Manure is a mix of animal feces, urine, water, and whatever bedding material the animals lived on. By weight, water makes up the majority, typically 60 to 80 percent of fresh manure. The remaining dry matter is a combination of undigested plant fibers, nutrients like nitrogen, phosphorus, and potassium, living microorganisms, and smaller amounts of minerals and salts. The exact makeup shifts depending on the animal species, what they ate, and how the manure was stored or processed.
Water and Organic Solids
Fresh manure is mostly water. The dry matter fraction varies by species and diet, but in fresh livestock manure, organic matter typically makes up 69 to 84 percent of that dry weight. “Organic matter” here means carbon-rich compounds: undigested plant cellulose, proteins, fats, and other biological material that passed through the animal’s gut without being fully broken down. As manure ages or composts, microbes consume those organic compounds for energy, and the organic fraction drops. Fully composted manure may contain only 29 to 54 percent organic matter in its dry weight, with the rest converted to stable mineral forms.
The Key Nutrients
The reason manure has been used as fertilizer for thousands of years comes down to three nutrients: nitrogen, phosphorus, and potassium. Every animal produces manure containing all three, but the concentrations vary dramatically by species.
Poultry manure is the most nutrient-dense by a wide margin. It delivers roughly 10.6 percent usable nitrogen, 12.1 percent phosphorus, and 15.7 percent potassium in the year it’s applied. Pig manure comes in second, with about 3.8 percent nitrogen, 4.5 percent phosphorus, and 6.2 percent potassium. Sheep manure is comparable at 3.0, 3.1, and 8.2 percent respectively. Beef cattle manure runs around 2.5 percent nitrogen and 3.0 percent phosphorus, while dairy cattle manure is slightly lower. Horse manure sits at the bottom of the scale, with just 1.2 percent nitrogen, 1.4 percent phosphorus, and 4.6 percent potassium.
These differences come down to diet and digestive efficiency. Chickens have short digestive tracts and excrete a concentrated waste product. Horses, by contrast, are relatively inefficient digesters despite eating large volumes of fibrous material, so their manure is bulkier but more dilute in nutrients.
Nitrogen Changes as Manure Ages
Not all nitrogen in manure behaves the same way. Raw manure contains high concentrations of ammonium, a soluble form of nitrogen that comes largely from urine and feed supplements. Ammonium is immediately available to plants, but it’s also volatile. Left on the surface, much of it escapes into the air as ammonia gas, which is what gives fresh manure its sharp smell.
During composting, microbes convert that ammonium into organic nitrogen, a more stable form that releases slowly into the soil over months. This is one of the most important chemical changes manure undergoes. Composted manure won’t burn plant roots the way raw manure can, and its nutrients stick around longer. The tradeoff is that the total nitrogen content drops during composting because some is lost as gas during the decomposition process.
Bedding Materials and Carbon Content
In practice, manure rarely comes as pure animal waste. It’s almost always mixed with bedding from the barn floor. Common bedding materials include straw, sawdust, wood chips, sand, and peat. These materials are rich in carbon and very low in nitrogen, so they raise the carbon-to-nitrogen ratio of the final product. Straw alone has a carbon-to-nitrogen ratio around 212 to 1. Sawdust is even higher, at roughly 833 to 1.
This matters because soil microbes need both carbon and nitrogen to do their work. When manure has a very high carbon-to-nitrogen ratio from heavy bedding, microbes actually pull nitrogen out of the surrounding soil to break down all that carbon, temporarily starving plants of nutrients. Manure with a lower ratio, closer to 10 or 12 to 1, releases its nitrogen to plants more readily in the first growing season.
Microorganisms in Manure
Manure is a living ecosystem. A single gram contains billions of bacteria and fungi, many of which play useful roles in soil health. During composting, nitrogen-fixing bacteria like Pseudomonas become dominant players, helping convert organic material into stable, plant-available forms. Carbon-degrading fungi break down the tougher structural fibers that bacteria can’t easily digest. These microbial communities are a major reason composted manure improves soil structure, not just nutrient levels.
But raw manure also carries pathogens. The most concerning are E. coli (including the dangerous O157 strain), Salmonella, Campylobacter, and Listeria. These organisms originate in the animal’s gut and can survive in manure for weeks or months depending on temperature. At room temperature (around 25°C), E. coli O157, Salmonella, and Campylobacter drop below detectable levels after about 90 days of storage. Composting speeds this up considerably. When manure heaps reach temperatures above 55°C, those same pathogens are eliminated within a month or less. This is why composting guidelines emphasize sustained high temperatures throughout the pile.
pH and Soil Chemistry
Different manures have different effects on soil acidity. Poultry manure is the most alkaline, with a pH around 6.3. Pig manure falls in the middle at roughly 5.2, and cattle manure is slightly acidic at about 4.6. This makes poultry manure particularly useful on acidic soils, where it can raise the pH and reduce aluminum toxicity. The organic matter in manure also binds to aluminum in the soil, further protecting plant roots from damage even beyond what the pH shift alone would accomplish.
Antibiotics and Hormones
Manure from conventionally raised livestock contains trace amounts of veterinary drugs. Antibiotics used to treat or prevent infections pass through the animal and end up in the waste. In broiler chicken manure, researchers have detected concentrations of the antibiotic doxycycline as high as 78,516 micrograms per kilogram of dry weight. Hormones are also present at detectable levels. These residues can persist in soil after manure application and contribute to antibiotic resistance in soil bacteria over time.
Composting reduces but does not always eliminate these compounds. The concentration and type of residues depend on what drugs the animals received, how recently they were treated, and how long the manure was aged before use. Manure from animals raised without antibiotics or added hormones avoids this issue entirely.