In order to generate higher yields, farmers supply nutrients to crops in the form of fertilisers. One of the most important of these nutrients is nitrogen. Some of the supplied nitrogen leaves the farm in the form of foods such as grains, whilst the remainder ends up in the environment, which it may pollute in the form of ammonia, nitrate and nitrous oxide (see also Potential N-Losses).Nitrogen input minus nitrogen output yields the nitrogen balance. In the calculation method used here, losses to the environment are not counted as output and are therefore included in the balance.
Since some of the losses are unavoidable, the N-balance in yield-oriented agricultural systems should be positive in order to compensate for these losses, provide the plants with sufficient nutrients and ensure good yields. Losses to the environment can, however, be minimised by keeping the N-balance as low as possible (in other words, by avoiding over-fertilisation).
The N-balance is calculated based on the farm data from the SAEDN using the OECD method. This method records total nitrogen (Ntot), not only the plant-available nitrogen (Navail) as with the Suisse-Bilanz method. The aim, therefore, is not an even balance, but rather (as described above) as low an excess as possible.
In general, the cropping year, which begins after the harvest of the last main crop and ends with the harvest of the current main crop, serves as the reference period. Farmyard manures (slurry, dung), mineral fertilisers, recycling fertilisers (compost, digestate), seed, atmospheric deposition and biological N-fixation serve as inputs for N-balance. The calculation of atmospheric deposition uses a representative reference value for Switzerland as a whole. With biological N-fixation, a distinction is drawn between permanent meadows, temporary leys and arable legumes. The plant foods (e.g. bread grains, table potatoes) and feed (e.g. grass, hay, fodder grains) produced as well as the straw removed from the field represent the output from the system.