Zomboid Composter and Compost Guide (B42.21)

Short answer: a composter turns rotten food, animal droppings and poppies into compost. On default settings it takes about 2 weeks, and once it holds 10% or more you can take out 10% with an empty sack. The compost bag you get has 4 uses, and it works on a crop once per growth stage.

This article is for Build 42.21. Build 42.21 went to Stable on September 28, 2026 (confirmed on Steam news). 42.22 is on the Pre-Release branch (a test branch) since October 27 and is not Stable yet. Neither the 42.21 Stable post nor the 42.22 Pre-Release post mentions compost or fertilizer (we did not read the full patch notes).

★A note on sources. The numbers come from four pages of the official-community Project Zomboid wiki (pzwiki): Composter (version tag 42.13.2), Compost Bag (42.3.1), Fertilizer (42.4.1) and Agriculture (42.18.0). We did not verify any of it in game. Also, the wiki pages disagree with each other on some points; they are listed under "What we could not confirm" below.

Getting a composter

• Build it: Carpentry level 3 (Agriculture page, Fertilizing section). The Composter page's infobox lists the carpentry level as "3, 6" for two sprites, probably a shoddy and a regular version, but the wiki does not say which needs which level.
• Find one: some backyards on the map have one (Agriculture page).
• Size and capacity: one tile, container capacity 20. Any item up to 20 encumbrance fits, but only rotten food and similar is turned into compost (Composter page).

What to put in, and how much compost you get

• Time to compost: about 2 weeks on default settings (adjustable in the sandbox options; Agriculture page).
• If you lack input: rotten berries and mushrooms from foraging, or rotten food from restaurants and fridges, work well. Once your farm runs, put the surplus in (Composter page).
• Don't let it fill up: at 100% the composter does not waste rotten food (it waits for space and composts it as soon as there is room), but worm production stops while it is full.

Taking compost out

1. Right-click the composter to see the compost amount as a percentage.
2. With 10% or more, use an empty sack to take it out ("Get Compost" in the English UI).
3. Each time 10% is taken and the sack becomes a Compost Bag.
4. 2% of capacity is 1 unit, so a composter holds up to 50 units, which is 10 bags' worth.

• A Compost Bag has 4 uses. When used up it turns back into an empty sack, which you can fill again, or use as a container (Compost Bag page).
• It weighs 5 (Compost Bag page).

Rules for using it on crops

This section follows the Agriculture page (version tag 42.18.0). Fertilizer (NPK) and compost can be used together.

• Effect: on the first use in a growth stage, 40 hours are removed from the current stage's remaining time (20 hours if the crop has weeds) and the crop gains 10 health. If less than 40 hours remain, the crop moves straight to the next stage and the extra hours are lost.
• Bonus yield chance: used in growth stages 1 to 3, it can trigger a bonus yield (if the crop is not cursed and has no weeds). Each Agriculture level adds 5% to that chance (the wiki flags that skill section as needing verification for Build 42).
• Using compost a second time in the same stage gives no benefit and does no harm (Agriculture page; fertilizer is different, see next section).
• It has no benefit in winter, or on a crop already in growth stage 6 or 7.
• Weed first: weeds halve the time removed and rule out the bonus yield.

NPK fertilizer: don't treat it like compost

• A second use of fertilizer in the same stage neither speeds growth nor adds health, but removes 25 health each time and curses the crop (Agriculture page).
• A cursed crop cannot be restored. A crop fertilized with fertilizer 3 or more times is cursed (same page).
• Fertilizer is found mostly in gardening shops, farm sheds and garages. You can't make more, so think of compost as the safe supply and fertilizer as the reserve.

How worms multiply

• With two or more fresh worms in the composter, a new worm appears each time rotten food or droppings are converted.
• The worms stay fresh as long as the composter is not "too cold".
• Once the composter is 100% full, worm production stops.

What we could not confirm

• Wiki disagreement: is a second compost use harmful? The Agriculture page (42.18.0) says a second use does no harm, but the Compost Bag page (42.3.1) says a compost bag "behaves exactly like NPK Fertilizer" and that using it too many times may make the plant rot. We could not tell which is current. This article follows the Agriculture page, which has the newest version tag, but it is safer to avoid a second use in the same stage.
• Wiki disagreement: how much time one use removes. The Fertilizer page (42.4.1) says 20 hours per use and that 5 uses kill the crop. The Agriculture page (42.18.0) says 40 hours, +10 health, and that a second use costs 25 health and curses the crop. We used the Agriculture page because its version tag is newer.
• In-game names: we did not check the in-game names of the composter, the take-out action or the Compost Bag in any language other than the English wording used here.
• Non-default settings: the 2 weeks is the default-settings value and changes with sandbox options.
• Another article on this site: our farming guide says fertilizer restores crop health and compost stops natural health loss. That differs from the wiki text used here (faster growth, +10 health). We could not confirm which is right, and that article is waiting for review.

Sources

• pzwiki: Composter (version tag 42.13.2)
• pzwiki: Compost Bag (42.3.1)
• pzwiki: Fertilizer (42.4.1)
• pzwiki: Agriculture (42.18.0)
• Steam: Build 42.21 Stable Released (September 28, 2026)
• Steam: Moving on Up: 42.22 Pre-Release (October 27, 2026)

Summary

• Composter (Carpentry 3): put in rotten food, animal droppings and poppies. About 2 weeks on default settings.
• At 10% or more, take it out with an empty sack. 10% per bag, 4 uses per bag.
• Once per growth stage on a crop. Used in stages 1 to 3 it can raise the yield. No effect in winter or in stages 6 to 7.
• Fertilizer (NPK): a second use in the same stage costs 25 health and curses the crop. Treat a second compost use as wasted too (the wiki disagrees on this).