Answer: Wait until the seeds germinate and develop 3 to 5 true leaves (not counting the two "seed leaves" that come up first). Then you can begin to water them with a diluted solution of liquid plant fertilizer. Emerging seedling have all the nutrition they need stored in the seed and can in fact be easily "burned" with fertilizer.
Note that plants may differ in their fertilizer needs and in the best time to apply it. So make sure and tailor you fertilizing to the plant species and your soil test results.
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