All three kinds of dirt, including different sizes
Dirt is a common obstacle in the game that Shovel Knight must clear to progress. Dirt appears as a 1x1 or 2x2 block and comes in different materials. Yellow/gold dirt requires only 1 strike to clear, and Shovel Dropping on it will cause Shovel Knight to bounce. Light yellow sandy dirt, also requires only 1 strike to clear, but Shovel Dropping will not cause Shovel Knight to bounce; he will drop straight through it. Grey dirt requires 2 strikes to clear.
In the Stranded Ship and Frigid Flight levels, the dirt is replaced with light-blue snow which behaves identically to regular dirt. Also in the Tower of Fate levels all dirt is replaced with a light-purple block which also behaves identically to regular dirt.
For Specter Knight, he may fall through sand blocks and land on other blocks, but if he tries to jump off any block, they will take 1 hit.