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Mushroom Compost: A Complete Guide

Mushroom compost provides slow release nutrients to your plants and helps retain soil moisture. It can also help break up heavy clay soils.

Mushroom compost is a fairly inexpensive soil additive that can not only improve the condition and structure of your soil but can also provide some nutrients to your growing plants.

In addition, mushroom compost can also aid in soil water retention and help to increase microbial activity in the soil.

What is mushroom compost?

Compost | Plant care
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Mushroom compost is a by-product of the mushroom growing industry.

It’s generally comprised of raw organic materials such as straw, hay, horse and poultry manures, cotton seed hulls, and corn cobs.

Some mushroom composts may also contain peat moss, gypsum, soybean meal, and lime.

Once the base ingredients are mixed up for the compost, it’s then steam pasteurised in order to kill both harmful pathogens and weed seeds. This process generally happens after the mushroom spawn has been added to the mix.

After the mushrooms have all been harvested, the remaining compost is bagged up and sold as a fertiliser or general soil additive.

You can usually buy mushroom compost at most garden centres and at hardware stores like Bunnings.

Mushroom compost benefits

Although not a complete fertiliser, mushroom compost has numerous benefits in the garden.

It’s a great way to improve the condition of the soil by digging it in when preparing garden beds or vegetable growing plots.

This is especially useful for clay soils because the compost helps to break up the clay particles.

Here are some of the major benefits:

  • Soil conditioner
  • Breaks up heavy clay soils
  • Helps retain soil moisture
  • Provides a slow release of nutrients to your plants
  • Increases soil microbial activity

How to use mushroom compost

Compost heap | Plant care
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One of the best ways to use mushroom compost is to incorporate it into the soil when you’re establishing a new garden bed.

To do this, it’s just a case of spreading a layer of the compost on the top of the soil and then digging it in, down to a level of about 10 to 30 cm.

Once you’ve done this, it’s a good idea to let the bed sit for around a week to let the compost blend well with the soil.

Another way to use mushroom compost is as a clay breaker.

Thanks to the straw content in the compost, it can be very beneficial in breaking up some of the tightly held clay particles.

This also means that the nutrients that are often tightly bound in clay soils will be more readily available to the plants. Just follow the same process as above to incorporate the compost into the clay.

The third common way to use mushroom compost is to use it as a mulch around your existing trees and shrubs.

It will keep the soil cooler and will help to retain moisture for much longer.

Just avoid using it around acid-loving or native plants and only place a thin layer around other plants.

Mushroom compost pH

In general, mushroom compost is considered alkaline because lime is normally added during the composting process.

However, the pH can vary quite considerably from one batch to another because it is essentially an organic material.

Most mushroom growers and mushroom compost resellers will state the pH at around 6.7 to 7.

This is quite important to understand when adding this compost to your soil so that you don’t end up with overly alkaline soil.

It’s also the reason that mushroom compost should not be used around acid-loving plants.

Plants that like mushroom compost

carrots | Plant care

Mushroom compost can be used on most plants except those that prefer acid soils.

This makes it ideal for incorporating into vegetable growing plots and garden beds.

Here is just a selection of plants that do like mushroom compost:

  • Vegetables such as tomatoes, carrots, lettuce, cabbage, broccoli, potatoes, spinach, squash, capsicum, kale, beetroot, sweetcorn, and peas.
  • Lavender
  • Jasmine
  • Lilac
  • Hosta
  • Daisies
  • Roses

Plants that don’t like mushroom compost

Magnolia tree flowers in bloom | Plant care

In general, mushroom compost is not suitable for plants that like more acidic soil.

The compost is actually quite alkaline and contains a high level of soluble salts.

Therefore, it should not be used in areas where acid-loving plants are growing, either as a soil additive or a mulch.

Plants that really don’t like mushroom compost include:

  • Azaleas
  • Rhododendrons
  • Camellias
  • Hydrangeas
  • Gardenias
  • Magnolias
  • Blueberries
  • Juniper
  • Ferns
  • Celery
  • Cucumber
  • Beans

Additionally, you shouldn’t use mushroom compost in or around plants that don’t like moist conditions, such as succulents and cacti.

Due to its formulation, mushroom compost does increase the water-holding capacity of the soil. This is not ideal for plants that like to have the soil dry in between watering.

It’s also advisable not to use mushroom compost around Australian native plants as most of these are sensitive to soluble salts.

FAQ

Can you buy mushroom compost at Bunnings?

Yes, mushroom compost is available in 25-litre bags at Bunnings. 

Is mushroom compost good for tomatoes?

Yes, mushroom compost can be used around tomatoes even though they generally prefer more acidic soil. Tomatoes are particularly fond of the calcium found in mushroom compost. In saying that, you should only use a small amount of mushroom compost as a mulch around your tomato plants.

Will mushroom compost burn plants?

Yes, the high salt content in mushroom compost can burn the roots of some plants. It should generally not be applied around plants that are acid-loving or sensitive to high levels of soluble salts.

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Annette Hird

Annette Hird is a gardening expert with many years of experience in a range of gardening related positions. She has an Associate Diploma of Applied Science in Horticulture and has worked in a variety of production nurseries, primarily as a propagator. She has also been responsible for a large homestead garden that included lawn care, fruit trees, roses and many other ornamental plants. More recently, Annette has concentrated on improving the garden landscape of the homes that she has lived in and focused a lot of energy on growing edible plants as well. She now enjoys sharing her experience and knowledge with others by writing articles about all facets of gardening and growing plants.

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