1. temperature Onions have strong cold resistance. The suitable temperature for leaf growth is 10-25℃, and the optimum temperature is about 22℃. When the temperature drops below 10℃, the growth is slow. When the temperature rises to a higher temperature of 20-28℃ in spring, bulbs begin to swell, leaves gradually wither, and when the temperature continues to rise, they enter the physiological dormancy period to escape the heat.
2.light. Onions are intolerant of shade. Suitable for high light intensity, light compensation point is about 2500 lux, and saturation point is 25000 lux. Short sunshine can promote the growth of leaves, and long sunshine can promote the formation and expansion of bulbs.
3. Moisture. Waxy surface of leaves can reduce water transpiration, but the absorption capacity of roots is poor, so it is necessary to ensure the water needed for growth and development. High soil moisture should be maintained during the growing period. Onions are not tolerant to waterlogging, so pay attention to timely drainage and waterlogging prevention after rain to prevent root retting.
4. soil. Red onion is not strict with soil, and loam or sandy loam with deep soil layer, good drainage and rich organic matter is preferred. Neutral soil with ph around 7 is suitable for onion growth. Onion is sensitive to nitrogen fertilizer, and applying nitrogen fertilizer will generally have a significant yield increase effect; It is necessary to apply more potassium fertilizer, and pay attention to supplement magnesium, sulfur and other trace elements.