did you look into a mechanical tree planter?
In Iowa, you can go to the State Forestry office, USDA ag office;DNR Forestry Division, Pheasants Forever, Natl Turkey Federation etc. and either use (no Chge) or rent.
You need a 3 pt tractor and another live body. One drives, the other sits on the seat on the planter. The planter slits/furrows the soil, you drop in your seedlings, the planter real wheels "crimp" the soil around the seedlings
you can plant 400 - 1000 seedlins per hour
A "hole" lot faster than PHD
example:
http://www.iowadnr.com/forestry/pdf/planting.pdf
G5009 Mechanical Tree Planters, MU Extension