The mix is too hot from the extra fertilizer, plus new organic potting mixes can have up to 3900 ppm of Total Dissolved Salts per gallon of soil. When you have that much salts and you water you release the nutrients with every watering. If the roots are very tiny thats why, because there is no need to search for nutrients. You would be better off using a smaller container in the beginning so you can release less nutrients with each watering. Germinate your seeds in small pots first and then up pott 2 inches at a time for best results.