Abaye notes that mushrooms do not obtain their sustenance from the soil. so the bracha is "shehakol."
However, today we know mushrooms obtain their nourishment from soil - shouldn't the bracha be amended?
harvey babich, NY, NY
Here is how the Kollel addressed this question (in Background to the Daf for Nedarim 55b), I hope you find it helpful:
22) [line 13] MIRVA RAVU ME'AR'A; MEINAK ME'AVIRA YANKEI - they grow out of the ground, but unlike plants that absorb nutrients dissolved in water in the soil, a mushroom or fungus finds its own food from its immediate environment. Enzymes that the fungi excrete dissolve the food, and the nutrients are then absorbed directly through the thin cell walls. Chazal use the term "me'Avira Yankei" (lit. they get their nourishment from the air) to refer to plants that do not take their nourishment from the ground, as is apparent from the Gemara Eruvin 28b. Since fungi grow on rocks and dead wood, they also fall into this category.
Best wishes,
M. Kornfeld and Y. Tavin,
Kollel Iyun Hadaf
Dear Rabbi Kornfeld:
Actually, I had a different idea -- If an individual wanted to grow mushrooms in his basement, the substratum used to support growth is compost, which is prepared from horse dung and straw (note, not from "soil"). The horse dung is allowed to undergo natural microbial fermentations, thereby yielding nutrients and insoluble lignins and celulose. It is then placed in flat beds and after a week, it is inoculated with mushroom spores (usually, Agaricus bisporus). However, after the spores are introduced into the fermented compost, a shallow casing layer of soil is spread over the compost; this casing layer provides a nutrient-deficient layer which does not encourage vegetative growth but does encourage the reproductive stage (i.e., it now produces the structure that is consumed by humans). (Source: Moore-Landecker, E., 1972, Fundamentals of the Fungi, Prentice-Hall, Inc., Englewood Cliffs, NJ). Also see: Rabbi Z. Blech, A fungible feast, mushrooms in halacha
(http://www.kashrut.com/articles/mushroom) --> "The key to successful mushroom production is finding an ample supply of decaying material (compost) to be used as the mushroom bedding and finding a way to inoculate this bedding with the desired fungal spores. The compost used in mushroom production is generally made by mixing a variety of less savory ingredients - horse manure, 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." Again, note, soil is not mentioned.
In the times of the Gemora, farmers lived in close proximity to their cattle, sheep, horses, chickens, etc. The most usual area on these frams in which to see mushrooms sprouting was, most probably, in the fermenting manure from these animals. That may be the reason why the Gemora notes that mushrooms do not get their nutrients from soil (i.e., soil is not that rich in organics to support mushroom growth).
Thank you,
Harvey Babich
Stern College for Women
Yeshiva University