What's a good starter fertilizer for evergreens?

I've got 50 thuja green giant seedlings that are 15~20 inches tall and in 3" pots. The soil here has alot of clay and rock in it. I planned on digging out a "milk jug" sized hole for each seedling and filling it with some good garden soil, then planting the evergreens in the good soil. What's a good fertilizer blend to add? I have a 5 gallon bucket of urea (46-0-0) in my garage, along with a bucket of DAP (18-48-0) and potash (0-0-60). I'd rather use what I have instead of purchasing something.