I start my seeds in Pro-mix BX, or Pro-mix HP, or Roots Organic potting mix, depending on the seed I'm planting. The trick is to not water them after they first sprout. In the photo the seeds are just beginning to sprout (1dayold) and that's the time to hold back the water. When using regular potting mix the nutrients are released with every watering and when you limit your water, you have better control of nutrients. I have waited as long as 12 days before watering the second time with super results. The plants roots only grow in search of nutrients or water and when you hold that back the root grows a lot faster producing bigger plants. .