Answer: You're both right, says Bill Wolf, president of Necessary Trading Company, a supplier of organic gardening products in New Castle, VirginiA. Used mushroom soil isn't high in nutrients, but it's still a great source of organic matter for soil conditioning, he says. And it is very attractive to soil insects and microbes, which feed on the mushroom compost, gradually releasing some of the nutrients locked up in the organic matter to plants all through the season. In a sense the mushroom soil is adding fertility by feeding the microorganisms. The high organic matter is mushroom soil also helps hold more moisture and increases the air spaces in this soil, according to Wolf.
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