Amending Peat for Cannabis

Using Peat for Indoor Grows

Making your own growing medium is really simple and will save you a load of cash. The following information should help you get started.

Peat Moss

Peat Moss is the main ingredient in most growing medium mixes as is it cheap, readily available, and retains water well. In addition peat has a mid to high CEC meaning it can hold on to nutrients for later use.


Perlite is used to help provide drainage and increased aeration of the root zone. A good ratio to use in your mix is 2:1 so about 2/3rds peat to 1/3rd perlite but it doesn’t have to be perfect. When making your mix just keep in mind that the more perlite there is the quicker the containers will dry out.

Yucca (optional)

Yucca is made from the plant Yucca Schidigera. It can be used as wetting agent for nutrient solutions, foliar sprays, and growing mediums and is great for flushing excess salts from the root Zone. When used in growing mediums like peat, it helps water and nutrients penetrate deeper and more evenly into the root zone and helps prevent peat from repelling water which can happen over time if gets too dry between waterings.


Lime is used to buffer the natural acidity of peat. Amending peat with lime will allow you to balance the pH of your mixture towards an optimal 6.4. The 2 types of lime used for buffering are calcitic and dolomite. These 2 types of lime have different elemental concentrations and breakdown at different rates. Only dolomite should be used since it is slower to breakdown and release. Calcitic lime can toxify your medium when over applied because it is immediately available for uptake.

Buffering Peats pH

Peat is naturally quite acidic (pH 3.5-4.5) and will drift to become even more acidic over time. The pH of a growing medium like peat is partially regulated by the mediums ability to hold nutrients which is known as the CEC or Cation Exchange Capacity. But because peat moss has a naturally low pH it will need to be buffered to both raise and stabilize its pH so that nutrients in the root zone remain available for uptake by plants throughout the season.

Types of Garden Lime

Only purchase powdered lime for you mixes, pelletized lime is for lawns and does not buffer growing mediums very well. It can be tricky finding bags of powdered dolomite lime at the big box stores. I often find it at random times during summer months and it is usually on a palette floating in the aisle or near end caps.


Dolomitic Lime

Dolomitic Lime is the ideal lime to use when buffering growing mediums and will seldom require further adjustment during the season. Dolomitic lime has a decent quantity of Magnesium in it, typically giving you a mixture that is around 65% Calcium and 35% Magnesium. On average when only using dolomitic lime you will use 2 tablespoons per gallon of growing medium or 3/4 to 1 Cup per cubic foot.

  • Extended Release
  • 65% Calcium & 35% Magnesium
  • Lasts About 4 Months
  • Balance The Lime To Peat Ratio To Obtain A pH of 6.4
  • Target Nutrient & Water pH 6.4
  • Average Application 3/4 to 1 Cup per Cubic Ft.
  • Over Applying Can Be Difficult Since It Is Slower To Release

Calcitic Lime (not recommended)

Calcitic Lime is almost entirely made of Calcium, the Magnesium content is negligible. Calcitic lime is immediately available to the plants but will only last about 4-6 weeks in the growing medium. Use sparingly and start on the low side at 1/8 cup per cubic foot of growing medium and see how well your soil maintains its pH buffering throughout the season by doing regular runoff testing for both pH and ppm.

  • Immediately Available
  • Almost Entirely Calcium
  • Only Lasts 4-6 Weeks
  • Balance The Lime To Peat Ratio To Obtain A pH of 6.4
  • Target Nutrient & Water pH 6.4
  • Average Application 1/8 Cup per Cubic Ft.
  • Over Applying Will Toxify the Soil

Helpful Measurement Conversions

1 Cup =
16 tablespoons

1 Cubic Foot =
7.5 gallons

Oyster Shell

Oyster Shell is an all-natural and high-quality source of calcium and can also be used to buffer peat growing mediums. Oyster shell amendments are ground into small, medium and coarse particles, these irregular shapes provide a sustained release of calcium as well as other micronutrients and help to chelate phosphorus while improving structure of soils and building optimum tilth. Typical application is 1-2 tablespoons per gallon of medium, which works out to about 1/2 to 1 cup per cubic ft. of medium or .5-1 lb per cubic yard.

Midseason Supplementation

Calcium is an important nutrient that cannabis uses a lot of (read more about cannabis nutrient ratios) and is used in all stages of development to build strong vascular systems in plants. Over time plants will deplete the calcium in the medium and cause the pH to drift. If calcium supplementation is needed midseason your only option comes from the nutrients you are using so it’s important to get your medium mixed right before the season starts. Attempting to mix powdered dolomite lime with water and supplementing that way doesn’t work well due to the compacting nature of peat over time, it cannot penetrate the medium in any meaningful way.

Soil Microbiology

It’s a good idea to use a Mycorrhizae soil amendment, preferably one that contains some sort of nutrient salt tolerant Bacillus bacteria that can address any contaminants in the growing medium that may find their way in like fungus gnats or anaerobic microbes. Mycorrhizae fungi form a symbiotic relationship with plant roots that will encourage strong healthy plants and increase nutrient uptake. Always use an additive that contains powdered molasses, this will feed the fungal an bacterial colonies.

Pro-Mix HP

Depending on where you live, there are some excellent peat based growing mediums available like Pro-Mix HP. It can get kinda pricey when buying in volume, but it will save you the hassle of mixing peat, lime, perlite etc. which can be a dirty job. Pro-Mix HP’s pre-blended lime amended peat that contains a little calcitic lime to cover the 1st 30 days of growth and the remainder is dolomitic lime which breaks down slowly over time and will last an entire 4+ month growing season. The Pro Mix HP peat blend usually has a pH of 5.5 to 5.8. The company suggests that nutrient solutions and water should be pH adjusted to 5.8 to 6.2 when using their HP product with chemical/salt based hydroponic nutrients.

grow room, grow rooms, grow room setup, grow room lighting, grow room equipment, grow room lighting, grow room environment, grow room security, grow room light, grow room light ballasts, grow room fans, grow room air filters, grow room carbon filters, grow room gardens, grow room gardening, grow room garden, grow room checklist, grow room ventilation, grow room gardening basics, grow room pests, grow room diseases, grow room insects, grow room maintenance, grow room location, grow room electricity, grow room reflectors, grow room nutrients, grow room plants, grow room cannabis, grow room marijuana, grow room papaver somniferum, grow room poppies, grow room poppy, grow room potting soil, grow room hydroponics, grow room faqs, grow room questions, grow room forum, grow room help, grow room soil, grow room seeds, grow room seed storage, grow room cleanliness, grow room propagation, grow room flowering, grow room vegetative growth, grow room air, grow room humidity, grow room temperature, grow room schedule, grow room mylar, grow room panels, grow room plans, building grow rooms, grow room space, grow room containers, grow room timer, grow room Q&A, grow room problems