For optimal growth, the pH level of the soil for marijuana plants should be between 6.0 and 7.0, with a slightly lower range of 5.5 to 6.5 preferred for hydroponics or non-soil mediums.

The pH of the growing medium should ideally be between 6.0 and 7.0 for cannabis plants. This range is considered neutral and allows the plant to absorb the necessary nutrients from the soil or growing medium.