I would particularly agree with an ice field rune. But about being the only one missing, I got a wild theory here related to that: Technically poison was considered an earth damage only after the update on 2007 (8.1). Before that, we had poison as a different kind of damage, not directly related to the earth damage. So we can think that an "earth field" is something that we had only because poison became part of it. Cipsoft never implemented maybe because they never really thought about adding a field for each of the four basic ones at all (although the earth damage got one field as a collateral effect of the adjustment of poison integrating the element earth), but it is something up for discussion.
More details about 8.1 can be found here: https://www.tibiawiki.com.br/wiki/Updates/8.1
Tibia Summer Update 2020 - private test server invitationsMay 18, 2020
Interview with Benjamin Zuckerer - managing director at CipSoftMay 07, 2020
Improving posts is now rewarded with reputation pointsApril 11, 2020
Looking for a community managerJune 09, 2019