Hi MJSJ,
Which variety are you growing?
I have sown 5 different varieties, starting out in january-february, and have planted out the seedlings under various conditions, by way of experiment. At this moment, the plants that receive most direct sunlight look the best. Those that have shade in the afternoon are doing considerably worse, even with better compost.
The patch that is doing best is a raised bed (30cm high, stone) filled with mushroom compost. At this time of year, it receives full sun from sunrise until 4pm. I have watered today, but last time was 3 weeks ago. Multisown and the size is competitive with bio-products from the supermarket.
At a second place is a patch, raised bed (20cm high, stone), filled with 10cm soil and topped with 10cm mushroom compost. It has decaying wood below ground level, and full sun from 9am till 7pm.
At a third place, a patch with a few cm of compost on top of soil. Full sun from 9am till 7pm. Small onions, but acceptable.
Last place, a patch with full sun from 8am till 1pm, 10cm compost on top of soil. Poor size, what you might call spring onion 🙂
Shade early in the growing process in not necessarily disadvantageous, as this will stimulate the plant to begin bulb formation earlier than “normal”. Once bulb formation has commenced, more light will usually result in larger bulbs.
Our soil here is very poor and sandy. I don’t have to dig a full meter to arrive at sand of “beach quality”.