Sustainable Nitrogen Management in Agriculture: An Action Programme towards Protecting Water Resources in Walloon Region (Belgium) - Water and Agriculture: Sustainability, Markets and Policies - page 401

The reference value for the soil surface balance is calculated on the basis of the
different nitrogen fertilisers applied and their respective gaseous or de-nitrification
losses, of the nitrogen storage in humus in relation to the relative importance of
meadows in the total surface, the presence of catch crops, the efficiency of manure and
a quantity unavoidably lost in the soil profile. If the surplus determined for each farm
is superior to the reference value, it is an indicator that the nitrate pollution risk is high
for the considered farm.
By combining the two indicators (from the farm gate and soil surface balances), it is
possible to classify the farms in four categories:
Those that use nitrogen efficiently and present little risks of nitrate leaching
Those that use nitrogen efficiently but present a risk (due to the type of speculation
they undertake)
Those that could use better their nitrogen resources but present little risk, and
Those that could use better their nitrogen resources and present a risk of nitrate
pollution.
This typology allows prioritising the action on categories 2 and 4 presenting
important risks for the environment, with special focus on category 4 where it is possible
to lower this risk by decreasing the nitrogen inputs to the farm. For category 2, it is not
possible to reduce the nitrate leaching risk without modifying the speculation or lowering
the expected production volume. However, the implementation of a catch crop, the lower
use of organic fertilisers or the relative increase of the acreage devoted to grassland could
decrease this risk.
With these two balances, combined with the nitrate residue measures, the Walloon
region has not only an objective tool to assess the performance of the farmers in regard to
nitrate pollution, but also a clear indication of where the possible problem lies and how it
could be solved.
Water and agriculture: other potential challenges in Walloon region
Since the region uses its renewable water resources quite intensively, it is all the more
important that their quality be irreproachable.
That is why, on top of the nitrate pollution, the Walloon government investigates also
other issues linking agriculture and water, and particularly the problem of pesticides and
phosphates in our water tables.
On the side of pesticides, the main problem in underground waters concerns the
active substance atrazine and its metabolite desethylatrazine . Their concentrations vary
between 25 ng/l and 100 ng/l in a quarter of the sampling sites, and special activated
carbon filters have been installed in a few harnessing sites to make sure the concentration
decreases below detection level. Fortunately, this active substance was forbidden in 2002;
it was in fact far more widely used by the municipalities and the national railways
company than by farmers.