Horse manure sits near the lower end of the nitrogen scale among common livestock manures. Fresh horse dung typically contains around 0.5 to 0.7 percent nitrogen by weight, roughly a third to a quarter of what you’d find in poultry litter. That makes it useful as a soil conditioner and a gentle, slow-release source of nutrients, but calling it “high in nitrogen” would be misleading. The story gets more interesting once you factor in bedding material, diet, composting, and how nitrogen actually becomes available to plants.
How Horse Manure Stacks Up Against Other Manures
The simplest way to understand horse manure’s nitrogen status is to compare it with what comes out of other animals. Poultry manure is the heavy hitter. Research comparing poultry and horse manure-based soil amendments found that poultry amendments consistently maintained higher nutrient concentrations than horse amendments across multiple measured parameters.1PubMed. Protection from wintertime rainfall reduces nutrient losses and greenhouse gas emissions during the decomposition of poultry and horse manure-based amendments Chicken and turkey manures can run anywhere from 1.5 to over 4 percent nitrogen, depending on the bird type and how the litter was managed. Cattle manure lands closer to horse manure but generally edges it out slightly, sitting around 0.5 to 1 percent nitrogen.
This ranking matters for practical reasons. If you’re looking for a quick nitrogen boost for heavy-feeding vegetables, horse manure alone probably won’t deliver enough. On the other hand, its lower nitrogen concentration makes it harder to over-apply, which is a genuine advantage for home gardeners who don’t soil-test regularly. You’re much less likely to burn plants or create a nitrogen runoff problem with horse manure than with poultry litter.
Why Diet Changes the Equation
A horse’s diet directly shapes how much nitrogen ends up in its manure. Horses fed forage-only diets (hay or haylage with no grain or protein supplements) tend to excrete more nitrogen in their dung compared to horses on supplemented diets. That sounds counterintuitive, but it’s because the additional protein from supplements shifts nitrogen excretion toward urine rather than feces. A Finnish study found that protein-supplemented diets significantly increased urine nitrogen concentration, while supplementing hay diets with oats or protein feeds actually decreased the amount of nitrogen ending up in feces.2PubMed Central. Protein Source and Intake Effects on Diet Digestibility and N Excretion in Horses—A Risk of Environmental N Load of Horses
A separate study showed the same pattern from a different angle. When horses were fed soybean meal as a protein supplement, fecal nitrogen and ammonia both increased significantly compared to a control diet, confirming that the type and amount of protein in the feed pushes more nitrogen into the waste stream.3Journal of Equine Veterinary Science. Dietary Protein affects Nitrogen and Ammonia Excretion in Horses And at a broader level, nitrogen intake, nitrogen absorption, and urinary nitrogen all increase in a straight line as dietary protein goes up.4Journal of Animal Science. The impact of dietary protein levels on nutrient digestibility and water and nitrogen balances in eventing horses
For gardeners collecting manure from a specific barn, this means the nitrogen content isn’t fixed. Manure from performance horses on high-protein rations will differ from manure produced by easy-keeping pasture horses on hay alone. You won’t know the exact composition without a lab test, but understanding that diet is the main lever at least tells you which direction to expect the variation.
The Bedding Problem
Almost nobody handles pure horse dung. What leaves a stall is a mix of feces, urine, and whatever bedding material was used, whether that’s straw, wood shavings, sawdust, or pelletized pine. Bedding is overwhelmingly carbon-rich and nitrogen-poor, so mixing it in dilutes the overall nitrogen concentration of the pile substantially. A heap of stall waste might have a carbon-to-nitrogen ratio of 30:1 or even 50:1 or higher, depending on how generously the bedding was applied. For comparison, materials with a ratio above about 25:1 will tend to temporarily tie up nitrogen in the soil rather than release it, because soil microbes need to consume nitrogen to break down all that carbon.
