Saltpetre

Saltpetre is a members-only item on the OSRS Grand Exchange in the other category.

In-game examine text: "A power that can be added to compost to make fertiliser."

It has a Grand Exchange buy limit of 13,000 every four hours, a high alchemy value of 3 gp, and item ID 13421.

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Saltpetre shows an active flip setup with a margin of +1 gp and 1,032,377 trades over the last 24 hours.

At the buy limit of 13,000 every four hours, max profit per cycle is about 13.0k gp — high margin with healthy liquidity.