growing conifers in containers

[conifer001]

Hello all. This is my first posting so please forgive me if this has been covered previously.

I'm growing some conifers (blue spruce, concolor fir and white pine) from seed in small plastic containers (zone 5 - Michigan). This is their second growing season and they are between 3 and 5 inches tall with fairly small and delicate roots. My goal is to grow them in containers for several years until they are large enough to plant in our yard.

For soil I'm using a mix of organic potting soil, perlite, mini pine bark nuggets and turkey grit. The seedlings seem to like the mixture and water drains fairly quickly from the plastic pots.

I'm confused by fertilizers. There are many theories on WHAT to use, how MUCH to use and WHEN to use it. So far I'm using Miracle Grow water soluble 20-20-20 about once a week. I'm open to suggestions for other alternatives or suggestions regarding fertilizing conifers in containers.

Thanks everyone!

Dave