**3. Conclusions**

A particular field is a basic unit for crop production. The key challenge for farmers and their advisors is to define and then elaborate a set of agronomic measures oriented towards covering the nitrogen gap. This is the most effective way to realize the 'twin objectives' of sustainable crop production managemen<sup>t</sup> within a particular field. The spatial variability of yields can be ameliorated, at least, by the division of a field into homogenous productive units. The criteria of a field's division can be based on physical or chemical soil properties, including the whole zone rooted by grown plants. They can also be delineated using satellite spectral images of both bare soil and crop biomass in a well-defined stage, which is decisive for yield component development. The developed agronomic decision support system (ADSS) should be adjusted to the yield potential of the homogenous field unit in order to optimize the efficiency of the applied inputs, mainly focusing on nitrogen fertilizer.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
