How to top-fertilize vegetables in seedling period?

First, topdressing time. Generally speaking, it is appropriate for the seedlings to come out and the cotyledons to flatten; during the day, it is better to be at 10: 12 am on a sunny day. Second, the method of topdressing. Both chemical fertilizer and organic fertilizer can be applied as topdressing fertilizer; for topdressing, nitrogen fertilizer should be the main fertilizer, and compound fertilizer should be better. It can be sprinkled first and then watered, or it can be poured with water, or the ratio of chemical fertilizer to water is controlled at 0.1 / 0.3 / 100. If the amount of chemical fertilizer is too large and the concentration is more than 0.5%, it is easy to burn seedlings. Apply organic fertilizer, use more mature human feces, urine or circle fertilizer, first dilute the fertilizer with water by 10 to 20 times, and then pour it into the seedling bed; no matter after applying chemical fertilizer or organic fertilizer, you should spray clean water after fertilization. to wash off the fertilizer attached to the leaves, prevent leaf burning, and timely ventilation to prevent harmful gases (such as ammonia, hydrogen sulfide gas, nitrous acid gas, etc.) from escaping, endangering vegetable seedlings. Third, extra-root topdressing. When the seedling bed temperature is low and the fertilizer absorption capacity of seedling roots is weak, root topdressing should not be carried out. 0.1% potassium dihydrogen phosphate or 0.3% urea solution or 0.2-0.3% superphosphate leaching solution can be sprayed on the leaf.