How to make large spruce tree like spreading of block at base on a custom tree?

pardon me if the title seems confusing .

This is my first actual question

basically I want to make a tree that will spread a block like podzol when grown like on a large spruce tree (2×2 trunk one)

I can't use structures as the I only want a certain blocks like dirt not all kinds of blocks

I suspect I can use a spreading bloc that'l spread the block in a radius IDK really I need help.