## 3 Results and Discussion
### SPAD Index
On average, regardless of the variety and year of the experiment, the lowest SPAD index values were noted in the plants fertilized with nitrogen at a dose of 120 kg N\(\cdot\)ha\({}^{-1}\) without P and K (Tables 2 and 3 and Figure 2). On average, they amounted to 571 in the first date, 585 in the second date, and 586 in the third date. When nitrogen fertilization was supplemented with P and K, the SPAD index value increased significantly in all the measurement time points--by 20, 13, and 29 units, respectively. This means that the supplementation of fertilization with phosphorus and potassium, which are key minerals, resulted in better uptake and use of nitrogen by plants [32; 33]. Figure 1: Boxplot of the SPAD index of two maize varieties in two years of the study (mean from fertilization treatments). When the dose of nitrogen fertilization was increased from 120 kg:ha\({}^{-1}\) to 200 kg:ha\({}^{-1}\), and the dose of P and K remained constant, there was a further significant increase in the leaf greenness index value. The fertilization with nitrogen at a dose of 160 kg:ha\({}^{-1}\) resulted in the average SPAD index of 612, 639, and 653 on the subsequent measurement dates. When the highest dose of nitrogen, i.e., 200 kg:ha\({}^{-1}\), was applied to maize plants, it \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fertilization**} & \multicolumn{4}{c}{**Variety Delitop FAO 240**} & \multicolumn{4}{c}{**Variety Magitop FAO 270**} \\ \cline{2-10}  & **1 st Date** & **2 nd Date** & **3 rd Date** & **Mean** & **1 st Date** & **2 nd Date** & **3 rd Date** & **Mean** \\ \hline
120 kg N:ha\({}^{-1}\) & 548 & 559 & 545 & 550 & 566 & 589 & 618 & 591 \\
120 kg N:ha\({}^{-1}\) + PK & 566 & 571 & 586 & 574 & 603 & 598 & 634 & 611 \\
160 kg N:ha\({}^{-1}\) + PK & 584 & 621 & 634 & 613 & 621 & 638 & 652 & 637 \\
200 kg N:ha\({}^{-1}\) + PK & 615 & 635 & 648 & 632 & 634 & 667 & 681 & 660 \\ LSD\({}_{0.05}\) & 6.268 & 1.413 & 2.706 & x & 4.828 & 5.827 & 5.413 & x \\ Mean & 578 & 596 & 603 & 592 & 606 & 623 & 646 & 624 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The leaf greenness index (SPAD) of two maize varieties depending on the fertilization variant in the first year. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fertilization**} & \multicolumn{4}{c}{**Variety Delitop FAO 240**} & \multicolumn{4}{c}{**Variety Magitop FAO 270**} \\ \cline{2-10}  & **1 st Date** & **2 nd Date** & **3 rd Date** & **Mean** & **1 st Date** & **2 nd Date** & **3 rd Date** & **Mean** \\ \hline
120 kg N:ha\({}^{-1}\) & 577 & 586 & 582 & 581 & 593 & 607 & 601 & 600 \\
120 kg N:ha\({}^{-1}\) + PK & 582 & 588 & 596 & 588 & 615 & 637 & 644 & 632 \\
160 kg N:ha\({}^{-1}\) + PK & 604 & 642 & 655 & 633 & 642 & 653 & 672 &resulted in their most optimal nutrition with this component. The average values of the greenness index readouts in both varieties and years of the study: 632 (1 st date), 660 (2 nd date), and 672 (3 rd date) (Figure 2) were the highest in this treatment. The results of our experiment were in line with the results of other authors [30; 31; 34; 35; 36] who fertilized maize with different nitrogen doses. These authors unequivocally stated that the amount of nitrogen fertilization was the most significant determinant of chlorophyll content in leaves expressed in SPAD units. [PERSON] [36] reported the lowest SPAD index values (305.5) in maize not fertilized with nitrogen, whereas the plants fertilized with nitrogen at a dose of 120 kg ha\({}^{-1}\) had the highest chlorophyll content, as evidenced by the SPAD index readouts (344.6). The author did not find statistically significant differences in the SPAD index values between nitrogen doses of 60 and 120 kg N\(\cdot\)ha\({}^{-1}\). The results of the research by [PERSON] et al. [25] showed that the highest SPAD index value measured at the phase of full flowering and milk maturity was observed when maize plants were fertilized with nitrogen at a dose of 240 kg\(\cdot\)ha\({}^{-1}\).