In different areas of the Kena: Bridge Of Spirits you will find Hay Piles scattered on the map. These Hay Piles need to be cleared to reveal valuable things inside, such as rot, to increase your army of spirits.
In Kena: Bridge Of Spirits, if you see a Hay Piles with some glowing particles floating on it, it means that there is something under the Hay Piles. Note that they can resist light, heavy, and charged melee attacks, as well as Spirit Bow, and your Spirit Bomb.
To destroy the Hay Piles, you must use Forest Tear. Whenever you see a Hay Piles, there is always a Forest Tear near by. Activate the Forest Tear to temporarily bring the Rot back to its powerful true form, then attack with rot using a spin or slam attack.
This will clear the Hay Piles so that you can find what is buried underneath and you can use forest tear as many times you want. Also not every Hay Pile has something hidden inside sometimes they are empty but they can have rot hidden inside too.