Like any good question: it depends! Mushroom compost can clump, has lots of organic matter, has a high pH, and is high in some minerals. Each plant will respond differently to mushroom compost.
Here’s a helpful breakdown that I found on a poster at a local community garden. Thanks community garden organizers!
This table comments on the nutrient, acidity, and pH contents of compost. Given that every compost source is different, its a good idea to test your soil and compost so you know what you’re changing by adding compost.
| Veggie | Mushroom Compost? | Notes |
| Tomatos | ✅ Yes | Benefits from the nutrients and moisture retention. |
| Peppers | ✅ Yes | Thrives with improved drainage and nutrients from compost. |
| Cucumbers | ✅ Yes | Benefits from the nutrients and moisture retention. |
| Squash (Zucchini, Pumpkin) | ✅ Yes | Benefits from the nutrients and moisture retention. |
| Corn | ✅ Yes | Heavy feeder – mushroom compost provides a good nutrient boost. |
| Lettuce, Leafy Greens | ⚠️ Caution | Small amounts help, but too much can make soil too rich or alkaline. |
| Spinach | 🚫 No | Prefers neutral to slightly acidic soil; mushroom compost is often too alkaline. |
| Carrots | 🚫 No | Compost can cause forked roots; best in looser, sandier soil. |
| Beets | ⚠️ Caution | Can tolerate a little, but does not like high acidity. Excess nitrogen, promotes leaves, not roots. |
| Radishes | ⚠️ Caution | Use in moderation to avoid excess nitrogen, which promotes leaves, not roots. |
| Potatoes | 🚫 No | High lime content in mushroom compost increases risk of scab. |
| Onions & Garlic | ⚠️ Caution | Tolerate small amounts but prefer leaner, well-drained soil. |
| Beans & Peas | ⚠️ Caution | Can grow in it, but prefer less rich, neutral soil. |
| Brassicas (Kale, Broccoli, Cabbage) | ✅ Yes | Thrives in the high nitrogen environment from the compost. |
Nutrients Profile of Mushroom Compost
Mushroom compost contents can vary depending on where you get it and what types of agriculture are near by. Typically, compost is made using three types of ingredients: a source of carbon like straw, corn cobs, or poultry litter, and a source of nitrogen like manure.
The compost is mounded up and soaked with water so it begins decomposing. Then it’s heated up to inoculate it and prepare it for mushrooms. Once the grower has used the compost for mushrooms, the spent compost is sold at your local garden centre or given away if you’re lucky.
According to the American Mushroom Institute mushroom compost on average is comprised of:
- 2.29% calcium
- 1.24% potash (potassium),
- 1.12% nitrogen
- 1.07% iron
- 0.67% phosphate (phosphorous)
- 0.35% magnesium
Most plants prefer a slightly acidic soil with a pH between 6.0 and 7.0. A pH of 7 is neutral. On average, mushroom compost has a pH of 6.6.
The carbon to nitrogen ratio of soil is important because it indicates the balance between energy sources for beneficial microbes (carbon) and nutrients used to build proteins (nitrogen).
High carbon content will result in higher microbial activity that competes for nitrogen with the plants because the microbes need the nitrogen to consume the carbon-based plant material in the soil.
Soil for growing food should have a carbon to nitrogen ratio of around 24:1. Mushroom compost has 13:1 ratio which means it is high in nitrogen and has a reasonably good content.
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