There are 17 soft soil patches in Majora's Mask. 5 of them are on a wall. You can only release a bug into each soft soil patch once since you last played the Song of Time. All of them give you something when you release a bug into it. Unlike Ocarina of Time, releasing one into a soft soil patch triggers a cutscene of them crawling into the hole so you can't recapture one of the 3 bugs before it goes into the hole. You can also plant a magic bean into any soft soil patch that's on the ground. After you plant it, if you water it, it will turn into a platform and the soft soil patch will disappear. That platform will not move until you get on it and once you do, it will move in a certain way then return where it started, enabling you to get to places you're not supposed to be able to get to another way. If you're on it when it returns to where it started, it won't move again until you get off and back on it. Here's what each soft soil patch gives you:

Location What it gives you Ground or wall patch
Termina field stump Red rupee Ground
Clock Town outside wall 3 blue rupees Wall
Termina field pillar 30 arrow drop Ground
Inside observatory fence 3 blue rupees Ground
Deku palace grotto 3 blue rupees Ground
Outer palace garden 3 blue rupees Ground
Left wall of second room swamp spider house Gold skulltula Wall
Right wall of second room swamp spider house Gold skulltula Wall
Under boulder in swamp spider house 3 blue rupees Ground
Second floor of second room swamp spider house Gold skulltula Wall
Room beside swamp spider house tree room 30 arrow drop Ground
Romani Ranch 3 blue rupees Wall
Dog race track Red rupee Ground
Great Bay cliffs Red rupee Ground
Inverted Stone Tower 30 arrow drop Ground
Higher area of inverted Stone Tower 30 arrow drop Ground
Secret shrine 3 blue rupees Ground