I started in MG potting mix last year, no problem. Not sure about your mix but I'm sure it won't kill the plants..

Yes, it is a potential problem. Young seedlings can be easily killed by high levels of nitrogen in the mix - they just aren't strong enough to handle it and are burned. And the use of any dirt/manure can lead to damp-off, a soil borne fungus the causes the young seedlings to rot at the soil line. Soil can also compact too much and suffocate the fragile young roots and can retail excess water and lead to root rot.

It is for these reasons that the use of a soil-less starting mix is recommended so that these problems can be easily avoided.

From the FAQ How to Grow Tomatoes from Seed:

#2 Get a Seed Starting Mix
Garden soil is not a good choice, as it compacts too easily and can harbor organisms that cause diseases. A commercially prepared seed starting mix, usually a combination of peat moss, vermiculite and perlite, is recommended. Avoid mixes that have a high fertilizer content, as this causes more problems than good. Commonly used and recommended mixes are Jiffy Mix, ProMix, MetroMix and Fafard. Many other brands, or even homemade mixes, can be used.

Since you have already started them in this mix it will be important to watch them closely and transplant them out of the mix at the first sign of trouble. Keep the air circulating well around them and avoid over-watering at all costs.

You might also want to browse through the many FAQs here and on the Growing from Seed forum for more tips.

Good luck.

Dave

Here is a link that might be useful: FAQ: How to prevent and treat Damp-Off

ut-oh

ok going to read there

whoops