Seed Starter and Looking for Faster Growth

I've been getting nutrient burn on my seedlings and was looking for some advice on what to use to give them a good start.

Also taking long time for the plants to grow. I started two plants about 6 weeks ago and they are only about 6 inches high. I think nutrient burn has partly been responsible for their slow growth.

They need nothing but water for about the first month. For seedlings and clones I use spent soil mixed with worm castings; if it’s kinda mucky compost I will add some extra perlite and/or coco coir. Keep it simple.

Approximately how tall are your plants after a month growing in spent soil & castings ?

Straight up promix works well.

Just an update here. I didn't get much in the way of responses on this post, but I'm using Fox-Farm Light Warrior to start & grow seeds for the next grow. Light Warrior is primarily peat moss + perlite + EWC + My-chorrizae. It is a very light mix and allows for plenty of oxygen to the seed.

After the seed germinates, I plant in Light Warrior in a 16 ounce styro-foam cup (holes are distributed throughout the cup to create airflow). After a few days, I use Fox-farm Grow Big at 1 ML per quart, increasing the dose as the plant gets larger. I also use super-thrive a couple of times to give them a boost too. Plants are growing much faster now than before.

When the leaves reach the outer edges of the cup, I transplant into a 1 gallon smart pot and after the plant reaches 8-10 inches, I transplant one last time into a 15 gallon pot which is 100% organic.