Amending Peat for Cannabis

Using Peat for Indoor Grows

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 is a dirty job that creates a lot of dust that you don’t want to be breathing. Pro-Mix already did the work of balancing the correct lime types (Calcitic and Dolomitic) and the ratios of each, which can be tricky to get right. If the lime ratios and amounts are not right you will have problems with nutrient lockout. If you want to save some $$$ and want to learn in the process you should consider making your own peat blend, these are the 4 main ingredients you should start with:

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 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 6.0-6.4. When buffering a growing medium with lime you have to be aware that there are a few different types of lime and they are used differently. Getting the concentration of lime wrong can cause toxicity and nutrient lockout, so read on if you want to avoid disaster.

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.

There are a few peat buffering options available which will take some trial and error to get dialed in. The 2 common types of lime used for pH buffering of peat are calcitic and dolomitic lime, both work well but they are used differently. For best results you would want to use a combination of calcitic lime (quick release) and dolomitic lime (extended release) to buffer a peat growing medium for a full 4+ month growing season. Typical formulations are 1/8 cup calcitic lime + 1/2 cup dolomitic lime per cubic ft. of growing medium (2/3 peat + 1/3 perlite).


1 Cup =
16 tablespoons

1 Cubic Foot =
7.5 gallons


Calcitic Lime

Calcitic Lime is a pulverized/powdered lime that is almost entirely made of Calcium, the Magnesium content is usually negligible. Calcitic lime is immediately available to the plants but will only last about 4-6 weeks in the growing medium. After 4-6 weeks supplementation will be needed if it is used by itself for buffering (not recommended): Supplementation Formulas.

When only using powdered calcitic lime (not recommended), you should use sparingly and start on the low side at 1/8 cup per cubic foot of growing medium (2/3 peat+1/3 perlite) 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

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.

Pro Mix has done a superior job in recognizing the differences between the 2 common types of lime (calcitic & dolomitic) and use them both simultaneously in their products so that calcium and pH buffering are both immediately available (calcitic) and last for the duration of the growing season (dolomitic).

  • 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

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. This means that over time plants will deplete the calcium in the soil and affect the buffering ability of the medium. When both calcitic (immediate release) and dolomitic (extended release) limes are used in the medium, the buffering ability is maintained for the entire season and will seldom require further adjustment. When lime is applied at the correct ratio for buffering, targeting a pH of 6.4 for a peat based medium, and your nutrient solution/water is pH adjusted to 6.4, you should be in good shape. However, if you only use calcitic lime to buffer your peat, as plants use up that calcium you will need to compensate in the first weeks of flowering to prevent the root zone pH from drifting down. You will need to do some midseason supplementation to avoid the pH drift.

Adjusting pH Up

Be very careful with supplementation because applying too much calcitic lime via solution (or initially) will adjust the peat mediums pH to a hard 7.0 causing your nutrient solution or water passing through it to also get adjusted to a hard pH 7.0 and your plants will suffer in the root zone with nutrient lockout. Also, over applying calcitic lime initially isn’t an option because it will toxify the soil. The ideal amount of calcitic lime to use per cubic foot (7.5 gallons) of growing medium (2/3 peat + 1/3 perlite) is 1/8th cup. When supplementing you will want to break up the top layer of soil if it has become compacted to help the lime solution penetrate the medium. Depending on how compacted the peat medium has become, supplementing may prove futile due to the inability of the lime and water solution to penetrate the growing medium. For this reason we do not recommend a calcitic lime only solution for peat buffering. See the breakdown below for the amount of lime to use for supplementation.

6 tsp = 1/8 cup
6 tsp ÷ 7.5 gallons = .8 tsp per gallon of growing medium

If 100% depleted
1 gallon pot = .8 tsp mixed into 1/4 gallon water
2 gallon pot = 1.6 tsp mixed into 1/2 gallon water
5 gallon pot = 4 tsp mixed into 1 gallon water

If 50% depleted
1 gallon pot = .4 tsp mixed into 1/4 gallon water
2 gallon pot = .8 tsp mixed into 1/2 gallon water
5 gallon pot = 2 tsp mixed into 1 gallon water

* Apply the entire amount of solution

Adjusting pH Down

On the other hand if you added slightly too much calcitic lime and now your medium is closer to a pH of 7.0 you will need to adjust your nutrient solution/water pH down to compensate for that higher growing medium pH, but this can be difficult to fix since it depends on exactly how much lime was mixed into the medium. If double the amount of calcitic lime was applied it will be hard to pull that pH down with pH adjusted solutions. There is a point when the soil will become toxic due to excessive calcitic lime and the only way to fix it is to start over or dilute the mixture with more peat and perlite until you get runoff tests that show your pH is back in the correct range for nutrient availability.

Pro-Mix HP

Pro-Mix HP’s pre-blended lime amended peat 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.

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.

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