Question
QUESTION: The area I am planning on planting a Colorado Blue Spruce has only approximately 2 -4 inches of soil on top of approximately 20 inches of gravel then clay below that

The area is within 30 feet of our vegetable garden and similarly from our basement wall

Can we plant one there or is the soil too thin and too much gravel?

ANSWER: There is just not enough soil to even dig the hole for the root ball. Usually a rootball of balled and burlaped trees are about 12-18 inches and the hole will need to be 12-18 inches deep. Sorry this is not a good location for a tree.  

---------- FOLLOW-UP ----------

QUESTION: Any sugegctions as to how I could improve the area so I could plant one?

Answer
There is not really much that can be done--a tree needs about 2 feet of soil to grow well. Tree roots grow in the first two feet of soil.  It is possible to add that much soil  but you would need about a 20 foot circle. The tree has a width of about 20 feet when mature. It may not be worth it.  Sorry!