Agriculture Reference
In-Depth Information
the least and most reactive (Léon
et al.
1986). However, the relative effectiveness also depends
on the particular soil and plant factors and can result in a rock phosphate being as effective as
superphosphate or nearly inert (Khasawneh and Doll 1978).
There is potential to increase the relative effectiveness of poorly soluble minerals by
managing soil biological processes. As already mentioned, plant availability of rock phosphate
can be improved by AM fungi (Barrow
et al.
1977, Pairunan
et al.
1980), and some soil micro-
organisms can solubilise P (Richardson 2001) and increase the dissolution of silicate minerals
(Barker
et al.
1997). Rogers and Bennet (2004) showed that in a P limiting environment, micro-
organisms selectively colonised the surface of minerals containing P. The ability of these proc-
esses to increase nutrient availability to plants might be maximised if specific practices and
inputs can be found to target and increase populations of soil organisms involved. Alterna-
tively, plant species and varieties may be chosen that cause the greatest mineral dissolution
(Wang
et al.
2000). Also, because the mechanisms by which plants cause mineral dissolution
can be stimulated by nutrient deficiencies, breeding plant varieties in suboptimal nutrient
conditions may produce varieties with greater capacity to dissolve minerals and to form asso-
ciations with more beneficial AM fungi (Hinsinger 2001, Ryan and Graham 2002, Marschner
and Rengel 2003).
Other procedures for increasing the effectiveness of silicate minerals and rock phosphates
involve altering the minerals themselves. High-energy milling (ball-milling at high energy
intensities) causes changes to mineral structure and bonding, and is acceptable under organic
production standards. Compared to unmilled rock phosphates, high-energy milling increased
the relative effectiveness of five rock phosphates by up to three times (Lim
et al.
2003). High
energy milling for 120 minutes increased the release of Ca and Mg from basalt and dolerite
from 2% to about 18% (Priyono and Gilkes 2004). The rapidly dissolved K from feldspar was
increased from 0% to 27% by 120 minutes of high-energy milling. Co-composting rock phos-
phates and silicate minerals (Garcia-Gomez
et al.
2002) may also increase their relative effec-
tiveness, by increasing their exposure to microbial processes that cause their dissolution, but
more rigorous research is needed to confirm this. Others have demonstrated that shaking
silicate minerals in organic extracts that are readily available to farmers, such as brewer's yeast
and cattle urine, increased the amount of Ca, Mg and K released from silicate minerals (von
Fragstein and Vogtmann 1983).
Very little work has been published about micronutrients in organic farming systems.
Condron
et al.
(2000) concluded that organic farming might be unsustainable on soils defi-
cient in micronutrients unless measures are taken to supply them in fertilisers. Seaweed
extracts can provide plants with micronutrients and opportunities exist to optimise their use.
However, little is known about the optimum time to harvest seaweeds or the nutrient content
of different species and their nutrient content is not high enough to meet crop demands
(Verkleij 1992, Edmeades 2002). Furthermore, they may stimulate plant growth and yield
through action of plant hormones rather than nutrients (Verkleij 1992). Micronutrient nutri-
tion of livestock can be maintained by growing pasture species rich in trace elements (Condron
et al.
2000), but this requires further investigation.
Organic farming has the potential to exploit soil reserves of nutrients and may, therefore,
be considered unsustainable (Derrick and Dumaresq 1999, Kirchmann and Ryan 2004, Gosling
and Shepherd 2005). In some cases, nutrients 'mined' in organic farming systems were previ-
ously added as fertiliser when the land was farmed using conventional practices. Published
nutrient budgets comparing organic and conventional farms have focused on N, P and K. They
indicate that it is possible to balance nutrient budgets in organic farming systems (Fortune
et
al.
2001, Watson
et al.
2002b), but that budgets are often negative because nutrients are not
adequately replaced (Gosling and Shepherd 2005, Nguyen
et al.
1995).
