## 4 Discussion  
The effect of nitrogen sources had also previously been studied on Chinese Cabbage [25]. Ammonium nitrate, ammonium sulphate, calcium nitrate, ENTEC 26 and urea were tested by pre-seeding (150 kg N per ha) or split application (100 + 50 kg N per ha). The highest total yield of Chinese cabbage was found when urea was applied, which is in contrast to the results showed in this study. It was previously reported that the increase in the intensity of N nutrition significantly affected the yield of broccoli [26]. Although others have indicated that the N rate does not significantly improve the cabbage yield, it was observed that cabbage fertilised with the lowest N rate (196 kg\(\cdot\)ha\({}^{-1}\)) had the lowest yield value [27]. The correlation analysis of our results indicated that the yield of flowering Chinese cabbage was significantly correlated with the N content in the plants from pots (r = 0.87, \(p<0.05\)). Such a correlation was not observed in hydroponics. The increase in yield may be due to improved functioning of biochemical processes, including photosynthesis [28]. In previous studies, a strong relationship between increasing N content in plant tissues of leaves, leaf chlorophyll concentration and photosynthesis was found [29; 30]. However, in our study, the correlations between the yield and chlorophylls, as well as between the yield and the N content were statistically insignificant. As previously reported, the highest yields of flowering Chinese cabbage were achieved with N-90 ammonium nitrate [3; 4]. Similar results were observed in the present study for the pot cultivation system. However, the plants grown hydroponically had the highest yield at the N-70 dose of ammonium nitrate. Plant species are usually quite sensitive to excessive ammonium concentrations in the root zone when cultivated hydroponically in rockwood. The recommended doses of ammonium for hydroponic crops are lower (<14 mg N\(\cdot\)NH\({}_{4}\cdot\)dm\({}^{3}\)) than those used in our study for N-90 of NH\({}_{4}\)NO\({}_{3}\) (45 mg N\(\cdot\)NH\({}_{4}\cdot\)dm\({}^{3}\)). In spite of that, there was no visual effect of NH\({}_{4}\)\({}^{+}\) toxicity on the plants in the case of the highest dose of that fertiliser. This demonstrates the relatively high tolerance of this species to ammonium when grown hydroponically. The excessive ammonium concentration (high proportion of ammonium to nitrate, i.e., 75 to 25) reduced the biomass of flowering Chinese cabbage. At the ratio of 25% of ammonium to nitrate, the best balance between the plants' biomass and nutrient uptake was observed [31]. In our study, although the yield was lower at the N-90 dose than at the N-70 dose of NH\({}_{4}\)NO\({}_{3}\), the plant biomass was satisfactory. It had been previously observed that high N nutrition (above the N-90 dose) had no positive effect or even reduced the yield of flowering Chinese cabbage [4]. In field cultivation, the efficiency of applied nitrogen fertilisation is about 40-50%. According to [PERSON] et al. [5], the optimal N dose for purple Chinese cabbage was 440-490 kg/ha (i.e., 220-245 mg\(\cdot\)dm\({}^{-3}\)). In the case of soilless cultivation, as in this study, the yields achieved were satisfactory, in terms of both quantity and quality, with the application of almost three times lower doses. This could indicate the importance of hydroponic cultivation of plants for environmental protection as fertilisation can be significantly reduced in this system. Most plant species are quite sensitive to excessive sodium concentrations in the root zone [32; 33]. What is particularly interesting is that the application of NaNO\({}_{3}\) as an N source, regardless of the type of cultivation, did not significantly reduce plant yields in comparison to other N fertilisers. Along with this fertiliser, relatively high doses of sodium were introduced, which could indicate a rather high tolerance of flowering Chinese cabbage to sodium, which is the novelty of our study.