"Dolomite Lime." Dolomite is a good source of calcium and magnesium and can be mixed with your soil. The great thing about dolomite is it works slowly over the course of a few months. Dolomite has a neutral pH of about 7.0 and will help keep soil at the correct neutral pH range which is optimum for cannabis growth. You can buy Dolomite Lime online, but with shipping it's almost always waaaay cheaper to pick up a bag at a home improvement or gardening store such as Lowes, Home Depot, gardening centers, etc. If possible, try to get a finer grade of dolomite compared to something that is more coarse.
How to Use Dolomite Lime for Cannabis: When growing cannabis indoors, add 6-7 teaspoons of fine dolomite lime to each gallon's worth of soil. So if you're mixing enough soil to fill a 5 gallon container, you want to add 30-35 teaspoons (about 2/3 cup) of dolomite lime to the mix. Mix the dolomite lime and the dry soil thoroughly, then lightly water it with water that has been pH'ed to 6.5. After getting the soil wet, mix the soil well and wait a day or two to let the soil settle before checking the pH and adding plants. When growing in an outdoor garden, follow the dolomite lime manufacturers instructions.
The pH of your marijuana plant's root zone is very important because when the pH is too high or too low, the plant's roots will not be able to absorb nutrients properly. As a result, cannabis plants may start showing signs of nutrition deficiencies even if the nutrients are available in the growing medium. In fact, feeding your plant with the incorrect pH is one of the most common reasons that a plant is displaying a cannabis nutrient deficiency. If you water your plants with water that has been pH'ed to 6.0, and the water that comes out the bottom has a pH of 6.5, you can assume the pH of your root zone must be around 7.0, because the water that came out was 6.5, which is halfway between 6.0 and 7.0. When trying to adjust the pH of your root zone, make sure to do all changes slowly. It is actually worse for your plant to have the pH change drastically than it is for the plant to have roots in the wrong pH
note: info from growweedeasy.com