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It's not uncommon to see a bunch of bags of compost at the garden center sitting right next to another, all labeled and priced about the same, and sharing almost nothing in common. Some compost mixes are rich and dark, packed with deep nutrients, and will feed a container of tomatoes all season. Others are nothing more than barely broken down wood chips.
Many people think compost is compost, or organic material is organic material, so it doesn't matter which bag you grab. Many people just choose the cheapest or pick the one with the nicest photo of vegetables on it. This isn't necessarily wrong, and it won't always get you the wrong thing, but it can. Compost describes a process rather than a fixed ingredient list, and what goes into that process and how long it sits before bagging changes the result completely.
Why Compost Varies So Much From Bag to Bag
Organic material breaks down over time into dark, crumbly material that is full of nutrients. The process to make this happen varies wildly depending on the source, though, and it is good to pay attention to make sure you get what you're looking for.
A common source for compost material is yard waste like leaves, grass clippings, and chipped branches. Another source is manure, usually made from cow or chicken waste, and this one tends to have more nitrogen than other kinds. A lot of cities also turn food scraps and kitchen waste into compost through local programs. And some bags of compost contain biosolids, which come from treated sewage sludge. This type is legal to sell in many places, but regulators are currently reassessing this material because of the PFAS, 'forever chemicals,' that the treatment doesn't fully remove. A growing number of states have restricted or banned its use on land.
A newer option is mushroom compost. This is the leftover, or "spent," growing material usually used to produce mushrooms. The growing medium is often a mix of straw, hay, and manure that then gets pasteurized, used to grow a crop of mushrooms, and finally recycled once the mushrooms are harvested.
This process leaves the material high in soluble salts, since the same nutrients and minerals added to fuel mushroom growth stay behind in the leftover substrate. This compost can hurt salt-sensitive plants like azaleas, blueberries, and many seedlings. As the roots pull in those excess salts along with water, the plant reacts the way it would to being overwatered, and the tips and edges of the leaves turn brown and crisp.
The best way to avoid this is to let the bag of mushroom compost sit out in the open for a few weeks before use. Then, spread it out and water it occasionally so rain or irrigation can leach out the excess salts. In general, mushroom compost is best to use on established, salt-tolerant plants such as roses, tomatoes, and most vegetables.
The front of a bag of compost doesn't always say where the material came from. The label might just say "compost," "garden compost," or "premium compost." This label alone won't help you know if it's rich, manure-based compost in the bag or half-finished yard waste.
In addition to checking the ingredients, it is important to know the processing timing. This isn't something that's always easy to determine, but there are some clues. Compost that hasn't been processed long enough will still have material in it that hasn't finished breaking down.
The problem with this is that the material will keep breaking down once it's sitting in the pot. And, as it breaks down, it draws nitrogen from the soil instead of leaving it for the plant. A related risk with unfinished compost is ammonia. This gas builds up when nitrogen-rich material hasn't had time to break down fully, and it can burn the roots of young plants.
Why "Organic" Doesn't Tell You Much
The word "organic" on a bag of compost sounds like a quality guarantee, but it is not. The word "organic" has a narrow meaning in the world of compost, and it mostly means that the material came from plant or animal sources rather than synthetic chemicals. The label doesn't indicate nutrient content, how finished the material is, or whether it came from rich manure or half-composted sawdust. A bag can be labeled organic and still be low in nutrients.
Any manufacturer can print the word "organic" on a bag. The only quality guarantee that exists is through the Organic Materials Review Institute, a nonprofit group that checks products against real standards for organic growing. A product that passes this review earns the right to carry the OMRI Listed seal, and the manufacturer prints that seal right on the bag. This seal means far more than the word "organic."
Herbicide Carryover
Hay, straw, or grass clippings treated with certain persistent herbicides can carry that chemical straight through the composting process without breaking down. These two chemicals, clopyralid and aminopyralid, are the usual culprits, and both are common on pastures and hayfields used to control broadleaf weeds. These same chemicals can pass through an animal's digestive system without breaking down, which means manure made from treated hay has the same risk.
Broadleaf plants like tomatoes, beans, or peppers often come out stunted, curled, or twisted when grown in contaminated compost, even though the compost itself looks and smells fine. This kind of damage can take weeks to appear, long after the compost is already mixed into the soil.
If your plants are struggling and show any of these symptoms, the compost might be the culprit. To find out if the source material was ever treated with clopyralid or aminopyralid, contact the manufacturer. If that information isn't available, a simple planting test works as well. Plant a few bean or pea seeds in a small pot of the compost in question. If the compost is contaminated, they will turn stunted or curled within two to three weeks.
What to Actually Look For
- Check the ingredient list on the back of the bag. It should name the source (composted manure, forest products, or food waste)
- An OMRI seal
- Lift the bag up and check its weight. A heavier bag is a good sign, because finished compost is pretty heavy. A light bag usually means there's lots of wood fiber and air.
- If the store allows it, take a look inside. Good compost is dark, close to black, crumbly rather than clumpy or stringy, and smells earthy, like a forest floor after rain. A sour smell, an ammonia scent, or a lingering manure odor indicates that the compost hasn't broken down.
- If the bag is labeled "compost blend" and gives no further detail, it usually means the bag mixes real compost with a cheaper filler, like ground bark.