There are all kinds of information about spent mushroom compost on the web ranging from it is the most bestest thing to come down the pike to it is not worth anything even if its free.

Spent Mushroom Compost is what mushrooms are grown in and it can be a mix of many things organic, manure, straw, shredded leaves, ground wood, whatever the mushroom grower can get and has found will grow a profit. Spent Mushroom Compost is often steam sterilized so only what the mushroom grower seeds in there will grow. The compost is fairly rapidly repopulated with beneficial organisms or the mushrooms, fungus, would not grow.

The pH scale is a range from 0 to 14 to give an indication of how acidic (toward 0) or how alkaline )toward 14) a substance may be. 7.0 is neutral, neither acidic nor alkaline, balanced so to speak. A pH of 6.6 is slightly acidic and is about in the middle of the soil pH most all plants grow best in since most all plant nutrients are most readily available.

Chalk is a clay and is not inherently good or bad, but like many other things depends on the amount. Could the "white" stuff you see is perlite of vermiculite? That may be some "mold" the term some use for the

"Salt" is a generic term for sodium and sodium is the sixth largest element in the universe. Table salt, Sodium Chloride, is not good, in the garden, but that may not be what the article was about. The roots of fungi are known as mycella and are white thread like growths.

I do not have any personal experience with Natures Helper brands which are not sold in the is area, but I have used Spent Mushroom Compost with no problems in the past.

kimmq is kimmsr

Just like any other type of compost, there is good stuff and there is bad stuff. 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. It is steam pasturized to kill off any surface pathogens or pests then seeded with the mushroom spores. Once the mushrooms are grown and harvested, the compost or growing media is discarded as "spent" or no longer useful for mushroom production. But the remains are still a useful source of OM for the gardener.

Concerns about pH are valid - just as the quality can vary, so can the pH depending on the ingredients. How "composted" this material really is is at issue, as the ingredients are pretty much piled together fresh before the pasturization process and depending on the materials used and their concentrations in the mix, pH could be higher than expected - I'd test to make sure.

The salts they are referring to are soluble salts. Soluble salts are minerals/nutrients dissolved in water. Synthetic fertilizer dissolved in water becomes a soluble salt and that is how plants take up nutrients, as soluble salts. When water evaporates from the soil, the minerals orsalts stay behind. As the salts in the soil become more and more concentrated, it becomes more difficult for plants to take up water. The soluble salts in mushroom compost are often quite concentrated and while these concentrations may be okay for more mature plants, they are often too concentrated for seedlings or seed starting. Also members of the Ericaceae or heath family salt sensitive and not happy with high levels of soluble salts.

Chalk is lime and this is often added to MC, in part to offset a lower pH or the acidity of some of the ingredients. Some is fine, too much is not. Again, it depends on the ingredients and how they were processed.

As with any acquired or purchased soil or compost product, I would want to see and examine the stuff before purchasing to determine exactly what you are getting - that the texture and smell is good, that there is no additional 'stuff' (debris) lurking in the mix and that mix looks uniform.

One thing I would add to the above is that if it smells bad (like manure for example) it may not be fully composted so you may want to avoid that one. True of any compost. Fungi live on wood so the mushroom growing medium has shredded fibrous and woody stuff in it. That needs to be composted. Steamed manure may be free of extraneous fungi but steamed is not composted. So when it's 'spent' it may need to be composted for awhile in a pile. I don't know the handling practices for sure. But if it does not seem to be good earthy smelling compost, that's a good reason to move on.

Salt means the product of an acid and a base----any acid and base. Not necessarily Sodium Chloride, which is pretty much inimical to plant life.

I don't know why you would expect to see NaCl in a bag of compost unless it's manure from an animal that consumes a lot of NaCl. You may, however, see salt in a crudely made batch of compost, I.e. excessive nutrients that might burn your plants. That's can be an issue, but a different one.

I said soluble salt, which is not the same as table salt. I know that fertilizer contains soluble salt that if use in high amount can burn plants.