Stikstofwerking van organische meststoffen en hun relatie met gebruiksnormen

Manures supply nitrogen (N) to crops in the year of their application but also in later years. This mineralisation extends over several decades. Regular manuring therefore results in a gradual increase of the N fertilizer replacement values (NFRV) of manures. However, prescribed NFRV’s in the present regulations refer to ‘first year’s’ N supply in most cases. In view of the common regular applications of manures, it seems reasonable to consistently adopt long term NFRV’s in recommendations and regulations. We hence propose to increase the NFRV of liquid manures, including slurries, to 70-80% and those of solid manures to 55-75%. Attention is required for possible ‘double counting’ because residual N-effects of manures have in some cases become confounded with current crop-related N-recommendations.If the composition of a manure or the conditions under which manures are applied would change, a simple set of rules allows to make an informed estimate of the NFRV. However, there still is a need for a better insight into the synchronisation between mineralisation and the seasonal N uptake pattern of crops. Besides, we need to know more precisely when and where manures stimulate the production of elementary N and nitrous oxide.As far as policies and regulations are concerned, there are close relationships between legal NFRV’s and the crop specific N application standards. The premises behind the definition of N application standards of grassland and silage maize differ from those of other crops. That implies that the proposed raise of NFRV’s justifies a proportional raise of the N application standards for grassland and silage maize as that has no negative environmental consequences. For any other crop a change of NFRV’s does not justify a moderation of the N application standard