I use an soilless mix such as Promix for starting tomato seeds because it is sterile, and weed free. Few things are as frustrating to find the plant you have been nurturing for several months on your windowsill is in reality a large weed with no redeeming floral graces. That’s experience talking by the way.
It is also consistent from bag to bag so once you have a system for germinating that works for you in your garden, using a soilless mix eliminates a problem with having a soil problem. There’s not too much salt from compost or some bacteria from the soil that gets in the way of producing a great seedling.
If you really, really, want to use potting soil, put it into the pot leaving one half to one inch of space between the soil top and the rim of the pot and then slowly pour boiling water into the pot until the water runs freely out the bottom. Let the soil cool before seeding.
I don’t usually recommend potting soil because it comes in a wide variety of qualities and can compress during the growing cycle.
Do not use garden soil as it compacts terribly and seed germination rates will be much lower than in artificial soil.
Manure in the Soil
Here’s the real deal. Never use manure in a seed starting soil. So if you have one of those “wonder” soils in packages that contain it – do not use that soil for seed starting. There are simply too many problems that come along with using manure in this way. Or use the boiling water treatment above.
Ah. Here’s the deal with compost. If it is fully composted, if it has been hot composted to destroy pathogens, you can actually germinate and grow your seeds in 100% compost.
If it isn’t perfect compost (and you’ll seldom buy perfect compost I note) then using it may very well kill off your seeds/seedlings.
What If I Want To Try
If in doubt, try growing tomatoes or cress in a 100% trial pot of the stuff. If the cress grows well, it is OK to add. This is the most sensitive indicator.
Tomatoes will grow in almost-OK and if they grow, then most other things will be OK to have some of this compost added to the mix. If neither of these plants does well, do not add any of that compost to the mix.
There are many examples of poor compost killing seeds, seedlings and even young plants in fields.
It is much better to avoid using compost in seed starting mixes but there are some gardeners who swear by their methods. Not me. I want consistent soil starting and growing seedlings.
You can read other tomato and organic vegetable gardening tips here.