Agriculture Reference
In-Depth Information
improved compared to conventional practices (Locketertz
et al.
1981, Reganold
et al.
1987,
Reganold
et al.
1993, Lytton-Hitchins
et al.
1994, Siegrist
et al.
1998, Glover
et al.
2000, Shepherd
et al.
2002) due to the beneficial effects of increased organic matter inputs on soil organisms
and soil structure (Shepherd
et al.
2002). Organic matter does little to improve soil aggrega-
tion, without the activities of soil organisms (Watts
et al.
2001). Improved soil structure and
root growth may be critical in organic farming systems for effectively using soil reserves of
nutrients and poorly soluble fertilisers (Shepherd
et al.
2002), and for preventing nitrogen (N)
leaching from mineralising legume residues (Thorup-Kristensen 2001).
Organic farming practices do not always lead to improvements in all aspects of soil physical
fertility compared to conventional practices. In some situations, soil physical characteristics
are more dependent on soil type than on management (e.g. Drinkwater
et al.
1995). Droogers
et al.
(1996) found that despite higher potential productivity (Droogers and Bouma 1996) and
improvements in some measures of physical fertility, an organically managed soil had a higher
probability of being compacted than a conventionally managed soil unless the farmer timed
machinery traffic carefully. Although tillage practices can be similar for some organic and
conventional farming systems (e.g. Droogers
et al.
1996, Siegrist
et al.
1998), weed control
using cultivation in some organic farming systems can lead to soil structural decline, loss of
soil organic matter (Gosling and Shepherd 2005) and loss of topsoil by erosion (e.g. in inten-
sive horticultural systems).
Soil chemical fertility
The chemical fertility of soil ref lects its capacity to provide a suitable chemical and nutritional
environment to plants (Stockdale
et al.
2002) and to support biological and physical processes
(Abbott and Murphy 2003). The maintenance of soil chemical fertility in organic systems
depends strongly on processes that govern transformations from fixed to soluble forms of
nutrients (Stockdale
et al.
2002, Watson
et al.
2002a) such as mineralisation of organic matter
and dissolution of minerals. Although these processes also occur in conventional agricultural
systems, organic farms rely on them to a greater extent (Stockdale
et al.
2002).
Traditional methods for predicting fertiliser applications for building and maintaining soil
chemical fertility may not be appropriate in organic farming systems (Oberson
et al.
1993,
Condron
et al.
2000, Watson
et al.
2002a). Most tests of chemical fertility were developed to
predict nutrient release from highly soluble sources using relationships developed between
chemical extracts of soil nutrients and plant uptake (Price 2001, Watson
et al.
2002a). These
relationships may not be directly applicable to organic farming systems where:
1 nutrient sources are predominantly organic or poorly soluble and therefore slow to become
available;
2 nutrient availability is more dependent on dynamic soil biological processes (Watson
et al.
2002a); and
3 release and uptake of nutrients occurs without demonstrable changes in soil chemistry
because nutrients are rapidly taken up by plants or soil microorganisms without accumu-
lating in the soil solution.
For example, Drinkwater
et al.
(1995) found that in organic farms, although soil inorganic
N was one-quarter that of conventional farms, organic N mineralisation potential was three
times larger. Similarly, Oberson
et al.
(1993) concluded that routine soil tests would not have
been sufficient to predict phosphorus (P) fertility due to increased mobility of P ions in a bio-
dynamic system.
Options for monitoring soil fertility in organic farming systems may include ongoing (but
time-consuming) quantification of trends in soil and plant analyses (Watson
et al.
2002a).
