The Q&A Archives: mushroom soil for veggies

Question: I just moved to a new house, and have a 30' x 30' space that I would like to make into a raised bed (square foot gardening) garden. I made six 11' x 4' boxes to fill with the recommended peat moss, vermiculite, and compost mixture, but I can get mushroom soil in bulk, too. The garden is situated over a very hard, clay and sand ground. We are on a lagoon. Should I fill the boxes with mushroom soil and topsoil, and add the neccessary things to that, or use the compost/vermiculite/peat moss mix? Also, I can't find vermiculite, and even heard it contains carcinogens and is now illiegal, but don't know if it's true. I don't have a whole lot of money to spend on this, but want good veggies this year. What do you suggest?

Answer: In my experience, peat moss and vermiculite will be very expensive in that quantity. You are going to need a huge volume of material to fill all those beds. (The vermiculite sold for horticultural use does not contain asbestos -- it is mined in a different area -- however you should still take care not to breathe the dust. You should not breathe peat moss dust, either. Moistening these helps keeps down the dust, you should probably wear a mask when you work with it dry.)

I would suggest you explore some different alternatives to see what is available locally at acceptable cost. You might find that the cost effective options include mushroom compost, coarse sand (not the fine play sand or beach sand) or fine grit, possibly leaf mold if there is a nearby municipal source for that, or well aged, rotted down stable manure/bedding, and topsoil from a reputable supplier who can provide soil test results for it. Your local county extension may have suggestions as to local sources.

I would also suggest you work with them to test the mix and make sure it is going to provide adequate fertility, good drainage, good water holding capability, and has a suitable pH. The specific ingredients are not as important as creating a mix that meets the above criteria.

Also, you may be able to use some of the native soil mixed into the other ingredients rather than purchasing top soil. If it is an area that has flooded in the past, there may be some issues with salt in that soil. This is also something your local county extension should be able to advise you on.

You might also want to look into any chemical treatments used on the mushroom compost, to be sure there will be no residual effects that could influence your vegetable crops.

Keep in mind you will need to replenish the soil in the raised beds. As the organic matter breaks down, it will settle dramatically. Your best long term solution would be to make your own good quality compost and use that, along with using an organic mulch and possibly cover crops. Save and compost autumn leaves, herbicide free grass clippings, disease free garden debris, and so on for this purpose.

With raised bed soil mixes as with anything else in gardening, there is going to be some experimenting and learning as you go to find out what works best for your situation. I hope you have a better vegetable year -- I remember last year wasn't that easy!

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