Potting Soils

Soil Microbiology

It’s a good idea to use a Mycorrhizae soil amendment, preferably one that contains some sort of Bacillus bacteria that can address any contaminants in the growing medium that may find their way in. Mycorrhizae fungi form a symbiotic relationship with plant roots that will encourage strong healthy plants and increase nutrient uptake.

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What's a Good Growing Medium for Indoor Grows?

While there are some excellent options available for the indoor grower like the Pro Mix lineup, pre-blended store bought growing mediums can get kinda pricey. If you are growing indoors you should consider making your own, use a mixture of Peat + Perlite + Pulverized/Powdered Lime. Mixing these yourself will save a lot of money and provide you with 10x the amount of usable growing medium. You will not have problems with fungus gnats or soil born pathogens that lead to disease when using an inert medium like this unless you introduce them to your growing space.


Amending Growing Mediums

Peat is naturally quite acidic (pH 3.5-4.5) and will drift to become even more acidic over time. Lime must be added to raise the pH. On average you will use 2 Tablespoons of fine powdered lime per gallon of growing medium or 1/2 – 3/4 Cup per cubic foot (this usually works out to around 2/3 – 1 cup of Calcitic Lime per 10 gallons of peat + perlite mixture). When using powdered Calcitic Lime, which is usually the only one you will find locally in the powdered form, you might want to start on the low side and use 1/2 cup per cubic foot of growing medium and see how well your soil maintains its pH buffering throughout the season by doing soil pH tests every 30 days. When using 3/4 cup of Calcitic Lime per cubic foot you might experience some nutritional assimilation issues with young seedlings and clones, but that can vary by manufacturer.

1 Cup = 16 tablespoons
1 Cubic Foot = 7.5 gallons

The 2 common types of lime used for pH buffering of growing mediums are Calcitic Lime and Dolomitic Lime, both work well but they are used differently.

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 30 days in the growing medium. After 30 days, supplementation might be needed and if desperate can be given in the form of 1 tsp lime per gallon of RO water. The resulting mixture has a TDS of around 40ppm and will give a pH adjustment of about 1/2 point, but be very careful with this supplementation because applying too much Calcitic Lime via solution can reduce the assimilation of other nutrients during that week as Calcium has an effect on many other nutrients, check out the Mulders Chart on our Cannabis Nutrients Cheat Sheet page for clarification. If you find yourself having to add lime via solution mid-season be sure to adjust the amount of lime added in the beginning so that you can avoid doing so next season. Amending peat upfront with a little fast acting Calcitic Lime in combination with a slower release Dolomitic Lime is a better pH buffering solution and will keep the pH stable over the long haul.

Dolomitic Lime is usually only found locally in pelletized form and as a result is slower to release taking more time before becoming available for plant uptake but will last considerably longer in the growing medium which will seldom require further adjustment. Smart growing medium producers like Pro Mix have done a superior job in recognizing the differences between these 2 types of lime and use them both simultaneously in their products so that lime is available both immediately (Calcitic Lime) and for the duration of the growing season (Dolomitic Lime). Dolomitic Lime has a decent quantity of Magnesium in it, typically giving you a mixture that is around 65% Calcium and 35% Magnesium.

Without the addition of lime the pH of growing mediums like peat will drift to become too acidic for plants and will result in deficiencies in elements like Potassium, Magnesium and Calcium even though they may be plentiful in the growing medium or nutrient solution. The low pH root zone environment can also lead to increased uptake of some elements that can become more available at a low pH like Manganese which can then lock out other elements like Iron, sometimes observed during flowering. Monitoring the soil and nutrient solution pH closely is required if you want to be successful. The correct degree of acidity for growing mediums is a pH of around 6.5.

Learn more about testing your growing medium pH here

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