The Nmin System , the crop accompanying Nmin System (KNS) , and N-Expert are three methods for determining fertilizer requirement of field vegetables. These three systems have been developed sequentially. They build on each other and have a lot in common.
Nitrogen fertilization according to the Nmin system measures the mineral nitrogen (NO3-N + NH4-N = Nmin supply) in the soil layer that can be used by the roots and takes it into account when calculating N fertilization. Numerous field trials have shown that the Nmin supply of the soil is used by the plants as fertilizer nitrogen and therefore can be fully taken into account.
According to the Nmin system, the N fertilization requirement is calculated from:
+ Nmin target value - Nmin supply --------------------- = N-fertilizer requirement
The Nmin supply is determined by taking soil samples. The soil samples are taken from different soil depths, depending on how large the rooting depth of the plants is. The size of the Nmin target value depends on the type of crop to be fertilized and the expected yield. Nmin setpoints must be determined in fertilizer trials. However, these target values have so far been assessed only for a part of the commercially important vegetables. The reason for this is the high cost of carrying out experiments and the large number of cultivated species. In addition, more than one Nmin set point is required for some vegetables, as the fertilization needs to be adjusted to the cropping method or yield expectation. This is illustrated by the example of white cabbage: white cabbage produced for the fresh market has an N requirement of about 280 kg N ha-1. White cabbage grown for industrial processing can take up more than 400 kg N ha-1. Of course, this big difference has to be taken into account in N fertilization. Since vegetables are cultivated on soils with very different mineralization potentials, the derivation of an N-fertilization recommendation must also take into account the variability of N mineralization from the organic soil substance. Overall, there are so many factors influencing the need for N fertilizers that it seems impossible to carry out fertilization trials for all types of vegetables and for all cropping methods in order to determine Nmin target values.
As an alternative to performing experiments, it is possible to calculate Nmin target values as described in the crop accompanying Nmin System (KNS). With the KNS system, it is possible to calculate N-target values not only for the beginning of the culture, but also – crop accompanying – for any time points during plant growth, and thus flexibly adapt N fertilization to the needs of the plants.
The N-Expert system builds on the approaches of the Nmin system and the KNS system. However, in N-Expert – similar to the KNS system – the N target value is not determined in fertilizer trials, but calculated.
+ N in the crop + Minimum soil mineral N (Nmin) + N immobilization in soil organic matter + unavoidable N losses - N mineralization of soil organic matter - N mineralization from organic fertilizers - N mineralization from crop residues --------------------------- = N target value
Then, in N-Expert, as in the Nmin system, the N-fertilizer requirement is calculated by subtracting soil mineral N (Nmin) from the N target value.
+ Nmin target value - Soil mineral N (Nmin) --------------------- = Nfertilizer requirement