Mushroom Compost:

Spent or used mushroom compost is one of the most beneficial soil conditioners in the amateur garden. It is regularly used as a soil conditioner or mulch on the surface of the soil. It is usually marketed as “spent” mushroom compost, meaning that it has already been used as a growing medium for mushrooms in the past months.

It can be purchased in small quantities for a relatively low price making it all the more suitable for small scale soil improvement and enrichment by the gardener. Mushroom compost is the left over by product from the mushroom production and growth industry, used to cultivate mushroom spores and develop this crop for commercial purposes. It is readily available and can be purchased either bagged or loose from gardening stockists, nurseries or from many garden DIY stores in the UK. It is generally composed of wheat straw, dried blood, horse manure and chalk composted and blended together to a fine high nutrient growing medium. Spent mushroom compost can include a chalk content of up to 30% in contrast to fresh compost, due to the fact that the mushroom beds are enclosed with chalk.

It is a hugely beneficial source of nutrients and includes the three essential trace elements, Nitrates, Phosphates and Potassium. Most forms of this compost will include approximately 0.7% Nitrogen, 0.3% Phosphorous and 0.3% Potassium, contributing to its highly nutritious breakdown and its advantageous use as a soil conditioner. Due to its extremely high chalk content and being an alkaline, it should not be used extensively on acid-loving plants in the garden who will not tolerate such enrichment. It should never be used as lawn topsoil unless there is a specific need to reduce the acidity of the grass plot. It should be incorporated in combination to normal garden compost and manure which will have a neutral or acidic ph content to counterbalance the alkaline mushroom compost. In extreme cases, plants placed and growing in overly alkaline soils will become yellow, have stunted growth and exhibit the symptoms of lime-induced chlorosis

Mushroom compost is particularly useful on the vegetable garden as vegetable crops usually grow better and more productively in soil which is not acidic and exhibits alkaline properties. In chalky alkaline conditions, brassicas (such as sprouts, cabbage, kale and broccoli) are less likely to also be inflicted with clubroot disease too. This form of compost is unsuitable, however, for fruit crops, which will ideally enjoy a more acidic/neutral soil. In the greenhouse, due to mushroom compost’s high concentration of soluble salts, it is best avoided as a potting medium and reserved instead solely for use in borders in the garden.

With regard to the organic use of mushroom compost, there is a mild chance that the used compost will include a small amount of residual pesticides, such as organochlorides, used against the fungus gnat. Chemicals may also have been used to sterilize both the straw and compost pre and post production. The Department for Environment, Food and Rural Affairs (DEFRA) will be able to advise further on this issue.

Kindly written by Kevin Thorns