If you use fertilizers such as Basic Compost,Basic Fertilizer, or Miracle Food your plants can grow faster or even instantly - skipping any need to water them or prevent diseases.
Recipe
Go to the Cooking Pot on Mystery Island:
Step 1Combine Pile of Dung with Fish to get Basic Compost
--or--
Combine Pile of Dung with Bone to get Basic Compost
Step 2Combine Basic Compost with Berry to get Basic Fertilizer
--or--
Combine Basic Compost with Seed to get Basic Fertilizer
Step 3Combine Basic Fertilizer with Magic Vial to get Miracle Food

Dung can be replaced by most items with dung in it, like Dung Pizza, Chewing Dung, or Dung Slushie

Fish can be most fish hauled up from fishing, including Fish Puddle and Scarf Squid. But some fish cannot be used, such as Doomfish.

Bone can be used to substitute the fish, such as Muddy Bone, UFFH or Petrified Bone

Berry can be most berries, such as Conkerberry or Loveberry. Certain berries cannot be used, such as Rotten Berry or Half-eaten Berry

Seed can be any seed except Maraquan seed

Where to get Dung?

You can get dung at The Meridell Rubbish Dump for up to 10 pieces a day. Sometimes while Berry Picking you may come accross some dung as well.

Dung drops randomly throughout the day in batches, so if you see dung in the Rubbish Dump, grab it, and refresh to see if there are any more. I've gotten 8 dung at once before.

Where to get Berries?

You can pick up to 6 berries a day from the Pick your Own minigame by paying the 400np entrance fee.

You can buy more berries from other players with the shop wizard. The more common berries will be cheaper.

Any berry listed in the highscore table can be used except for the half-eaten and rotten ones.

Where to get Magic Vial?

You can get magic vial's from Kauvara's Magic Shop. They are fairly common and will drop about every 2nd or 3rd restock.

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