I used multi-purpose. I must say that last year, my Brussels Sprouts seedlings went the same way - doing well, then just collapsed. I was told to put them in a deeper tray for the roots to grow, which I did this year, and I have just planted all of them out this week. There is nothing that I can say really of what has gone wrong with your cabbage seedings!! A mystery indeed!!
I will try with seed compost and then if it goes wrong again I will buy some root trainer cells to see if it is the depth of compost that is the problem.This happened to me last year aswell but I am new to growing my own so I put it down to not knowing what I was doing,obviously learnt nothing in a year!!
I use disposable plastic drinking cups and 3/4 fill with general purpose composting soil. 1 seed per tub. Water a little,frequently,keep in the green house. No problems. Have just planted out into their final postions.
Why use a seed tray then a pot and then into their final postion.
Least disturbance for any plant is better!!
I know you use more composting soil but it does get the results you want.
Thanks for all your replys,I have planted some more seeds into cardboard toilet roll middles then I can plant them with no root disturbance at all as they will break down in the soil.
Thanks again Keith.