土 Tǔ means earth
Perhaps the cross is a plant growing out of the earth.
It is related to 王 wáng, as he is the one who links earth with heaven.
地 Dì actually has a similar meaning to 土, but is used in such common words as 地方 dìfang 'place' and 地图 dìtú 'map'. See also 也.
场 (traditional character: 場) is an open area, a place, as in 市场 shìchǎng 'market'.