Mushroom compost is NOT compost. It's partially rotted organic material such as straw. It has no nutrients. If you want to start seeds, buy a proper seed starting mix which is usually a sphagnum moss peat based material. I use Pro Mix BX but there are many others.

The proper term is Spent Mushroom Substrate (SMS). Search on that term and you will learn all about it. I use SMS to hill potatoes or as a mulch. You can dig it in to the garden but wait until the next year to plant or add nitrogen fertilizer to ensure your plants are not straved.

it's not a seed raising medium, you would have to germinate in trays or the like then transplant the seedling, you could try creating rows of a suitable medium then seed those rows.

len

Here is a link that might be useful: lens straw bale garden

I would agree that Spent Mushroom Subtrate is not a good choice for starting seeds. It is primarily intended as an ammendment tilled into soil, so I wouldn't recommend planting directly into it. The reason is that the salt levels tend to be up there, this is from a lot of good salts like N, P, K, and Ca among others. So I have to take issue with bluegoat because SMS does have a lot of nutritional value.
The reason it is sometimes called mushroom compost, and marketed that way, is because the material(straw, horse bedding, poultry manure, etc) is composted according to specifications from mushroom growers. After producing crops of mushrooms it is spent substrate. In my neck of the woods (Chicago) there is some variation at what I see at stores. Some products make claims of being mushroom compost but may not truely be 100% Spent Mushroom Substrate. If you can get it from a mushroom farm, or have material that you know is coming from a mushroom farm, you should have some good stuff.
There is a lot of good info on it from the Penn State and others. They created the website below.

Here is a link that might be useful: http://www.mushroomcompost.org/

well, i have learned this year. don't plant in the compost.........only use for mulch. the plants I've mulched are doing well. Thanks for your input.

Mushroom compost is made from horse manure, straw, chicken manure, and GYPSUM. Put your hand up if the soil test says putting gypsum on your soil is a good idea this year. Now, if you don't do the math, and use a bunch of it, you might end up having to do a recorrection with next year's soil test. In addition, mushroom compost is saltier than hell, chock full of sodium and chloride. It's not a good product, it's "garbage for martians". I've used it, and have banned it to the dungeons. No more waste of hard-earned cash on that concept....Sorry noone told ya already...sheesh...

Mackel

Some people have had good results when starting seeds in Mushroom Compost while others have not. My experience is that mushroom compost can be as difficult to get evenly moist as peat moss and that might affect germination about as much as anything else.

Here's an additional article about SMS.

The important thing to remember as a gardener is that it's not compost and will require further composting to reach the stage of mature compost.

One year I hauled a 1/2-ton load of of SMS for future use. It was stored in a large wooden compost bin a little of a cubic yard in size. After four days the temperature had reached 120F. Water was dripping off the lid and mushrooms were gowing on the surface. This is not mature compost.

Here is a link that might be useful: Spent Mushroom Substrate

"So the 'Spent Mushroom Substrate' is as much compost as any other mixture of decaying organic matter."- Kimmsr

It doesn't matter if mushroom compost has been aged for five years, eventually it'll be all gypsum, and produce measureable negative effects on nine out of ten soils. Kimmsr, people have called you out on various forums since the internet began. It hurts my head, you pretending to be the intellectual type, without getting the practitioner part accurate. I'm one of dozens of posters who have pointed this out to you all over the globe, and on different forums. You are a deeply flawed character, son.

Mackel

not sure where this debate is heading it certainly seems to be getting somewhat personal, not a good debating method to attack anyone's character, we don't know each other that well enough. a judgement can't be made unless one knows about who they are judging.

this whole spent mushroom compost seems to be getting off line, see our bale garden presentation like all gardens we do they are raised and like all gardens to date, we use spent mushroom compost in the beginning and when topping up is needed we use spent mush' compost again.

it's not so much about improving the soil that is there but it does that very well from our experience, i'm not sure about the gypsum factor in the mush' compost? but either way we end up with a good growing medium, that we plant in as soon as the garden is built, we have the pic's there unless someone is going to judge that we are slight of hand and snake oil sellers?

all the soils we get to work with here are clay and we use heaps of gypsum sometimes a sprinkle of dolomite as well but rarely, at around 12 months or more we dig by finger through our medium and down into the original soil without really noticing so some change for the better is occurring.

i listen rarely to science they only get involved when someone pays them to so they support the persons who pay them, in gardening KISS is the go, science has led our food growers down the wrong path and over here with their nothing but theories they are conspiring on this climate change do dat.

len

Here is a link that might be useful: lens straw bale garden