Agriculture Reference
In-Depth Information
1 the larger yields and more intense animal production in conventional systems increasing
organic matter inputs;
2 the smaller C:N ratio of organic leys causing them to decompose more quickly; and
3 more intensive cultivation for weed control.
Organic farming systems that fail to increase levels of soil organic matter cannot be
expected to increase overall soil biological activity. Further research is needed to investigate
the role of organic matter and soil biological fertility in the sustainability of organic farming
systems in semi-arid environments with highly weathered soils, especially in relation to organic
certification standards.
Fertilisers
The fertilisers permitted in certified organic farming systems, and in some cases the amount
that may be applied, are restricted by organic certification standards. They are loosely divided
into two categories: (i) naturally occurring geological resources (minerals) and (ii) organic
materials. Minerals permitted as fertilisers in organic farming systems include lime, gypsum,
rock phosphate, guano, elemental sulfur (S), dolomite and various ground silicate minerals.
Organic materials include those produced on farms such as green manure, animal manure and
compost, as well as off-farm sources such as fish, blood and bone meal, seaweed extracts and
microbial products. Complete lists of fertilisers permitted in organic farming systems can be
found in the various organic certification standards. Some restrictions may not be based on
scientific evaluations. For example, the allowable level of organic matter (e.g. as compost) may
be less than that required in poor sandy soils. In Australia, the National Association for Sus-
tainable Agriculture Australia (NASAA) limits off-farm manure and compost applications to
15 and 20 t ha
-1
y
-1
respectively (NASAA 2003) and the European Union standards limit
manure application to 170 kg N ha
-1
y
-1
(Watson
et al.
2002b).
Estimation of the relative effectiveness of nutrient sources can be a useful way in which to
estimate their efficacy as fertilisers (Barrow 1985). The relative effectiveness of alternative
nutrient sources is usually calculated by comparing the yield plateau of the response curve of
the fertiliser in question to a soluble source of the same nutrients (Barrow 1985). For minerals
used as nutrient inputs in organic farming systems their relative effectiveness is almost always
<1 due to low solubility in soil. Organic matter inputs can also be evaluated in terms of their
relative effectiveness based on their recalcitrance, but of equal importance is the extent to
which they are physically protected from degradation in soil aggregates (Strong
et al.
1999),
which would be different in different soil types.
Silicate minerals and rocks composed of silicate minerals are used in organic farming as
fertilisers and soil ameliorants. Silicate minerals are the main components of igneous and
many metamorphic rocks and vary in their composition and dissolution rates. Dissolution is
favoured by minerals of small grain size (Gillman 1980, Niwas
et al.
1987, Gillman
et al.
2001
2002,) and large surface area and is greater in soils with low pH, high moisture and tempera-
ture and soil solutions that are not in equilibrium with mineral surfaces (Harley and Gilkes
2000). Silicate minerals are most suitable as fertilisers in highly weathered, tropical soils where
acidic, nutrient deficient soils and heavy rainfall events favour dissolution and also in leaching
soils, particularly sands (Leonardos
et al.
1987, Coroneos
et al.
1996, Hinsinger
et al.
1996,
Harley and Gilkes 2000). In addition to a soil's capacity to dissolve silicate minerals due to
inherent chemical and physical characteristics, plants and microorganisms increase silicate
mineral dissolution (Barker
et al.
1997, Hinsinger
et al.
2001). They release organic ligands
which attack mineral surfaces and form complexes and lower soil pH by releasing H
+
ions and
organic acids into the soil. Nutrient uptake by plants prevents equilibrium between minerals