NO3− Requirement and the Quantitative Management Method of Nutrient Solution Based on NO3− Supply in Hydroponic Culture of Radish Plants

The absorption of NO3 and growth of radish (Raphanus sativus L. ‘Yukikomachi’), and the timing and interval of NO3 supply were examined to evaluate quantitative nutrient management (QNM) of nutrient solution in a hydroponic culture of radish plants. In experiment 1, the amount of NO3 required for growth to a marketable size (30–35 g FW of thickened axis) was presumed to be approximately 1000 mg/plant by the direct measurement of NO3 absorption of radish plants grown with the EC-based control management method (EC-based control method) of nutrient solution containing different concentrations (2, 4, 6, 8, and 10 me·L-1) of NO3. In Experiment 2, plants were supplied with the total amount of NO3 (1000 mg/plant) at the beginning of the experiment or with 1/5 of the total amount of NO3 (1000 mg/plant) repeatedly 5 times every 4 days, and then their fresh weight and nutrient absorption were compared with the plants grown with the EC-based control method. Significant differences in the growth of thickened axes and leaves were not obtained among plants grown by three different methods. However, plants appeared to be supplied with an excess amount of nutrients because EC and NO3 levels were high at the end of cultivation. From the experiment in which plants were supplied with the whole amount of mineral nutrients containing 900, 800, and 700 mg/plant of NO3 at the beginning of the experiment in December, it became apparent that 800 mg/plant of nitrate would be sufficient for radish growth in the cold season. In conclusion, we propose the QNM method supplying the whole amount of nutrients required for crop growth at the beginning of cultivation so that radish plants could be produced without draining nutrient solution containing a large amount of NO3 from the hydroponic system into the environment.