Complete guide to mushroom compost

What is mushroom compost?

Mushroom compost, often called spent mushroom compost, is a by-product of the edible mushroom industry. Mushroom farmers cultivate mushrooms on a blend of composted straw and well-rotted animal manure – usually horse and chicken manure. After harvesting two or three crops of mushrooms, the compost is sterilised at high temperate to remove any pests, diseases or weeds. It’s then sold, either in bulk or in bags, as a soil conditioner and mulch for ornamental and vegetable gardens alike.

Why should you use mushroom compost?

Use mushroom compost to improve your soil so that it provides the air, moisture and nutrients that plants need to grow well.

Mushroom compost is high in organic matter, which enhances all soils, from heavy clay to light, free draining sand. Organic matter improves the structure and aeration of soil, at the same time creating greater moisture-retaining and drainage capability. Traditionally, manure or compost would be incorporated into soils through single- or double-digging. More recent no-dig approaches advocate preserving the existing soil structure, spreading the compost as a thick mulch over the flower or vegetable bed and allowing it to be gradually drawn into the soil through the action of soil organisms such as worms, beetles and millipedes.

Mushroom compost also provides the nutrients that plants need to grow well. The three main plant nutrients found in the soil are nitrogen, phosphorus and potassium, while magnesium, calcium and sulphur are also important. These nutrients work together to help plants grow strongly and produce flowers and fruit. They are particularly important on the vegetable patch as vegetable crops need a lot of nutrients. Invertebrates, bacteria and fungi in the soil work together to break down organic matter such as dead plant material and manure to make these nutrients available to plant roots. Mushroom compost is slightly alkaline, with a pH of 6.5-7, which is suitable for a wide range of ornamental plants. It is also good for a wide range of vegetables, including brassicas (vegetables in the cabbage family), as the higher pH defends against club root, and tomatoes, as the higher calcium levels act against blossom end rot).

Mushroom compost may be lower in nitrogen that other composts, as this nutrient will have been depleted by the mushrooms that were grown on it. However, lower nitrogen levels, will promote the production of flowers and fruit, as high nitrogen can stimulate the plant to put its energy into leaf production.

Where should you use mushroom compost?

Use mushroom compost as a general mulch, spreading a 5cm layer on flower beds, herbaceous and shrub borders and around young trees and specimen shrubs. Mushroom compost can also be used to improve waterlogged or clay soils as well as light, free-draining soils.

Avoid using mushroom compost as a mulch for so-called ‘acid-loving’ or ‘ericaceous’ plants, like rhododendron, camellia, heather and citrus, as the higher pH means they have difficulty accessing the nutrients they need from the soil.

Use mushroom compost as a 5cm deep mulch on your vegetable beds and allotment, but not on soft fruit, such as blueberries, raspberries and currants, which need neutral to acid conditions.

As it has raised concentrations of soluble salts, mushroom compost is not suitable for use on its own as a potting compost, or for growing seeds or cuttings. However, you can use it to grow outdoor plants in containers by mixing one part mushroom compost to three parts garden soil.

When to use mushroom compost

Use mushroom compost in spring as a mulch and soil conditioner, spreading a 5cm deep layer over the surface of the soil. If spreading it on vegetable beds, leave for a week after application before sowing seeds.