Is spent mushroom compost any good?

Spent mushroom compost is what is left after a commercial mushroom crop: a composted straw and manure base with a chalk-rich casing layer mixed through it. That casing layer is why it is worth understanding rather than just buying — the chalk makes it an excellent material for one kind of garden and a slow mistake in another.

Step by step

1. Understand what is actually in it.A base of composted straw and manure, usually horse or poultry, plus the casing layer of peat or peat substitute mixed with ground chalk or limestone. It is well composted by the time it is spent, and it is usually cheap because the growers need it gone.
2. Check your pH before you decide.This is the whole question. On acid soil, its alkalinity is a bonus and does part of a liming job for you. On neutral soil, occasional use is fine. On soil already at pH 7.5 over chalk, regular use pushes it further and locks up iron and manganese.
3. Keep it away from acid-lovers, entirely.Blueberries, rhododendrons, azaleas, camellias, pieris, heathers, and to a lesser extent raspberries and blueberry-adjacent fruit. It will make them chlorotic. This is the most common mistake made with the material.
4. Use it where it is genuinely good.Bulk improvement of clay, where the calcium content also encourages the clay particles to flocculate. Vegetable beds on acid soil, particularly the brassica part of a rotation which wants a higher pH anyway. General mulching of ornamental borders on non-chalky ground.
5. Stack fresh material before using it on anything sensitive.Fresh spent mushroom compost can carry high soluble salts, which scorch seedlings and young plants. Two to three months in a heap, open to the rain, leaches most of that out. If you are mulching established shrubs, you can skip this.
6. Do not use it in seed or potting compost.Too alkaline and too salty for young roots, and its structure is not right for a container. It is a soil improver, not a growing medium.
7. Ask about herbicide residue like anything else.The base is manure and straw, so the same aminopyralid question applies. Growers vary in what they can tell you. A bean test before it goes on a vegetable plot is the same sensible precaution as for any manure.
8. Judge the quantity by its alkalinity, not just its bulk.An annual 50mm mulch of mushroom compost is a liming programme as well as a soil improvement programme. Test your pH every two or three years if you use it regularly, and stop if the number is climbing.

Tips

• On heavy clay in an acid area, mushroom compost is close to the ideal material — bulk, calcium and organic matter all at once, and usually the cheapest per cubic metre of anything available.
• It is often sold loose by the trailer load near mushroom farms at a fraction of bagged compost prices. Availability is very regional.
• Its nutrient content is modest and mostly already used by the mushrooms. Treat it as a conditioner and feed separately.
• Some spent mushroom compost is peat-based in the casing layer and some is peat-free. If that matters to you, ask.
• It suppresses weeds reasonably well as a surface mulch at 50mm, and it is usually free of viable weed seed because of the heat in the process.

Common mistakes

• Mulching a blueberry or rhododendron bed with it — It raises pH and locks up iron. The plants go yellow between the veins over one or two seasons and the cause is rarely connected back to the mulch.
• Using it every year on chalky soil — You are liming ground that is already alkaline, and the compound effect over five years is a soil where nothing sensitive to pH grows well.
• Sowing directly into fresh spent mushroom compost — The soluble salt content in fresh material scorches germinating seed. Stack it first, or sow into soil and mulch afterwards.

Questions people ask