Essence of Earth Farming Locations (Classic - WotLK)
Essence of Earth is one of the least expensive essences and if you're looking to farm it for a recipe - check the Auction House first. You'll save a buttload of time just purchasing it from the AH and farming something else more profitable instead. One of the reasons that this essence is so cheap is because it's commonly found in Molten Core.
In addition to farming for this essence, you can also create it through Alchemy using Transmutations. Both Essence of Fire and Essence of Life can be turned into Essence of Earth, typically Essence of Life isabout the same price (or cheaper) than Essence of Earth depending on your server.
Tip: While farming for Essence of Earth, make sure you hold onto all Elemental Earths that you find. These are used in crafting Greater Nature Protection Potions which are used like candy in the Ahn'Qiraj raids, especially while guilds are still progressing through them.
Silithus - Essence of Earth Farming Location
Without a doubt the best location for you to farm Essence of Earth is The Crystal Vale in the northwestern portion of Silithus. Surrounding The Crystal Vale you'll find a bunch of air elementals, which are the only enemies in the game you can farm Essence of Air from. Inside The Crystal Vale you'll find a couple dozen earth elementals, all of which drop Essence of Earth.
Felwood - Essence of Earth Farming Location
The second best location for you to farm Essence of Earth is in Felwood and it's the same exact location that you farm Essence of Fire/Elemental Fire at as well. Surrounding the large pits of green goo you'll find large infernal monsters roaming around, they're elite but in the lower 50s which means they shouldn't be too difficult to solo at 60.
Also in this area you will find green fire elementals which have a chance of dropping both Essence of Fire and Elemental Fire. Both of these items will be worth more than the Essence of Earth that you're farming so I strongly recommend you hunt these enemies while you're here as well.
Un'goro Crater - Stone Guardian/Essence of Earth Farming Location
In Un'goro Crater the enemies you'll want to farm for Essence of Earth are Stone Guardians. These enemies are level 59 - 61 elites and unless you're the right class or spec they'll be extremely difficult for you to solo. Out of all the locations on this page, this one is the worst if you're just looking for Essence of Earth... However if you're looking to make money - this location is probably the best.
Stone Guardians have a chance of dropping an item by the name of Guardian Stone which is used in a few high end tradeskill recipes. You can sell this item for much more than Essence of Earth which is why this location is the most recommended one if your main goal is to make gold.