Instead of buying expensively: It's best to mix your own potting soil for planting. It's a mix of sand, soil and compost if you make it yourself.
This creates nutrient-poor soil in which young plants can grow well before they can be repotted or planted in the garden. According to Oekotest.de, ornamental and vegetable plants in particular need a special growing soil in order to thrive as well as possible in the early growth stage. You can easily make the growing soil for young plants yourself.