When I transplant my young seedlings into 4 or 6 packs I use a mix of potting or even peat and mix it with some of my good strained garden soil! It seems to work great and stays moist longer than sraight potting mixes!
I got very few weeds even in my seed boxes that have the bottom half of the box soil strained garden soil! When I am done with the boxes of soil I dump it on the compostor around my garden plants!
+++ Remember when I started my seeds as early as feb 12 I had a mix of leaves, paper and blood meal at the bottom of my boxes covered with strained garden soil with 2 inches on top of good potting soil! Also I mixed pine shavings in with my strained garden soil! It seemed that the pine shavings held the moisture in the soil for the plant roots and kept the garden soil from caking or getting muddy! I guess there are lots of good ways of doing things and when it works use it!