## 4 Conclusions  
The mean values of the SPAD index of maize depended on nitrogen dose. When the dose was increased from 120 kg-ha-1 to 200 kg-ha-1 and constant P and K fertilization, the value of the leaf greenness index increased significantly.  
The plants of both maize varieties fertilized with nitrogen at a dose of 120 kg N\(\cdot\)ha\({}^{-1}\) without P and K were characterized by the lowest values of the SPAD index, regardless of the year of the experiment. When the nitrogen dose was supplemented with P and K, the plant nutrition with nitrogen improved significantly. The values of the SPAD index in maize plants of the variety Magitop were significantly higher than in the plants of the variety Delitop.  
The coefficients of correlation calculated for both maize varieties were highly significant at all measurement dates. Therefore, it can be concluded that the SPAD leaf greenness index can be used as an important, reliable, and non-invasive indicator for DMY prediction of maize biomass.  
Nutrition of maize plants with nitrogen proved to be the main determinant of the variability in biomass yield. For this reason, it was the basis for the verification of individual mineral fertilization variants.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Analysis Dates**} & \multicolumn{2}{c}{**Delitop FAO 240 Variety**} & \multicolumn{2}{c}{**Magitop FAO 270 Variety**} \\ \cline{2-5}  & **Regression** & **R\({}^{2}\)** & **Regression** & **R\({}^{2}\)** \\ \hline
1 st date & Y = 0.443x + 49.49 & 0.70 & Y = 0.164x + 120.08 & 0.86 \\
2 nd date & Y = 0.324x + 14.4 & 0.68 & Y = 0.125x + 141.83 & 0.69 \\
3 rd date & Y = 0.287x + 34.35 & 0.81 & Y = 0.141x + 129.5 & 0.75 \\ \hline \hline \end{tabular}  
Y—DMY of maize biomass, x—SPAD index value.  
\end{table}
Table 6: Straight-line regression equations describing the relationship between SPAD values measured in successive time points and DMY of whole plants.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{4}{c}{**Delitop FAO 240 Variety**} & \multicolumn{4}{c}{**Magitop FAO 270 Variety**} \\ \cline{2-9}  & **DMY** & **1 st Date** & **2 nd Date** & **3 rd Date** & **DMY** & **1 st Date** & **2 nd Date** & **3 rd Date** \\ \hline DMY & 1.00 & & & 1.00 & & & \\
1 st date & 0.94 ** & 1.00 & & 0.93 * & 1.00 & & \\
2 nd date & 0.92 ** & 0.96 ** & 1.00 & & 0.86 * & 0.96 * & 1.00 & \\
3 rd date & 0.95 ** & 0.94 ** & 0.97 ** & 1.00 & 0.88 * & 0.96 * & 0.93 * & 1.00 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Correlation matrix between the DMY of maize biomass and SPAD index values at successive measurement dates for Delitop and Magitop varieties (**-**-**\(p<0.01\); *-**\(p<0.05\)).  
The lack of phosphorus and potassium in the dose of mineral fertilizer limited the yield, regardless of the variety, as compared to the variant that was optimally balanced with nitrogen dose.  
Conceptualization, W.Z., B.W. and P.S.; methodology, W.Z., B.W. and P.S.; software, B.W.; validation, W.Z., B.W., H.B. and B.S.; formal analysis, W.Z., B.W. and P.S.; investigation, W.Z., B.W. and P.S.; resources, W.Z. and B.W.; data curation, W.Z., B.W. and P.S.; writing--original draft preparation, W.Z., B.W. and P.S.; writing--review and editing, W.Z., B.W. and P.S.; visualization, W.Z., B.W. and H.B.; supervision, W.Z., B.W. and P.S.; project administration, W.Z. and P.S.; funding acquisition, W.Z., P.S. and B.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
All data generated or analyzed during this study are included in this published article.  
The authors declare no conflict of interest.