This is the single biggest practical consideration for anyone trying to use horse manure as a fertilizer. The manure itself has modest nitrogen. Diluted with bedding, the available nitrogen drops further. If you’re spreading fresh stall waste on a garden bed, you may actually see plants go slightly pale and nitrogen-starved in the short term as microbes borrow soil nitrogen to decompose the wood or straw. Composting before application is the standard remedy, but it introduces its own nitrogen dynamics.
What Composting Does to Nitrogen
Composting horse manure transforms it from a high-carbon, low-availability material into something more plant-friendly, but the process doesn’t magically increase nitrogen. It concentrates it by burning off carbon as carbon dioxide while simultaneously changing the chemical forms of nitrogen present. A study tracking the microbial communities and chemistry through different composting phases found that ammonium content nearly doubled early in the composting process, jumping from about 16 milligrams per kilogram in fresh litter to roughly 31 milligrams per kilogram during the initial hot phase. Then it plummeted, dropping below 1 milligram per kilogram by the time the compost was fully mature.5PubMed Central. Dynamics of bacterial and archaeal communities during horse bedding and green waste composting
Meanwhile, nitrate levels followed the opposite trajectory, peaking in the middle phases of composting before falling back in the finished product. This matters because ammonium and nitrate are the two forms of nitrogen that plants can actually take up. The finished compost is stable and gentle, but much of its nitrogen has shifted into organic forms that release slowly over months as soil microbes continue to break them down. That’s a feature if you want steady, season-long feeding, but a drawback if you need a quick nitrogen hit for transplants.
How Storage and Weather Affect Nitrogen Losses
Leaving horse manure exposed to rain before you use it costs you nitrogen. A six-month field study comparing different manure types found that rainfall exposure decreased total nitrogen in horse amendments by about 37 percent.1PubMed. Protection from wintertime rainfall reduces nutrient losses and greenhouse gas emissions during the decomposition of poultry and horse manure-based amendments That’s better than poultry manure, which lost 59 to 74 percent of its total nitrogen under the same conditions, but it’s still a substantial chunk gone. Ammonium losses were even more dramatic, reaching 69 to 96 percent across all manure types tested.
Temperature and moisture during storage also play a role in determining what happens to the nitrogen that remains. Horse manure-based amendments stored at moderate temperature and lower moisture retained considerably more total nitrogen than the same material kept wetter or colder.6PubMed. Greater Impacts of Incubation Temperature and Moisture on Carbon and Nitrogen Cycling in Poultry Relative to Horse Manure-based Soil Amendments The practical takeaway is straightforward: cover your manure pile. A tarp or a roofed structure prevents rainfall from leaching away the nutrients you’re trying to preserve. Turning the pile periodically for oxygen promotes composting without the waterlogged conditions that drive the most severe losses.
How Quickly Plants Can Access the Nitrogen
Even after composting, horse manure doesn’t release all of its nitrogen at once. Researchers studying the mineralization of nitrogen from various organic amendments found that animal manures generally showed a slow initial release, followed by a faster phase, then another slowdown. The total nitrogen mineralized from animal manures ranged widely, from 176 to 617 milligrams of nitrogen per kilogram of soil, depending on the specific manure and conditions.7Journal of Environmental Quality. Mineralization of Nitrogen in Soils Amended with Organic Wastes That broad range reflects differences in manure composition, soil type, temperature, and microbial activity, but the general pattern of delayed release holds across the board.
This delayed availability makes horse manure compost better suited as a pre-season amendment mixed into beds weeks before planting, rather than a side-dressing applied when crops are actively hungry. Compared to a synthetic nitrogen fertilizer, which is immediately available, composted horse manure feeds the soil food web and builds organic matter over time. You’re investing in soil health rather than instant plant nutrition. For annual vegetable gardens, that often means pairing horse manure compost with a faster-acting nitrogen source during peak growth periods.
