Agriculture Reference
In-Depth Information
farming systems are relatively insoluble in soil (IFOAM 2002) and the processes that govern
their capacity to release nutrients are mediated or improved by soil organisms (Lampkin 1990,
Stockdale
et al.
2001). Organic farming may alter the function of the soil microbial community,
increasing its ability to release nutrients from organic and poorly soluble sources, thereby com-
pensating for the absence of soluble nutrient inputs (Oberson
et al.
1993, Penfold
et al.
1995,
AQIS 1998, Ryan 1999). This may be related to a 'priming effect' (Welbaum
et al.
2004).
However, Cookson
et al.
(2005a) found that despite differences in microbial community com-
position and functional diversity, there was little difference in gross mineralisation rates
between the organic and conventional arable farming systems. Other field work showed that
there was no difference in the response of crops grown in organically and conventionally
managed soil to rock phosphate application, indicating that organic management had not
increased the availability of rock phosphate to crops (Dann
et al.
1996, Ryan and Ash 1999).
Further research is needed to scientifically investigate the exact nature of biological activities
under long-term organic and conventionally managed farming systems, especially in soils that
are highly weathered.
Practices that enhance function of arbuscular mycorrhizal (AM) fungi and root nodule
bacteria have potential to improve some aspects of soil biological and chemical fertility. Plant
breeding for greater dependency on these symbioses could be equally important (Smith
et al.
1992, Ryan and Graham 2002, Marschner and Rengel 2003). AM fungi have the potential to
access P in soil pores unavailable to plant roots (Schweiger 1993), but it is not known whether
this mechanism has a significant effect in overcoming P deficiency. AM fungi can also improve
plant availability of poorly soluble P sources such as rock phosphate and P adsorbed in soil
(Barrow
et al.
1977, Pairunan
et al.
1980) by enabling plants to have access to P as soon as it is
released into the soil solution. Under organic farming practices, the extent of root colonisation
by AM fungi may be greater due to decreased use of soluble P fertilisers (Ryan
et al.
1994,
Mäder
et al.
2000, Ryan
et al.
2000) and the diversity of AM fungi can be greater (Oehl
et al.
2004b). However, the benefits of AM fungi to plant production and sustainability are very dif-
ficult to measure (Jakobsen
et al.
2001) and appear to be environmentally dependent
(Thompson 1987, 1990, Ryan and Graham 2002, Ryan and Angus 2003).
It may be possible to selectively increase the abundance and activity of certain soil organisms
using agricultural inputs. For example, simple organic compounds such as sugars and complex
humic substances may stimulate microbial activity, leading to short-term increases in biological
activity and, potentially, to nutrient release and improved physical fertility (Welbaum
et al.
2004). Gleeson
et al.
(2005a, 2005b) demonstrated that specific fungi and bacteria occurred pref-
erentially on different silicate minerals. This observation is very significant if microorganisms
preferentially stimulated in this manner have beneficial effects on soil fertility. While further
understanding of the dynamics and diversity of soil biological processes is necessary in all
farming systems, a soil may have 'too much' biological activity. This can occur if organic matter
is continuously disturbed and exposed to rapid degradation, with subsequent loss of its value as
a slow release nutrient source and contributor to maintaining or improving soil structure.
Managing soil fertility in organic farming systems
Underlying principles
The practices used to manage soil fertility in organic farming systems should be understood in
terms of the aims and underlying principles on which they are based. This has been discussed
in various reviews of aspects of organic farming systems (Stockdale
et al.
2001, Shepherd
et al.
2002, Stockdale
et al.
2002, Watson
et al.
2002a) and in organic production standards (e.g. Le
