temperature
Onions are highly adaptable to temperature. Seeds and bulbs can germinate slowly at 3-5°C, and start to accelerate at 12°C. The growth temperature of seedlings is 12-20°C, leaves are 18-20°C, bulbs are 20-26°C, and robust seedlings can withstand -6-7°C ℃ low temperature. Bulb expansion requires a higher temperature, bulbs can not expand below 15 ℃, 21 ~ 27 ℃ growth is the best. If the temperature is too high, it will grow and decline and go dormant.
illumination
Onion is a long-day crop, which requires more than 14 hours of long-day light conditions during the bulb expansion stage and the flowering stage of bolting. Under high temperature and short sunshine conditions, only leaves will grow, and shallots cannot be formed. The suitable light intensity for onions is 20,000 to 40,000 lux.
moisture
Onions should supply sufficient water during the germination stage, the seedling growth stage and the bulb expansion stage. However, in the seedling stage and before overwintering, the water should be controlled to prevent the seedlings from growing too long and suffering from freezing damage. Irrigation should be controlled 12 weeks before harvest to enrich bulb tissue, accelerate maturation, and prevent bulb cracking. Onion leaves are drought tolerant and suitable for 60% to 70% humidity. Too high air humidity is prone to disease.
soil and nutrients
Onion has strong adaptability to soil, and it is suitable for fertile, loose, and well-ventilated neutral loam soil. Sandy loam soil is easy to obtain high yield, but clay loam soil has full bulbs, good color and good storage resistance. The onion root system has a weak ability to absorb fertilizer, and sufficient nutrient conditions are required for high yield. Every 1000 kilograms of onion needs to absorb 2 kilograms of nitrogen, 0.8 kilograms of phosphorus and 2.2 kilograms of potassium from the soil. The application of copper, boron, sulfur and other trace elements can significantly increase production.
