Fertilizer

Fertiliser goes into the second slot of the Hydroponics Tray and its data variant, which the game explicitly names "fertiliser". It has slot type plant and stacks up to 10.

Every item carries three values, at base Cycles 2, HarvestBoost 1 and GrowthSpeed 1. According to the game code they work like this as soon as a plant is placed in the tray: HarvestBoost, rounded to a whole number, gives that many extra fruits per harvest. GrowthSpeed is a factor on growth and healing – 1 means no difference. Cycles counts down by one with every planting; below 1 the item is used up. Perennials take another dose each time they regrow.

Fertiliser from the large composter can be better: up to GrowthSpeed 1.25 from decayed food, up to HarvestBoost 2.5 (rounded 2 extra fruits) from plants and up to Cycles 7 from biomass. If you stack two items of different quality, the game averages their values. Put the fertiliser in the slot before you plant – a plant that is already growing gets nothing, only the next planting or the next regrowth does. The Greenhouse planner calculates without fertiliser.

You cannot buy it: no trader stocks it and no printer has a recipe for it. It comes from the Advanced Composter or the Portable Composter, made from food leftovers and biomass. That closes the loop: whatever is left over or rots in the greenhouse returns to the bed as fertiliser.

Fertiliser does not decay. It is flammable, though, with a flash point of 100 °C according to the game data – so do not store it next to a furnace. How compost and seeds work together is explained in Compost, fertiliser and getting seeds.