Most mulch is not made of poop. The standard bags of mulch you see at garden centers and hardware stores are made from shredded wood, bark, or other plant materials. However, some specialty products blur the line between mulch and compost, and a few do contain animal manure as an ingredient.
What Standard Mulch Is Actually Made Of
Commercial mulch is overwhelmingly plant-based. The most common types are shredded hardwood bark, cedar chips, pine bark nuggets, and cypress mulch. These are byproducts of the lumber and timber industry, chipped or shredded and sometimes dyed red, black, or brown for appearance.
Beyond wood products, mulch can also be made from straw, hay, grass clippings, shredded leaves, sawdust, newspaper, cardboard, or even wool. On the synthetic side, rubber mulch (made from recycled tires) and plastic landscaping fabric serve the same purpose. None of these contain animal waste of any kind. Mulch sits on top of the soil to hold in moisture, regulate temperature, and suppress weeds. It’s a protective blanket, not a fertilizer.
Where Poop Does Enter the Picture
The confusion usually comes from compost, which is a different product that sometimes gets used as mulch. Compost is decomposed organic matter designed to enrich soil with nutrients. For many years, well-rotted farm manure has been one of the most popular composting ingredients, and manure-based composts are widely available. When gardeners spread compost on top of their soil instead of mixing it in, they’re using it as mulch, which is a perfectly common practice.
Some products are explicitly marketed this way. Price Farms Organics, for example, sells “Zoo Brew,” a compost made from Columbus Zoo animal manure and bedding mixed with horse manure, recycled food scraps, yard trimmings, and coffee grounds. They recommend it both as a soil amendment and as a mulch. Their “Barnyard Café” compost uses hog barn manure and bedding blended with paper, leaves, and coffee grounds. Products like these do contain animal poop, but they’re composted first, meaning the raw waste has been broken down by microorganisms into a stable, earthy-smelling material.
Mushroom compost is another product that sometimes shows up in the mulch aisle. It’s the spent growing medium from commercial mushroom farms, often made with horse manure, straw, and gypsum. Once the mushrooms are harvested, the leftover medium gets sold as a soil conditioner or mulch.
What About Human Waste?
Treated human sewage, known as biosolids, is a real product used in landscaping, though it’s not typically sold as “mulch.” The EPA allows biosolids to be applied to agricultural land, parks, golf courses, forests, and even home lawns and gardens. Some municipalities sell or give away processed biosolids for residential use. These products go through extensive treatment, but they are derived from sewage. You’re unlikely to encounter biosolids in a standard bag of mulch at a store, but they do exist in the broader landscaping supply chain.
Why Some Mulch Smells Like It
If you’ve ever opened a bag of wood mulch and thought it smelled like manure, the culprit is fermentation, not actual poop. When large piles of wood mulch sit too long without airflow, moisture builds up and oxygen disappears from the center of the pile. This creates ideal conditions for anaerobic bacteria, which produce acetic acid, methanol, and other foul-smelling chemicals as they break down the wood. The result can smell sour, rotten, or vaguely like animal waste.
This is more than just unpleasant. Sour, fermented mulch can actually damage plants. If your mulch smells off or feels hot to the touch when delivered, spread it out in a thin layer and let it air out for a few days before using it. Once oxygen reaches the material again, the smell dissipates and the harmful compounds break down.
How to Tell What’s in Your Mulch
There’s no federal requirement for mulch manufacturers to disclose every ingredient the way food labels do. Your best bet is to read whatever packaging is available and look for specific language. Pure wood mulch will say “hardwood,” “bark,” “cedar,” or “pine” and nothing else. Products containing manure or biosolids will typically mention it, especially if they’re marketing the nutrient content as a selling point.
If you’re buying bulk mulch from a landscape supply yard, ask what’s in it. Yards that process compost alongside mulch sometimes blend the two, and the composition varies by supplier. A simple smell test works surprisingly well: clean wood mulch smells like fresh wood or earth. Anything sour, sharp, or barnyard-like suggests either fermentation or manure content.
For gardeners who want manure-based products, they’re easy to find. For everyone else, standard wood or bark mulch contains nothing but shredded trees.