Hoe does not til grass blocks, or dirt blocks with no grass

Crafted hoe (first wood, then stone, then iron) to use for farming. Tried to til the grass covered dirt block and it was harvested rather than tilled. Figured it might be because of the grass cover, so I took some dirt down to mine to make a wheat field. I laid the dirt down and tried to use the hoe (right clicked hoe on dirt block) and the dirt block was picked up after a few clicks. I'm managing so far with mushroom soup and animal carcasses, but I can't do any long term farming without being able to til the earth.