Effects of Nitrogen and Potassium Rates on Yield of Summer Chinese Cabbage

Chinese cabbage is one of the most widely grown leafy vegetables in China, a crop which can be grown year round. Nutrient management studies conducted in the past on Chinese cabbage were mostly targeted toward winter varieties. This project was started to determine the optimal K rates for some summer varieties of Chinese cabbage in Chongqing. The experiment consisted of six treatments including two N rates (300 and 375 kg N/ha), one P rate (90 kg/ha) and four K rates (0, 75, 150 and 225 kg K₂O/ha) replicated three times. Fertilizer sources used for N, P and K were urea (46% N), diammonium phosphate or DAP (11% N and 44% P₂O₅) and potassium chloride or KCl (60% K₂O). Nitrogen and K fertilizers were split applied at three times, viz., basal application at seeding and two top dressings at seedling and russet stages in a 30:30:40 ratio. Fertilizer P was applied only once basally.

Results showed that for each N rate, cabbage yield increased significantly with an increase in K rates without leveling-off. At 225 kg K₂O/ha, cabbage yields reached 41 t/ha at 300 kg N/ha and the highest yield of 42 t/ha was obtained at 375 kg N/ha. These yields were 4.7 t (13%) and 5.9 t (16%) higher than the yields obtained with K omission treatment. In terms of cabbage quality, the treatment with low N and low K obtained high contents of vitamin C, amino acids and soluble sugars, indicating that high quality does not always follow high vegetable yields. The treatment with high N rate and medium K rate achieved the highest N agronomic efficiency and N recovery. Minimum N content in leaves was observed six days after top dressing N. This indicated that a week after topdressing N might be the best time to apply the next dose of fertilizer N, especially during the fast growing period of summer Chinese cabbage in the region.