The proposed "thud" means the sound that is made when something heavy falls or hits something else. I don't suppose a shovel is heavy enough to make this kind of sound. "Crunch" could help to paint the picture of something crushed like the sound of gravel under your feet.If the ground was dry, it could do.Personally, I would choose thwack (the short loud sound of something like a stick hitting a surface). Anyway, native speakers will have to step in.

"Thwack" comes to mind. By the way, I really like Weiyang Luo's posts. Most italki questions could be answered by looking at a good dictionary or grammar website or simply googling, but Weiyang's can't. He asks the sort of thing that you can only find out by asking native speakers directly. This is how an international community should be used!

Can't think of a made up word. I'd probably use something like, 'his shovel hit the earth with a thud'.

Haha, honestly I don't know how to describe the sound with onomatopoeia, but here is my first instinct. It depends on the wetness and dryness of the earth. If the ground is dry, then it would make more of a "kwuh" sound, and if there were rocks, then perhaps a "cling" or "clang". If the ground is wet, then the shovel would go into the earth more easily and perhaps make a "wuh--thhh" sound.

"Thud" works. "Thump" is also possible. In comics you could also get "thwack" or "thunk".