Elemental Soils can be obtained in the "Elemental Spheres Quest", you need at least level 80 to start it.
And yes, you can enter the Realm and farm Soils as many times as you want. But keep in mind that each time you enter you'll have to pay the entrance fee, which is 20 enchanted stones, depending on your vocation:
- Knight: 20 Small Enchanted Amethysts
- Paladins: 20 Small Enchanted Sapphires
- Druids: 20 Small Enchanted Emeralds
- Sorcerers: 20 Small Enchanted Rubies
Once you have your stones, head to the Edron Magic Academia and ask Arkulius for the mission. (In case you never did it). Now go to the middle tower, lowest level (where you can find the elemental portals), here:
Turn OFF the machine and use all the 20 stones in it. After using all the stones turn the machine ON and you will be teleported into the Secret Realm.
Once inside you can stay there as long as you want (I'm not sure if you can logout there, tho)