**5. Conclusions**

The study showed the occurrence of two different season-site N managemen<sup>t</sup> strategies, resulting in a significant diversity in SD, the primary yield complement decisive for the final seed yield of WOSR. The first strategy was based on a high indigenous productivity of soil, as revealed in two of the six sites (Ko, Do). The second strategy was based on a high productivity of applied fertilizer N, which was revealed in two sites of the best quality of soil with respect to the soil agronomy class (Ve, Bu). The amount of N accumulated in WOSR seeds at harvest, i.e., seed nitrogen (Nse) was found to be the key yield driver factor. Its yield forming function is determined, however, by the SD and the N content in seeds, which in turn depends on N supply during SFP. The simultaneous increase of both these components is crucial for yield increase. A sufficiently high amount of Nse in WOSR seeds during the onset of pod and seed growth can be achieved provided there is a net N release from its soil resources during the growing season, but especially during SFP. The PFPN is a useful tool for discrimination of site productivity with respect to Nsemax, treated as the prerequisite of WOSR yield evaluation.

**Author Contributions:** Conceptualization, R.Ł. and W.G.; methodology, R.Ł.; software, R.Ł.; validation, W.G.; formal analysis, W.G.; investigation, R.Ł.; resources, R.Ł.; data curation, W.G.; 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 publication was co-financed within the framework of Ministry of Science and Higher Education programme as Regional Initiative Excellence" in years 2019–2022. Project No. 005/RID/2018/19.

**Acknowledgments:** This research was in part supported by UCC Uralchem OJSC.

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