**5. Conclusions**

The rosette stage of WOSR growth (BBCH 30) was revealed as the cardinal for seed yield performance. The highest yield was obtained, provided two basic conditions were fulfilled. The first refers to the N-NO3 content at this particular stage, amounting to 150 kg ha−1. The second was the considerable decrease in the N-NO3 content during the stem elongation phase of WOSR growth. The yield was significantly dependent on the nitrogen fertilization system and increased in the order of NFS: M, O, OM. The maximum yields of 3.616, 3.887, 4.195 t ha−<sup>1</sup> were achieved given the optimum N rates of 138.9, 171.4, 210 kg ha−1, respectively. The key factors limiting yield were significantly related to the particular NFS. In the case of M-NFS, yield was limited by the availability of Ca and Mg, but especially of K in the deepest subsoil layer (0.6–0.9 m). In the O-NFS, the key limiting factor was

the shortage of N-NO3. The highest yield obtained in the OM-NFS was due to the positive impact of Ca and Mg on both the amount of N-NO3 and its productivity. During the second cardinal stage of WOSR yield performance, i.e., at the onset of flowering, Mg and to a lesser extent the contents of Ca in the subsoil both resulted in an N-NO3 content increase—a direct growth factor. In can be therefore concluded that the higher the content of available Mg in the subsoil at the onset of WOSR flowering, the better the supply and utilization of N by plants, resulting in the highest yield. PCA analysis was found to be an effective tool in discriminating against nitrogen fertilization systems differing in the source of N (mineral vs. organic).

**Supplementary Materials:** The following Tebles and Figures are available online at http://www.mdpi.com/2073- 4395/10/11/1701/s1, Table S1a. Spearman's matrix of correlation coefficients for the pooled nitrogen fertilization systems at BBCH 30 of WOSR growth, *n* = 30; Table S1b. Spearman's matrix of correlation coefficients for the mineral nitrogen fertilization systems at BBCH 30 of WOSR growth, *n* = 10; Table S1c. Spearman's matrix of correlation coefficients for organic nitrogen fertilization systems at BBCH 30 of WOSR growth, *n* = 10; Table S1d. Spearman's matrix of correlation coefficients for the organic-mineral nitrogen fertilization systems at BBCH 30 of WOSR growth, *n* = 10; Table S2a. Spearman's matrix of correlation coefficients for the pooled nitrogen fertilization systems at BBCH 60 of WOSR growth, *n* = 30; Table S2b. Spearman's matrix of correlation coefficients for the mineral nitrogen fertilization systems at BBCH 60 of WOSR growth, *n* = 10; Table S2c. Spearman's matrix of correlation coefficients for organic nitrogen fertilization systems at BBCH 60 of WOSR growth, *n* = 10; Table S2d. Spearman's matrix of correlation coefficients for the organic-mineral nitrogen fertilization systems at BBCH 60 of WOSR growth, *n* = 10; Table S3a. Spearman's matrix of correlation coefficients for the pooled nitrogen fertilization systems at BBCH 89 of WOSR growth, *n* = 30; Table S3b. Spearman's matrix of correlation coefficients for the mineral nitrogen fertilization systems at BBCH 89 of WOSR growth, *n* = 10; Table S3c. Spearman's matrix of correlation coefficients for organic nitrogen fertilization systems at BBCH 89 of WOSR growth, *n* = 10; Table S3d. Spearman's matrix of correlation coefficients for the organic-mineral nitrogen fertilization systems at BBCH 89 of WOSR growth, *n* = 10. Figure S1. Principal components analysis (PCA) of nitrate N, available nutrients content and yield indices at the onset of WOSR stem elongation for (a) total NFS, (b) M-NFS, (c) O-NFS, (d) OM-NFS; NFS—nitrogen fertilization system; N30—nitrate N content at BBCH 30 (rosette); PFPN30—partial factor productivity of N-NO3 at BBCH 30; P—phosphorus, K—potassium, Mg—magnesium, Ca—calcium; a, b, c—soil layers of 0.0–0.30, 0.30–0.60, 0.60–0.90 cm, respectively, Y—yield. Figure S2. Principal components analysis (PCA) of nitrate N, available nutrients content and yield indices at the onset of WOSR flowering for (a) total NFS, (b) M-NFS, (c) O-NFS, (d) OM-NFS; NFS—nitrogen fertilization system; N60—nitrate N content at BBCH 60; PFPN30—partial factor productivity of N-NO3 at BBCH 30; P—phosphorus, K—potassium, Mg—magnesium, Ca—calcium; a, b, c—soil layers of 0.0–0.30, 0.30–0.60, 0.60–0.90 cm, respectively; Y—yield. Figure S3. Principal components analysis (PCA) of nitrate N, available nutrients content and yield indices at WOSR ripening for (a) total NFS, (b) M-NFS, (c) O-NFS, (d) OM-NFS; NFS—nitrogen fertilization system; N89—nitrate N content at BBCH 89; P—phosphorus, K—potassium, Mg—magnesium, Ca—calcium; a, b, c—soil layers of 0.0–0.30, 0.30–0.60, 0.60–0.90 cm, respectively, Y—yield.

**Author Contributions:** Conceptualization, W.G., and R.Ł.; methodology, W.G., R.Ł.; software, K.K.; validation, W.G.; R.Ł.; formal analysis, R.Ł.; investigation, K.K., R.Ł.; resources, R.Ł.; K.K.; writing—original draft preparation, R.Ł.; writing—review and editing, W.G.; visualization, K.K.; supervision, R.Ł.; 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.
