Install
Install the latest version of SMAPI.
Unzip the mod folder into <font style="font-family:Courier New;">Stardew Valley/Mods</font>.
Run the game using SMAPI.
Use
Just apply fertilizer to crop spaces like usual. Now you'll be able to apply a different type even if they're already fertilized.
Compatibility
Compatible with Stardew Valley 1.5.5+ on Linux/macOS/Windows, both single-player and multiplayer.
See also