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
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
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.
Dolomite Lime
Dolomite Lime is the ideal lime to use when buffering growing mediums and will seldom require further adjustment during the season. Dolomite 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 dolomite 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
Mixing The Ingredients
Mix your peat, perlite, and dolomite lime in a large tub of some sort and then add a lot of water and mix thoroughly. The peat will initially be hydrophobic and not mix with the water so you will want to cover your tub and let it sit for a few days. The cover will prevent insect infestation and the added water will both hydrate the medium and help the lime start to break down. After a week or more you will have a nice inert medium ready for seedlings or direct sowing. You can also use this medium in containers for vegetable plants outside by adding some time release fertilizer to the mix.
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 a strong vascular system. During the growing season plants will deplete the lime/calcium in the medium and cause the pH to drift. It is important to get the right amount of lime in your soil less mix prior to planting due to peat’s tendency to compact over time. I have read about people adding 1 tablespoon of dolomite lime per gallon of medium mixed into the top layer of soil as a common mid season supplementation for pH (half the amount you used when initially amending peat), and then watering as usual, but I think this will have little benefit due to the compaction issue. If calcium supplementation is needed mid season, a gypsum and water solution is common, but this offers no buffering benefit, and is only used as a nutrient supplementation.
Soil Microbiology
Mycorrhizae soil amendments that contain some sort of nutrient salt tolerant Bacillus bacteria can address some contaminants in the growing medium like fungus gnat larvae and 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 when using inert mediums like peat. If you are not having problems with fungus gnats then these amendments won’t provide much if any benefit.
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 dolomite 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.