The autumn soil nitrate content reflects the amout of mineral nitrogen in soil before the leaching period during the winter season has begun. Sampling should be done after mineralisation of plant residues has been accomplished and before the leaching has started (NLWKN, 2007).
Autumn Nmin is the less expensive analytical control method for ground water protection measures. In Flanders, this indicator is used as indicator for nitrate leaching. It is part of the Action Plan of Flanders for the national transformation of the Nitrates Directive. In water protection areas in Germany, autumn Nmin is used for monitoring the catchment area (Osterburg and Runge, 2007).
Autumn Nmin refers to a single plot or field. The determined nitrogen amount may be subject of immediate leaching, and therefore functions as a good indicator for the concentration of nitrates in the leachate and for the amount of nitrates washed out. A disadvantage is the strong relation to site and year, so results may not be directly compared (Osterburg and Runge, 2007).