The Kitchen Scraps That Are Actually Hurting Your Compost Heap

There is a certain smell that indicates that something has gone horribly wrong in a compost bin. In this pile, there is no damp-earth earthiness that indicates it is functioning. It tastes like something that hasn’t been plugged in for a week; it’s sharp, slightly sour, and has a hint of something that isn’t quite sure what it will become. Many people who encounter it believe the project was a mistake. Some blame the bin, the weather, or some nebulous incompatibility between their way of life and composting. It is rare for them to blame leftover pasta, citrus peels, or the generous amount of salad dressing they added on Tuesday night.

Composting is one of those tasks that seems straightforward at first glance, but has more regulations than expected. It is fairly accurate to say that organic matter decomposes into something useful. There is a large discrepancy between what belongs in a backyard heap and what qualifies as organic matter, which could endanger the entire industry. Despite the fact that the EPA has been fairly clear about which kitchen materials are problematic, the same errors continue to appear in bins in suburban neighborhoods, on allotment plots, and on urban gardens. By composting with the best of intentions, people are subtly undermining their own efforts.

In part because they seem so natural, the biggest offenders often surprise people. Cheese rinds, sour cream, and the last inch of milk in a carton decompose so slowly in an outdoor pile that they turn putrid, release odors, and alert rodents and raccoons nearby. It isn’t dairy’s inorganic nature that causes problems. It produces odorous compounds and retains enough moisture to upset the pile’s equilibrium because it breaks down differently than plant matter. The line between wet and dry is already blurred by a well-kept heap. Dairy pushes it to the wrong side of the fence.

Meat and fish pose a similar, but more serious problem. It is impossible to tolerate the rotting smell that raw or cooked animal protein produces in a compost pile. Practically speaking, it invites pests in. Raccoons seem to take a particular interest in containers filled with leftover meat. In addition, animal products can introduce pathogens that can persist long enough to find their way into finished compost, garden soil, or food crops. Many casual composters would change their behavior if they realized that the final product of their pile might coat the vegetables they’re growing. There is no hypothetical danger here. There isn’t much discussion about it.

Biologically and mechanically, cooking oils and fats impede the composting process. The oil also prevents microbial activity that causes decomposition from occurring on other materials in the pile. These substances can change a pile from active and aerated to dense, wet, and anaerobic when added in volume. Pizza boxes, butter scraps from pans, and salad dressing leftovers are examples. Anaerobic decomposition still occurs, but it occurs slowly and poorly, producing sulfur compounds and methane, which are largely responsible for the unpleasant odors associated with composting.

The composting community appears to be genuinely divided on citrus peels, which present a more complex issue. Orange peels and lemon peels in small amounts are not harmful. In greater quantities, especially in smaller, less thermally active piles, acidic citrus material can change the pH sufficiently to eliminate the beneficial worms and microbes. Citrus-rich environments seem to be avoided by worms in particular. Onions and garlic have a similar problem: they are not harmful or pathogenic, but they seem to repel the organisms that are necessary for composting to function properly. In real life, this may or may not be relevant, depending on how much garlic a household uses and how active the pile already is. The question remains whether adding onion skins really makes a difference or if being overly cautious is the problem. Moderation seems sensible in any case.

In composting guidelines, processed foods and bread have an odd place. They are technically plant-based. It appears that they are harmless. When consumed in small amounts, stale bread probably won’t cause any long-term harm. Unlike cooked rice, pasta, and other similar starchy leftovers, bread decomposes into a dense, wet mass that fosters mold growth and tends to compact into airless layers that obstruct airflow. Rats are attracted to it, practically speaking. Compost bin rats are not only undesirable in urban and suburban settings, but they are frequently a neighborhood issue with social implications far beyond the individual gardener’s pile.

Non-food items that are brought into the kitchen with food are a smaller category of compost contaminants. Produce stickers, the tiny, sticky labels found on almost all fruits and vegetables, are made of vinyl or plastic. During the composting process, they remain undamaged, become finished compost, and then accumulate over time in garden beds. Seals of many paper tea bags contain a small amount of polypropylene mesh that can withstand composting. Shiny or glossy paper towels that are frequently used near food can be coated with plastic films. A lot of what appears to be compostable in a kitchen is actually something else – something that will remain unaltered after everything else has decomposed.

Composting won’t solve most of these problems. You need to be more careful about what you put in. A typical backyard pile can be modified to address odor, pests, and slow decomposition all at once by keeping meat, dairy, and cooked foods out. Those households that produce large quantities of animal products might want to consider Bokashi composting, a fermentation-based process. The technique is well-liked in certain regions of Japan, and interest in it has been steadily increasing elsewhere. When added to soil, the pre-compost material decomposes more rapidly than aerobic composting. In spite of its flaws, it’s better than the sluggish, foul-smelling experiment most people are presently conducting without even realizing it.