Appropriate Amount of Nitrogen Fertilizer for Shading Cultivation of Tea Tree Seedlings

Nitrogen fertilizers are essential for productivity and quality improvement of all crops. However, excessive nitrogen can cause physiological disturbances. Therefore, depending upon the type of crop and cultivation method, controlling the amount of fertilizer is important. Typically, the proper amount of nitrogen fertilizer is determined by the organic matter content of the soil in which the plants (tea plants in our case) are grown. However, there is no difference in the amount of fertilizer applied during shading which leads to serious problems. The current study investigated the physiological activity of tea plants according to nitrogen concentration in oil cake during shading cultivation. The physiological activity expressed via photosynthetic ability showed that the photosynthetic ability remained steady without being significantly affected by nitrogen content up to 9 to 12% under non-shading conditions, whereas nitrogen excess disorder started to occur as the treatment period increased under shading conditions. After 3 weeks of treatment, the leaf chromaticity was the highest at 67.67 for 4.5% oil cake under shading condition, while the highest was 64.19 for 12% oil cake under the non-shading condition. In terms of inorganic nitrogen content in the soil, the NH₄⁺-N content and the NO₃⁻-N content under shading were 1.3 times and 1.9 times higher, respectively than those for the non-shading condition. With increase in the nitrogen content of oil cake, the inorganic nitrogen (NO₃⁻N, NH₄⁺-N) also increased.The total free amino acid content was 614 mg·100g⁻¹ at 18% nitrogen content when not shading, and 1,073 mg·100g⁻¹ at 18% nitrogen content when shading was the highest. There was a 2.0-fold increase for shading (401 mg·100g⁻¹) compared to non-shading (200 mg·100g⁻¹) that was not treated with oil cake. Overall, the total free amino acid showed a tendency to increase as the nitrogen content increased, but both non-shading and shading showed a relatively decreased tendency for the straight oil cake (N 12%) treatment. Considering the growth and component content, the most effective nitrogen fertilizer was oil cake with 9% nitrogen content for non-shading cultivation and 4.5% nitrogen content for shading cultivation.