Onion seedlings need a healthy, loose soil. Add well-aged manure or compost. In the rainy season you need to raise the seedbed so the onion roots will not rot. If you use quality seed, most of the seed will germinate and you need much less of it. Onion seedlings need room to grow, so do not plant them too close together. Put the seed in lines, 5 to 10 centimetres apart and 1 centimetre deep. Cover the seeds lightly with fine soil.