Update: I watered my seedlings with an iron solution (Optiferrum from MackAgrar) and sprayed once with a solution of iron, manganese and a trace of zinc. The plants are now looking a much fuller darker green. Two weeks later some plants are starting to show the original signs of iron deficiency (interestingly only the moderner cherry varieties), so I will spray again. The plants I have already plated out (I am now the proud owner of a frost-free passive solar greenhouse!) are looking very healthy. Only tomatoes show these signs. The cabbages, for example, look really good on the same compost. I have adjusted one batch with peat. I’ll upload some photos if anyone is interested.