems, but rather to discuss their effects on soil fertility.
Although it has been claimed that organic farming systems are fundamentally different
from other systems, this cannot be substantiated from a soil fertility perspective. The manage-
ment of soil fertility in organic farming systems is variously characterised by mixed livestock-
arable systems, crop rotations, legumes, organic matter inputs and the use of fertilisers that are
not readily soluble in soil (Stockdale
et al.
2001); similar practices also characterise conven-
tional farming systems that address sustainability issues, including the use of legumes to
manage plant and animal nutrition in broadacre livestock-crop production (Puckridge and
French 1983) and crop and forage rotations to manage pests and soil fertility.
Livestock
Livestock can directly improve soil chemical and biological fertility by introducing organic
matter and nutrients in manure and urine (Watson
et al.
2002a). Although animal traffic may
decrease some aspects of soil physical fertility (Watson
et al.
2002a), the reduced tillage, dense
rooting, increased root exudation and soil organic matter content associated with pasture
phases (Nguyen
et al.
1995, Murata and Goh 1997) increases soil aggregation and biological
fertility and reduces erosion (Robertson and Morgan 1996, Breland and Eltun 1999, Eltun
et al.
2002). Livestock can also help to control weeds (Penfold 1997) which may reduce chemical fer-
tility by competing with plants for nutrients. The use of manure helps to achieve the aim of
self-sufficiency in nutrients and organic matter, but it is not always available in large quantities
(Condron
et al.
2000) and its management during storage and handling needs to minimise
gaseous and leaching losses of nutrients (Lampkin 1990).
Rotations that include livestock can be a key component of sustainable farming systems. The
retention of mixed farming in organic farming systems and the associated increases in spatial
and temporal habitat heterogeneity was partly responsible for increased diversity of organisms
observed on organic farms, from soil microbes to mammals and birds (Hole
et al.
2005).
Without livestock, it can be difficult to manage nutrients, particularly N, in organic farms
(Lockeretz
et al.
1981, Lampkin 1990, Fortune
et al.
2001, ,Stockdale
et al.
2001) but dairy farms
may be an exception to this because of their susceptibility to N losses (Watson
et al.
2002a).
Legumes
Legumes are a fundamental component of organic farming systems (in pastures, green
manures, cover crops or food crops) because they reduce or eliminate the need for external N
