---
title: "How a manure lagoon works, and why it smells | Alchemal"
url: "https://alchemal.com/field-notes/how-a-manure-lagoon-works/"
description: "A manure lagoon is a treatment system, not a storage pit. Here is the biology it runs on, why it sours in warm weather, and what the smell is telling you."
source: "Alchemal — alchemal.com"
---
# How a manure lagoon works, and why it smells

Alchemal

Published August 10, 2026

A manure lagoon looks like a pond doing nothing, which is why it gets treated like one. It is closer to a wastewater plant with no moving parts. The pit under a barn stores waste; the lagoon is where that waste is supposed to break down, and everything about how it smells and how fast it fills comes down to which biology is doing the breaking.

## The two chemistries

Waste in water breaks down one of two ways. With oxygen present, aerobic bacteria digest it quickly and hand back carbon dioxide and water, which is why a healthy creek can carry a load without announcing it. Without oxygen, anaerobic bacteria do the work slowly and give off hydrogen sulfide (the rotten-egg gas), ammonia, and organic acids instead. The first chemistry is fast and quiet. The second is slow and loud, and it is the one a loaded lagoon defaults to, because manure arrives faster than oxygen does.

Most working lagoons settle into layers: anaerobic depths doing the heavy digestion, under an upper layer that stays aerobic where the water meets the air. That aerobic cap is the odor control. Odorous compounds rising from the depths are oxidized as they pass through it, so what reaches the surface is far milder than what was made below.

## Why it sours in warm weather

The cap holds only as long as oxygen enters the upper water faster than the load consumes it. Warmth works against both sides of that balance at once: warm water holds less oxygen, and warm biology demands more of it. A lagoon that behaved all winter can [sour over a few warm weeks](https://alchemal.com/problems/lagoon-anaerobic-souring/), which is why the worst complaints land on roughly the same schedule every year.

The same starved condition slows the digestion of solids, so they settle out and the [sludge blanket grows](https://alchemal.com/problems/lagoon-sludge-capacity-pumpout/), eating storage capacity and making pump-out harder. At the surface, floating solids can knit into a crust. Odor, sludge, and crust look like three problems and trace to the one shortage.

## What the smell is telling you

Hydrogen sulfide at the fence line means anaerobic breakdown is reaching the surface unoxidized, which means the aerobic cap has thinned or failed. That condition is measurable before the neighbors report it: oxidation-reduction potential, a probe reading of whether the water’s chemistry is running oxidized or reduced, tracks the sulfide chemistry directly, and [reading an ORP curve on a lagoon](https://alchemal.com/field-notes/reading-an-orp-curve-on-a-lagoon/) shows what the number looks like as a lagoon sours and as it recovers.

The working fix is to hold the upper column aerobic through the warm season, so the cap keeps doing its quiet work at full depth. [Aerobic odor control on a lagoon](https://alchemal.com/problems/lagoon-odor-sludge/) covers how that is done and what it changes about sludge, and the [livestock overview](https://alchemal.com/livestock/) walks the whole picture. An [assessment](https://alchemal.com/assessment/) starts with a baseline of your lagoon as it stands, taken before any commitment.

## Related field notes

- [Understanding your water How to read an ORP curve on a working lagoon Oxidation-reduction potential (ORP) tracks the chemistry behind a lagoon odor: a negative curve means sulfide, a rising one means the smell is receding.](https://alchemal.com/field-notes/reading-an-orp-curve-on-a-lagoon/)
- [Understanding your water Why stored irrigation water goes flat Warm, still storage strips the oxygen out of irrigation water two ways at once, and the bottom-drawing intake sends the flattest water to the root zone.](https://alchemal.com/field-notes/why-stored-irrigation-water-goes-flat/)
- [Understanding your water How to read a dissolved-oxygen curve The one chart that explains most pond problems: what a healthy daily oxygen cycle looks like, and the three shapes that mean trouble.](https://alchemal.com/field-notes/how-to-read-a-dissolved-oxygen-curve/)

## Tell us what your water is doing.

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