Is Salt in Mushroom Compost Harmful to Your Garden?

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Robert Pavlis

Mushroom compost is used a lot for gardens but it comes with a warning, don’t use too much because it contains a lot of salt. I have been aware of this for a long time, and I have even cautioned people about using too much. Salt can harm plants and it only makes sense not to use a product loaded with salt.

I always wondered why mushrooms would be grown with high levels of sodium. Are the fungi that different from plants? Do they need high sodium levels? A little online research started to make things clear. It turns out to be a good example of the confusion caused between salt and salt – something every gardener should understand.

Is Salt in Mushroom Compost Harmful to Your Garden?, photo by Highline Mushrooms
Is Salt in Mushroom Compost Harmful to Your Garden?

What is Salt?

It is always good to define your terms and in this case it is absolutely critical.

The general public uses the term salt to mean table salt, which is sodium chloride. This is also used in cold climates in winter to defrost ice, so most cold climate gardeners know that salt kills plants because they have seen the damage left behind in spring due to the salt trucks.

Chemists and other scientists use the term salt to refer to any compound that is made up of ions. Sodium chloride is one of many different types of salts. In water, it breaks up into sodium ions (Na+) and chloride ions (Cl-).

Compost Science for Gardeners by Robert Pavlis

Compounds such as ammonium nitrate and potassium phosphate, found in synthetic fertilizer, are also salts. Too much fertilizer can also damage plants, but the fertilizer salts are not nearly as toxic to plants as sodium.

Unfortunately, gardeners are caught half way between these two definitions for salt; sometimes it is fertilizer salt and sometimes it is table salt. When gardeners use the term, you have to stop and ask how the word is being used.

How is Mushroom Compost Made?

“The recipe for mushroom compost varies from company to company, but can include composted wheat or rye straw, peat moss, used horse bedding straw, chicken manure, cottonseed or canola meal, grape crushings from wineries, soybean meal, potash, gypsum, urea, ammonium nitrate and lime. ”

The ingredients are combined and mixed well. It is then left to sit and compost. The ingredients contain a fair amount of nitrogen so the pile heats quickly, producing a nutrient rich compost. When finished it is pasteurized to kill disease causing organisms and pests.

It is then placed of beds and inoculated with mushroom spawn (mycellium). Once the mushrooms are harvested the compost can’t be used again because it is not nutritious enough for a second batch. It still contains lots of nutrients for plants and is sold to nurseries or made available free of charge to gardeners.

As it comes out of the mushroom production facility, it is called ‘fresh’ mushroom compost. In most situations it then sits in piles for a few months to further age before it is provided to gardeners.

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Sodium Salt in Mushroom Compost

A recent study collected fresh mushroom compost from 30 facilities in Pennsylvania, USA, for lab analysis.

High levels of sodium can cause problems with soil structure and inhibit water absorption in roots. One way to determine if the sodium levels are high enough to cause a problem is to look at the SAR value, which compares sodium concentrations to those of calcium and magnesium. A SAR of 15 or more indicates a problem. The SAR of the mushroom compost was 0.4 indicating that the amount of sodium was very low.

The actual measured amounts of sodium, on a wet weight basis, were 0.1%.

PH of Mushroom Compost

A lot of lime and gypsum are used to make mushroom compost and there is a concern that it is too alkaline for garden use. The average pH in the above study was 6.6, which is perfect for most gardens.

Fertilizer Salts in Mushroom Compost

The average NPK of mushroom compost, in the above study, was 1.1-0.7-1.3. This is well within the range of other types of compost which tend to be around 1-1-1.

The calcium level is 2.3% which is a bit high, but not a concern in most soils.

If you purchased a bag of compost with these numbers you would not consider it to have a high salt content.

Keep in mind that all of these numbers refer to fresh mushroom compost. Once it sits for a while it will have even lower nutrient values.

This Post might Interest you: You Can Use Too Much Compost

Effect on Seedlings

Trying to grow seeds or young plants in straight mushroom compost will cause problems. The media is just to nutritious and will harm small plants. Mix it with 80% soil to make a good mix.

What About the Gypsum?

Some people are concerned about the added gypsum. Recipes use about 5% gypsum, which is calcium sulfate. The sulfur is a nutrient required by plants. The oxygen and water in it don’t affect anything. The calcium in the fresh mushroom compost is 2.3% and some of that is from all the plant material used to make the compost.

Gypsum is not a concern.

Does Mushroom Compost Contain Too Much Salt?

All mushroom compost is made differently, but the data above from 30 different facilities indicates that it is neither high in sodium, nor fertilizer salts.

The key is to understand that it is a rich compost, not a soil. Use it as a 1-2″ mulch, or add a bit to soil, and it will help build soil structure and feed plants. If you use too much you can burn plants but that is true of any manure, compost or fertilizer.


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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

17 thoughts on “Is Salt in Mushroom Compost Harmful to Your Garden?”

