Coffee grounds help to increase the acidity of the soil which is important if the cannabis plant is to thrive and give the required yield. Cannabis requires a slightly acidic pH environment of 6.0 to 6.5 to thrive and flourish and with coffee grounds, these levels can be reached. If the environment of the soil is too alkaline, coffee grounds can be added to help reduce the pH to favor acidity. Coffee grounds can also be added if the soil is not acidic enough to help the cannabis plants grow as it ought to.