Agriculture Reference
In-Depth Information
In areas where water is scarce (e.g. in the Mediterranean countries and in arid and semiarid
zones), excessive water use, particularly for irrigation, is a problem. National standards for
organic farming in Europe set up limits for irrigation in order to conserve water resources
(David
et
al
. 1996).
Nutrient use and balance
The general conclusion that can be drawn from the literature is that organic farms often have
smaller nutrient surpluses than conventional farms. Obviously, the same farm types need to be
compared because livestock farms often have higher nutrient surpluses. It follows that in
organic farming, the risk of water and air contamination as a consequence of nutrient sur-
pluses tends to be lower (Stolze
et
al
. 2000, Shepherd
et
al
. 2003).
Restrictions in organic standards include a ban of mineral N fertilisers and a limitation of
livestock density. More characteristic of organic farms, therefore, is that N tends to be a
minimal factor, particularly on arable farms. Since the opportunity costs to produce N on-
farm in organic systems can amount to from seven to 16 times the cost of mineral N fertilisers
(e.g. Stolze 1998), it is of particular economic interest to avoid N losses. As far as nutrient defi-
ciencies are concerned, Unwin
et
al
. (1995) argue that the medium-term effects of an unbal-
anced nutrient supply are likely to take the form of a reduction in economic performance
rather than environmental detriment.
Nitrate leaching
As a result of fertiliser or manure applications as well as N fixation by leguminous crops, N
accumulates in the soil. Nitrate leaching occurs when the amount of nitrate in the soil exceeds
the plant's requirements and when water from rain, irrigation or snowmelt moves through the
soil into the groundwater. Nitrate in water can lead to toxic contamination of drinking water
for humans and animals, as well as an eutrophication with excessive algal growth. The most
common parameters used are the nitrate leaching rate and the potential for nitrate leaching
(Stolze
et
al
. 2000).
Variation in leaching from individual fields is large both in organic and conventional agri-
culture. Many organic systems operate at a lower level of N intensity than conventional systems
because of lower stocking rates and fertilisation levels. Another reason for lower N losses in
organic farming is that their application is bound to organic manure and its corporation into
the soil. Straw-based manure, common in organic farming, reduces the nutrient availability
and the risk from run off in comparison to slurry. Other organic farming practices which
minimise losses are wide crop rotations, soil cover during winter, intercrops, underseeds and
fallows of several years (Nocquet
et
al
. 1996, Dabbert and Piorr 1999, Shepherd
et
al
. 2003).
Nitrate leaching in meadows, where herbage is removed from the field, are generally small.
Greater losses occur where pastures are grazed because of the large returns of N in excreta. The
f flush of N mineralisation following the ploughing up of leys is another feature of organic
systems that possibly increases the risk of nitrate leaching (Stopes and Philipps 1992, Scheller
and Vogtmann 1995). Leaching from arable land is increased where fertiliser rates exceed crop
requirements. In particular, losses are associated with the temporary nature of annual crops
and, sometimes, the lack of synchrony between release of N from organic matter and crop
uptake. Improving the fertility of organically farmed soils by building up the content of SOM
and incorporating organic residues and manures may increase this risk (Shepherd
et
al
.
2003).
Taking all these factors into account, overall leaching losses from organic farms tend to be
less than from conventional farms (Edwards
et
al
. 1990, Younie and Watson 1992, Eltun 1995).
Using a modelling approach, Condron
et
al
. (2000) found that conventional dairy farms in
