[DanHulton]

Dirt/grass _absolutely_ compress. Easy way to see - take a large hammer outside and hit the dirt real hard. It'll leave a dent. You want to leave that dent, because that's force transferred into the dirt, not you.

But also, it makes sense when you think about it a bit, too. Dirt is a mostly heterogeneous mixture of _stuff_ that tends to clump, which leaves great pockets of air in between that can get compressed out. Water, on the other hand, is a homogeneous liquid with _no_ air in between. There's practically nothing to compress!