Living organisms in soil refine and mix the soil, add organic matter, increase the amount of nutrients in the soil, and often make soil uninhabitable for other organisms. Mycorrhizae are mutually beneficial associations between roots and fungi. Mycorrhizal fungi increase the absorption of nutrients and tolerance of environmental stresses.
Mutualistic relationships may have evolved as organisms traded resources. Many plants, especially legumes, establish mutually beneficial relationships with nitrogen-fixing bacteria. The bacteria convert atmospheric N2 to ammonia, which the legume uses for growth. These relationships have great benefits for farmers around the world.
Many fungi are parasites of crop plants and forest trees. Why might it be a bad idea to spray a fungicide (which kills all fungi) on a forest to control a parasitic fungus?
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