Effects of Nitrogen Concentration on Growth and Quality of Hydroponically Grown Purple Kale (Brassica oleracea var. sabellica)

This study evaluated the effects of nitrogen concentration in hydroponic nutrient solution on the growth and quality of purple kale (Brassica oleracea var. sabellica). The standard Knop solution (170.8 mg N/L) was used as the control, and six experimental treatments were established by increasing N levels to 1.5, 2, 2.5, 3, 3.5, and 4 times that of the control. The results showed that, 512.4 mg N/L (T4) promoted the best growth, with a plant height of 36.70 cm, 13.60 leaves, 68.93 g fresh weight, and 2.71 mg/dm² chlorophyll at 70 days after sowing. In contrast, 341.8 mg N/L (T2) yielded the best quality, with total organic acid content of 55.61 mg/g, vitamin C content of 58.66 mg/100 g, reducing sugar content of 4.46 mg/g, and anthocyanin content of 0.65 mg/g. Importantly, nitrate content at T2 (785.54 mg/kg fresh weight) remained below the European Union safety threshold. These results provide a scientific basis for optimizing nutrient solutions to balance growth and quality in hydroponic purple kale cultivation.