**6. Conclusions**

The sustainable intensification of agriculture is a concept which relies on the assumption of realizing two, seemingly contradictory goals, i.e., increasing food production but without a negative impact on local or global ecosystems. This twin objective can be achieved if solutions strictly oriented towards amelioration of the yield gap are put into agricultural practice, which will involve a reliance on increasing efficient nitrogen use. The inefficiency of nitrogen in the soil/crop plant system, i.e., the Nitrogen Gap, requires a three-dimensional diagnosis. The first, as a matter of fact, the major variability refers to the in-season variability in the nitrogen requirements of the currently grown crop. This gap can be filled by a synchronization of the demand of a grown plant for nutrients, in fact, driven by nitrogen applied in the required Nf dose and at the right time. The recorded inefficiency of in-season applied nitrogen is due to the spatial variability of its supply. The main reason for the occurrence of the NG is both spatial and vertical variability in soil characteristics responsible for N uptake and its subsequent utilization by plants. The recognition of the size and strength of a plant's growth constraints, the disturbing formation of yield, is the prerequisite for dividing a field into zones of homogenous levels of productivity. Because the capacity of soil to soil water storage depends on the content of colloidal particles, the first goal of soil fertility amelioration is to increase the humus stock. This production objective is especially important for field zones naturally poor in mineral colloids. The subsequent steps in the action oriented to soil fertility amelioration depend on a farmer's ability to recognize the level of the subsoil fertility, including the content of available water and nutrients, and on the access of the currently grown crop to these resources during the growing season. A diagnosis of the soil fertility status is important for the development of application techniques of both nitrogen and nutrients, supporting its use efficiency. Classic and remote sensing techniques should be simultaneously applied to delineate the field into units of the homogenous level of productivity. Remote-sensing is the most efficient tool for the in-season diagnosis of the N plant status, provided it can discriminate the productivity of each field unit.

**Author Contributions:** Conceptualization, W.G. and R.Ł.; methodology, W.G.; software, R.Ł.; validation, R.Ł.; formal analysis, W.G.; investigation, R.Ł.; resources, W.G.; data curation, R.Ł.; writing— original draft preparation, R.Ł.; writing—review and editing, W.G.; visualization, R.Ł.; supervision, W.G.; project administration, R.Ł. All authors have read and agreed to the published version of the manuscript.

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

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