"Sense soft earth nearby"
In Oracle of Seasons, the ring can be found in Gnarled Root Dungeon, the first dungeon in the game. From the dungeon entrance, head up a screen, right a screen, and up a screen. It is found in the room with several Floormasters.
In Oracle of Ages, the ring can be found in Spirit's Grave, the first dungeon in the game. From the dungeon entrance, head left two screens and up one screen. The Seed Ring can be found within the Treasure Chest. If you are going through the dungeon for the very first time, you won't be able to head west two screens immediately, as you will need the Power Bracelet to lift the pots. Just naturally going through the dungeon, you will have to pass right by this treasure chest.
When the Discovery Ring is worn, a sound will be made when Link is on a screen with a Soft Soil patch. There are 16 Soft Soil locations in Holodrum, along with 8 Soft Soil spots in both the Past and Present of Labrynna.