Mycelium spreads on Dirt ID 3:1

Mycelium spreads in the dirt block with Data ID of 1
Grass does not spread on it, But Mycelium does.

Here is a screenshot
http://mbcx2.net/files/downloads/2013/09/0906075402K9QoEdmdYlK1FNlDHPwPqSo4nq46ggH84sBrc7B87J1Yj205hejP5crDV6m.png