Gricoller's fertiliser is an item only available within the Tithe Farm minigame.
It automatically is added to your inventory at the start of the minigame granted the player has enough space.
After used on a planted seed, it will decrease the time it takes for the seed to grow by half.
Gricoller's fertiliser can be useful at the end of a Tithe Farm session when only a small number of seeds are left over (e.g. 8 seeds out of 100); the fertiliser will help speed up the last growth periods of the smaller batches of plants.