  1. I had wondered about that for years too. Thanks for doing the reading for us. Interesting that part of the misunderstanding is from something as simple as people not understanding the difference between total salts and sodium chloride.

  2. The method of making mushroom compost you describe is very old and more in line with how button mushroom compost is made. Most local growers of wood loving mushrooms, of which I am one, don’t use any animal waste, nor is there a composting process involved.

    A simple recipe is: wood chips, 2% gypsum and water. This is mixed to field capacity and steam sterilized. This is a suitable substrate for Oyster mushrooms, Lion’s Mane, Black poplar, Shiitake, and all other wood lovers I am familiar with, with the exception of Chaga, which is a bit of an oddball in that it has to parasitize a living tree (typically birch.)

    So you are right in terms of the contents of store bought spent mushroom substrates, which typically come from bulk button growers, but the recipes used for wood lovers, which most local growers cultivate due to price point and a lower concern regarding contaminants, is somewhat different.

      • Like many of your answers on here, it depends.

        Grain spawn is produced using grain, which is a fertilizer in our use case given its higher nutrient density relative to wood. Grain spawn typically makes up between 5-20% of larger grows by volume. However, grain spawn is optional. Direct injection of mycelium into a wood chip/sawdust substrate also works for most wood loving mushrooms, though yields will probably be lower (speculation on my part, I have not seen a study on that topic.)

        As to fertilizing the wood substrate intentionally, rather than as a byproduct of inoculating it with grain spawn, some growers add bran in a 10% by volume ratio. Another common mix is called master’s blend, which is 50% hard wood pellets and 50% soy hulls by weight. Both have been shown to improve yields over wood alone. Both also increase the risk of contamination and crop loss, unless sterility is well maintained or antifungals are used. The richer a substrate is, the more prone to contamination by trichoderma or other molds it will be. For starting hobbyist growers, and established growers with time and space, it often makes sense to accept lower yields in order to avoid higher risk of crop loss caused by imperfect sterility.

        While the use of more nutrient dense substances as fertilizers is common where sterility can be easily maintained, additions of this kind are optional, just as they are in conventional gardens. Apart from the fact that mushrooms grow in the wild without any such amendments, the classic example of producing mushrooms on wood alone originates in Japan with Shiitake log farms. In this process wooden dowels are colonized by Shiitake mycellium, holes are drilled in logs, dowels are placed in the logs and covered in wax. Then the growers wait, once colonization is complete, fruiting occurs. Here is a US based example of this process:

        Personally, I use grain spawn, gypsum and wood chips, but nothing else. The cost of keeping a perfectly sterile environment outweighs yield increases for me.

        Oh, and thank you for your work. Accurate and reliable information is very hard to find, if I saw another website claiming that watered down fish poop was a magic elixir, without any supporting evidence, I fear I’d have pulled my hair out. Your site is a relief to read, as it doesn’t include magical thinking.

  3. I have used Mushroom Compost for years and recently someone told me it had a lot of salt in it. This article explains it so well. I see it is best to let it mellow by placing it in the garden in the fall. Thanks for taking the time to do this research and post it.

  4. Is most generic compost in big box stores or nursery and agriculture type stores the same as the spent growing media you’d get from a large scale farm ? Say if you had a connection to gather their discarded blocks.

  5. Great information! In regards to planting seeds in straight mushroom compost, I had great success with a 900 sq. ft section growing native wildflowers (perennials and annuals) from seeds in straight mushroom compost (not mixed with soil). It might just be my variety of mushroom compost perhaps, or specifically what I was planting, though all of mine comes from a PA mushroom farm that participated in that study that you referenced. I’ve also had great success in germinating native seeds in small trays of mushroom compost over winter months.

  6. Thanks Robert,
    You’ve saved me from believing many myths – now I know mushroom compost doesn’t have too much sodium.

  7. Glad to find this site. Would love to hear your thoughts on Neem oil (which sounds suspiciously like a lot of hype to me), as well as whitefly control – been getting a lot of those buggers lately.

    • Have not research NEEM oil. I once compared it to baby oil to control scale on orchids and found no difference.

    • I sometimes get whitefly in the greenhouse, and I’ve found the sundews, Drosera spp, attract and kill them very effectively. I keep them in tubs of rainwater. It must be rainwater, and don’t let them dry out.

    • This year for the first time I used neem oil, and as recommended , obtained pure neem. The only insect I specifically targeted was the red lily beetle, and the neem oil was very effective. I expected a resurgence, but the population did not reestablish. Hope this helps.

      • Three years ago, I did nothing. The population did not reestablish. This is a true story – I have not seen them for 3 years – no idea why.

        Without controls – you can’t conclude anything.

  8. Your article on mushroom compost was reassuring, but you do not address the insecticides used to control mushroom flies,whose tunneling would make the mushrooms unsaleable. I would appreciate your comments on this.
    David Marshall

    • Insecticides in compost are not going to harm other plants. The amount you use per volume of soil is very small – I doubt it is a concern.


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