Earthworms and the Hidden Boost
One under-appreciated aspect of horse manure’s role in the garden is how it interacts with soil biology, particularly earthworms. When earthworms process horse manure, their casts end up enriched in mineral nitrogen relative to the surrounding soil. Research found that the amount of nitrogen accumulated in earthworm casts reflected the nitrogen content of whatever organic matter the worms were eating, and that horse manure-fed casts had elevated nitrate concentrations in the range of 80 to 100 micrograms per gram of dry weight.8Soil Biology and Biochemistry. Nitrogen transformations associated with earthworm casts In other words, earthworms act as biological concentrators, turning the manure’s modest nitrogen into more plant-available forms right in the root zone.
This doesn’t change the fact that horse manure starts with less nitrogen than poultry or even cattle manure, but it does mean that in a biologically active soil, the effective nitrogen delivery from horse manure may be somewhat better than the raw chemistry suggests. Gardens and pastures with healthy earthworm populations will extract more value from a horse manure application than sterile or heavily compacted soils where earthworm activity is minimal.
Leachate and Environmental Concerns
Because horse manure’s nitrogen is relatively modest and slow to release, it might seem like environmental risk would be negligible. That’s mostly true, but not entirely. A study simulating rainfall on various compost feedstocks found that horse bedding compost produced some of the highest nitrate-nitrogen levels in the leachate that washed through it, comparable to municipal sludge compost.9The Journal of Solid Waste Technology and Management. A Comparison of the Quantity and Quality of Leachate Generated by Five Compost Feedstocks Exposed to Simulated Rainfall It also had some of the highest total phosphorus in leachate. That said, the same study concluded that a simple grassed filter area would be capable of absorbing both the nitrate and phosphorus in the runoff, so the risk is manageable with basic siting precautions.
The bigger environmental concern with horse manure isn’t nitrogen at all but rather the volume. A single horse produces around 20 to 25 kilograms of manure per day, and when bedding is included, the total waste output can reach 10 to 12 tons per year per horse. Properties with several horses and limited acreage can quickly accumulate more material than the land can absorb, leading to nutrient loading in nearby waterways even if the nitrogen content per unit is low. Proper composting, covered storage, and spreading at agronomic rates (matching the amount applied to what crops or pasture can realistically use) are the standard defenses.
Weed Seeds in Horse Manure
A practical concern that has nothing to do with nitrogen but everything to do with using horse manure in gardens is the presence of viable weed seeds. Horses digest their feed less thoroughly than ruminants like cattle, which means seeds from grasses, weeds, and hay pass through the gut and remain capable of germinating. Those seeds then get deposited in a bed of nutrient-rich feces, which gives them a head start over surrounding vegetation. For gardeners, spreading uncomposted horse manure can introduce a wave of weeds that outweighs any fertility benefit.
Proper hot composting kills most weed seeds if the pile sustains temperatures above about 55°C (130°F) for several days and is turned so that all material, not just the core, reaches that temperature. Cold piles or casually managed heaps are much less reliable at seed destruction. If you don’t have the infrastructure to compost thoroughly, aging the manure for at least a year and allowing weed flushes to germinate and die before incorporating the material into beds is a lower-effort alternative, though it means accepting more nitrogen loss during the waiting period.
When Horse Manure Is the Right Choice
Horse manure’s moderate nitrogen content is actually its selling point in several situations. For clay-heavy soils that need organic matter more than they need nutrients, composted horse manure opens up soil structure without the risk of nitrogen overload. For perennial plantings like fruit trees, berry bushes, and ornamental beds, the slow nitrogen release aligns well with the steady, low-level feeding those plants prefer. And for anyone building a no-dig garden or layered raised bed, horse manure and bedding provide the bulk carbon-and-nitrogen mixture that breaks down into beautiful growing medium over a season.
Where horse manure falls short is in high-demand annual vegetable production, particularly for nitrogen-hungry crops like corn, brassicas, and heavy-producing tomatoes. You can compensate by applying larger volumes, but that brings excess phosphorus and potassium along for the ride, since those nutrients are present in horse manure at ratios higher than most crops need. Over years of heavy application, phosphorus in particular can build to levels that cause environmental problems. A soil test every few years keeps this in check and tells you when it’s time to ease off on manure and supplement nitrogen separately.