Mushroom Compost: The Unlikely Material That Could Make Your Vegetable Garden Thrive

Are you looking for a gentle soil amendment for your vegetable garden? There's one lesser-known option that is weakly acidic to alkaline, high in soluble salts, and less nutrient-dense than fertilizer. The amendment in question is mushroom compost, which recent research has shown helps vegetables thrive — although not all vegetables, as you'll see if you read on. The alkalinity of this material appeals to many of the edible plants you are probably growing in your veggie patches and may even help fight disease.

Mushroom compost is the leftover substrate used in mushroom farming. It is also known as spent mushroom substrate (SMS) or spent mushroom compost (SMC). Since mushroom compost starts life as an agricultural growing medium, the ingredients can vary from one farmer to the next. However, most contain some combination of straw, hay, poultry manure, ground corn cobs, cottonseed hulls, gypsum, soybean meal, and other nitrogen-containing compounds. Once the substrate is spent and will no longer grow mushroom crops, it is hot-composted for 30 days to kill weed seeds, pests, and pathogens. It is then pasteurized using steam, packaged, and sold to gardeners.

Characterized as both a soil amendment and a soil conditioner, mushroom compost may significantly improve garden soil in several ways. Primarily, it boosts organic matter in the soil so that you can create healthy growing conditions. It also encourages the proliferation of beneficial soil microorganisms and provides the macronutrients — nitrogen, potassium, and phosphorus — plants need to thrive. Secondarily, mushroom compost helps the soil retain water and improves its structure. It may even gradually ameliorate clay soil.

As noted, mushroom compost is particularly beneficial for some vegetables over others. Tomatoes (which especially like the calcium in mushroom compost), peppers, lettuce, cucumbers, and eggplant thrived when cultivated with spent mushroom compost (SMC), per a 2024 research review published in the Journal of Material Cycles and Waste Management. Outside of the nutritional benefits, the plants were better able to resist disease. The alkalinity of mushroom compost benefits vegetables in the Brassica genus, such as cabbage, cauliflower, and kale. You can also apply this amendment lightly to beds planted with lettuce, leafy greens, onions, and garlic.

To prep your soil for successful planting, top-dress with aged mushroom compost in spring or summer. Other options include making a microorganism-rich tea with a four-to-one ratio of water to compost, then using it as a foliar spray or adding it to your watering can. You can also till about 3 inches of compost into the top 6 inches of your garden soil.

There are cons to using mushroom compost in your garden. It's alkaline and high in soluble salts, so be careful not to use it on salt-sensitive or acid-loving plants (like spinach) or on seedlings. Use just a little mushroom compost on beets (they dislike high pH soils) and beans and peas (which prefer a neutral soil). It should not be used at all on potatoes (they can't take the lime) or carrots, which prefer loose, sandy soil. Quality can vary in mushroom compost: avoid products with a bad odor or weed seeds. Ideally, choose compost certified by the U.S. Composting Council, which comes with a Seal of Testing Assurance (STA). Don't go overboard, either — limit your use to no more than 9 cubic yards per 1,000 square feet.