Because mycelium (the body to which mushroom is a fruit) makes up 10-30% of soil. When you dig out the soil, you break the mycelium connections between the soil you removed and the rest of the earth. Now water and nutrients are not going to move as easily to the replaced soil untill ge mycelium repairs itself. Almost like ripping a house out of the ground, putting it back and wondering why the plumbing doesn’t work
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