This is a great question for the Discussions side and you will get more responses. Not many people even remember there is a Conversations side and it is usually used for personal and OT posts. Wish I had an answer for you.

Hi there, You say that it is your radishes turning yellow, I had the same problem a mini crop of cherry tomatoes, i Tried to solve the problem by taking the plants out with all the soil still around the roots and added more potting soil with topsoil, sometimes i have found that the compost and the potting soil is too strong for the seedlings too handle ( to acidic) or even just not enough nutrition. But i managed to save the plants, well most of them at least, some of them were too far gone, and what i would do is also too secure the roots a bit more, once sprouted i would gently compact the soil around the base too better absorption of water and minerals. And also I would try a small shade net as well from say 9 in the morning till about 1500, then remove so it still gets sun but doesn't burn to a crisp or just get too much sun. Maybe you've tried everything i mentioned but everything has worked for me in the past. So goodluck, hope you have success with this.