Agriculture Reference
In-Depth Information
New Zealand had higher annual losses than organic dairy farms. Farm comparisons in Europe
presented by Stolze
et
al
. (2000) show that nitrate leaching rates in organic farming in most
studies are significantly lower than those of conventional systems. However, if the nitrate
leaching rate is related to the output of grain and milk, organic systems tend to perform simi-
larly or even worse.
The farm level data are supported by the results of nitrate screening. Large-scale surveys in
water protection areas in Germany and Denmark indicate that organic farming results in a
lower or at least similar potential for nitrate leaching into ground and surface water. The
absolute values generally did not exceed critical levels. As a result of improved conventional
(integrated) management of mineral N fertilisation, or systems using extensification measures
especially in water reclamation areas with strict regulatory standards and extensive control
measures, differences between organic and conventional systems have become smaller recently
(Piorr and Werner 1998).
Even though scientific results from other climatic zones are scarce, positive effects of
organic farming on the nitrate leaching risk can be reported from a citrus farm in Cuba. Under
organic fertilisation management based on composting with 60 kg N ha
-1
, the farm achieved
exactly the same yield level as under conventional fertilisation management with 200 kg of
mineral N. This example shows that organic fertilisation management can help reduce the risk
of nitrate leaching, especially under extreme climatic conditions (Kilcher 2001).
Although not all the results showed that organic farming results in less nitrate leaching
than conventional farming, a strong tendency towards a decreased risk of nitrate leaching can
be deduced. Losses after ploughing of leys for instance, can be large. But the growing con-
sciousness of problematic phases in crop growth or cultivation has resulted in improvements
in organic management practices. The remaining differences may decline as conventional fer-
tiliser practices improve under increasing regulatory pressure (Stolze
et
al
. 2000, Shepherd
et
al
. 2003).
Phosphorus
The main pathway for P losses is by movement of soil particles (i.e. together with soil erosion).
Leaching is a much smaller and a more site-limited effect. Although the quantities of P lost
from farmland are usually small in agricultural terms, losses of a few kilograms of P per
hectare are sufficient to be of environmental concern (Shepherd
et
al
. 2003). Leaching is most
likely on deep sandy soils or high organic matter soils with little capacity to adsorb P (Sims
et
al
. 1998, Haygarth and Jarvis 1999).
Data on P leaching and runoff from organic agriculture are scarce. As nutrient balances for
organic farms rarely show a significant surplus of P, losses are assumed to be small (Edwards
and Withers 1998). A more reliable indicator to determine losses could be the differences in
the dominant loss pathways in livestock and arable farming.
Because of the relatively infrequent use of P and potassium (K) fertilisers in organic farming
(e.g. rock phosphate, sugar beet processing waste), organic systems have been criticised for
exploiting reserves of P and K built up by conventional farming (Nguyen
et
al
. 1995, Greenland
2000, Oehl
et
al
. 2002). Some authors have reported a decline in the concentration of extracta-
ble P and K in soils after conversion to organic management (Haraldsen
et
al
. 2000, Løes and
Øgaard 2000), but this decline does not always occur, even where budget deficits of P and K are
measured (Watson
et
al
. 2000). Gosling and Shepherd (2005) have researched long-term (over
15 years) changes in soil fertility in organic farming systems in England. Their results support
the argument that organic arable systems are mining reserves of P and K. Therefore, they state
that changes to organic management practices are required in order to increase inputs of P and
K, if long-term declines in soil fertility are to be avoided. In the case of broadacre organic
