Having read the other replies I ought to add the following to my earlier
post
I do not grow seedlings in conventional seed trays (too fiddly for my big
hands!) I use those large modular seed trays with a 1" square "pot" for each
seed. In hindsight this obviously allows for better air circulation. These
are then transferred into the 3" pots until planting.

As for sterilised soil, well I do sometimes get toms growing where marigolds
were sown! Hey that is all part of the fun. I even get toms in my hanging
baskets...and they fruited.

AndyP
--
"Wisest are they that know they do not know." Socrates
"If more of us valued food and cheer and song above
hoarded gold, it would be a merrier world" JRR Toliken, The Hobbit