## 4 Discussion
[44], it was shown that Mg fertilisation increased the levels of N and P in plant seeds, which is consistent with our results. The authors found that Mg nutrition improves the Mg status of plants by significantly affecting the seed yield, seed quality, seed vigour (germination) and increasing the nitrogen and phosphorus uptake in oilseed rape plants. In our study, fertilisation with magnesium nitrate resulted in a positive effect of Mg on the N, K, Mg, Na, Fe, Mn and Cu contents and a negative effect on the P and Ca contents in the substrate system. However, statistically significant correlations were revealed only between Mg and N (r = 0.553, \(p\) = 0.032), Mg and K (r = 0.541, \(p\) = 0.37) and Mg and Zn (r = 0.515, \(p\) = 0.05) in the pot system. In the case of hydroponics, Mg had a negative effect on N (significant correlation) and a positive one on P, K, Ca, Mg, Mn and Cu in Chinese cabbage. In general, it can be assumed that the effect of nitrogen nutrition on the content of nutrients in plants can vary, and the relationships may be different for different plant species [45; 46; 47]. When calcium nitrate was used as a source of nitrogen, the rate of Ca (balancing for N) increased at the same time. According to [PERSON]'s chart, Ca has a negative effect on the uptake of P, K, Mg, Fe, Mn, Zn and Cu. In our study, the effects were different depending on the cultivation system, namely an increase in the Mg, Fe and Mn contents for the plants grown in pots and a decrease in the P, K, Mg, Fe, Mn, Zn and Cu contents for the plants grown hydroponically. [PERSON] et al. [12] indicated an increase in the content of such nutrients as P, K, Mg, Ca, and S in the leaves of pepper under increasing concentrations of urea. Such an effect was not found in our study in pot cultivation. [PERSON] and [PERSON] [34] observed that Ca and Mg concentrations were the lowest after the application of urea as the only N source. In our study, the reducing effect of urea on the N, P, Ca, Mg and Na contents of Chinese cabbage was confirmed in comparison to the other N forms when plants were grown in pots. The pigment contents (chlorophylls and carotenoids) could be related to the nitrogen doses, but the final response could vary depending on the season and type of cultivation of flowering Chinese cabbage [4]. In our study, in the pot cultivation system, the highest chlorophylls and carotenoids levels were obtained in the samples treated with urea at the N-90 dose, in the hydroponic system--in the samples treated with magnesium nitrate at the the N-70 dose. However, in pot cultivation, the pigment content was not correlated with the colour parameters (L*a*b*), whereas in the hydroponic system, Chl b correlated with L* (r = -0.65, \(p\) = 0.02) and b* (r = -0.72, \(p\) = 0.01). Generally, the pigment content in the pot system positively correlated with the content of Mn in the leaves, the microelement which is involved in the process of photosynthesis [48]. [PERSON] et al. [49] reported that the application of different N levels could affect pigment contents in crops, including Chl a and Chl b. It is well-documented in the literature that Chl a and Chl b are crucial pigments involved in the photosynthesis process in plants, and the optimal pigment contents can lead to better growth and yield [50; 51; 52]. In a study by [PERSON] et al. [53] who tested different nitrogen forms in nutrient solutions (monoammonium phosphate, potassium nitrate, calcium nitrate, ammonium sulphate, ammonium nitrate, urea and monosodium glutamate), it was reported that ammonium nitrate (151.277 mg/L) can increase Chl a and Chl b while urea (113.502 mg/L) can increase carotenoids and xanthophylls. Furthermore, [PERSON] et al. [54] found that the application of N concentration (ranging from 0 up to 12.6 mg/L) and N sources (Ca(NO\({}_{3}\))\({}_{2}\), KNO\({}_{3}\), CH\({}_{4}\)N\({}_{2}\)O) differentially affected the concentrations of photosynthetic pigments and biomolecules in Mexican marigold (_Tagetes erecta_ L.) in a hydroponic system.