The kanji 土 depicts a plant or object rooted in the ground. The bottom line represents the flat earth, and the top line with a cross is something growing or embedded in it.
'DO' (ド) — Imagine you DO get your hands dirty with SOIL, or like the DOor is opening to a place where you can find SOIL.
This character looks like a simple cross marking a spot of EARTH or a pile of SOIL, indicating a location on the ground.