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It can be used in soil, coco, rockwool, some even use it in hydro. The more important thing is the style of growing you use, and whether or not that will impact the life of the micro-beasties negatively. The style most suited for use of microbes would be no-till/supersoil/ amended soils such as
foxfarm ocean forest or
happy frog. The next best thing would be when your using BOTTLED ORGANIC NUTRIENTS such as Bio-bizz,
Roots organics, or
Nectar for the gods.
all this being said, you CAN use myco if you're using salt based (or synthetic) nutes, it just wont be as effective. The job of the myco is to reach out with its fungal hyphae to tap into nutrients where the plant cant reach. The plant signals that its hungry, and the myco transports the food to the plant.
If your using salt/synthetic nutes, this isn't really needed since your basically force feeding your plants with readily available forms of nutrients.