I got two small japanese maple last year. I live in Kansas so I understand the clay soil. When I dug the holes, I mixed the dirt I took out with compost and a little manure which I used as back fill. The varigated green one has one of those rubber mulch tree rings and put on more growth then I expected. The red one has pine needles around it and is doing great. The red one doesn't get as much sun but the new leaves start out green then turn red as they grow.

Amending a planting hole in clay soils is asking for trouble. Picture a bucket filled with compost or good quality potting soil.....what happens to the excess water after you water or it rains?

Japanese maples do not necessarily mind clay soils - they tend to be nutrient rich - but they do resent poor drainage. In fact, it is pretty much the kiss of death. When dealing with clay soils, if you want to amend, amend the entire planting area, not just individual holes. Otherwise, dig a wide but shallow planting hole, place the tree in high (top of rootball above grade) and mound excavated soil up to the top of the rootball, creating a small berm. Any amendments you wish to add can be used as a topdressing.

Vibrant color depends on cultivar and sunlight. Filtered or light shade is good for any variety, as is morning sun. That is usually sufficient to develop very satisfactory coloring on any cultivar.

You should be aware that most JM's have at least 3 distinct colorations - early spring or any new growth (which can occur anytime), mature midseason leaf color and in fall. Of the three, midseason coloring tends to be the least dramatic with new growth and fall color competing neck and neck for intensity, depending on specific cultivar.