Key Technical Points for Spring Planting and Care of Onions
Scientific fertilization: After the spring greening, the first fertilization should be carried out to supplement nutrients for the growth of the onion root system and at the same time lay the foundation for the vigorous growth of the aboveground part later. Fertilization should be based mainly on fully decomposed organic fertilizer, applying 1000 kg per mu, with 20 kg superphosphate and 10 kg potassium sulfate added. For plastic film mulching, generally only 15 kg diammonium sulfate and 10 kg potassium sulfate are applied per mu together with watering.
When the onion plants grow to 8-10 true leaves and the bulbs begin to swell, start applying bulb-promoting fertilizer, applying it 2-3 times in succession, with 10-20 kg ammonium sulfate per sq m. The middle stage of bulb expansion is the key period for fertilization, and the amount applied at this time should be large. When the bulbs begin to expand, do not apply too much nitrogen fertilizer, so as to prevent the plants from “staying green”. During the bulb expansion period, potassium sulfate should be added during fertilization, with an amount of 5 kg-10 kg per mu, because potassium deficiency in onions can lead to poorer storability.
Timely watering: For onions planted before winter, spring greening water should be applied. The greening water should be applied at the right time. Watering too early will lower the soil temperature and is not conducive to onion growth, while watering too late will also inhibit plant growth and cause dry leaf tips.
When the onion bulbs begin to expand, water should be controlled to promote seedling hardening. The length of the seedling hardening period should be flexibly controlled according to the soil, climate, and plant growth conditions. For sandy soil or arid climate, the seedling hardening period should be appropriately shortened; for clayey soil, low-lying terrain, and rainy conditions, it should be appropriately extended. The purpose is to promote the transition of onion growth toward bulb expansion. Generally, this stage takes about 10 days. It is generally believed that when the mature tubular leaves of the onion turn dark green, the leaf mesophyll becomes thick, the waxiness on the leaf surface increases, and the color of the heart leaves deepens, this is the sign that the seedling hardening period should end. After seedling hardening, generally water once every 5 days to promote rapid bulb expansion. When some plants in the field lodge, stop watering to avoid poor storability after bulb harvest.
Cultivation and earthing up: When onions are not covered with plastic film, cultivation must be carried out before seedling hardening. The number of cultivations in heavy clay soil should be greater than in sandy soil. The cultivation depth should be controlled within 3 cm, shallower near the plants and deeper away from the plants. After the plants close the rows, stop cultivation. While cultivating, appropriate earthing up can be carried out.
Harvesting and storage: In southern Shandong, harvesting is generally carried out in mid-June. The main signs of onion bulb maturity are that in the late stage of bulb expansion, no new leaves fill the leaf sheaths, the false stem becomes hollow, and the plant falls over from the base of the false stem. The timing of bulb maturity is related to the characteristics of the onion variety, planting time, and climatic conditions. For varieties with a short dormant period and poorer storability, harvest should be appropriately earlier; when half of the plants have lodged, harvesting can begin. For mid-late maturing varieties, the harvest period is later, and it is suitable to harvest when 70% of the plants have lodged.
Harvesting should be carried out on sunny days, preferably with several consecutive sunny days. At harvest, pull out the whole plant and spread it out to dry for 2-3 days. When drying, only dry the leaves, not the heads. The bulbs should be covered with leaves, which can promote after-ripening of the bulbs and dry the outer skin. Then remove the soil, trim off the fibrous roots and dead leaves, and they can be stored. Some also do not remove the leaves after harvest and directly braid or bundle them for storage.