GET STARTED

Guide to the Best Potting Cannabis Soil

There are many reasons why you may grow cannabis in pots instead of directly outdoors – you have restricted space, you wish to control the lighting better, you want to be able to move your plants. Whatever your reason, it is very important that you choose carefully what kind of soil you drop your seeds or sprouts into, since the right soil can multiply your plants’ size and production.

Before we go into the soil in detail, let’s talk about quantities for a moment. While a cannabis plant can grow into a pot as small as 1,5 gallons, you want to give them as much room as possible for roots to grow and expand. 3-gallon pots are a good size, and if you have space for 5-gallon pots, do invest in more soil and bigger pots. The cannabis plant tends to grow long roots, and they need room to expand during the vegetative stage so they can later absorb lots of water and nutrients for successful and abundant blooming. Now, the size of the pot will not automatically result in a bigger harvest – you still have to be very mindful of light, water and nutrients. What a bigger pot does is to enable greater growth, so your plants can reach their full potential.

Drainage

Experts say that occasional under-watering will not do any harm, but overwatering is a silent killer. While you would never overwater on purpose, a poorly draining soil will have the same effect, which is keeping too much water around the roots. Making sure your soil and pots have good drainage will ensure your cannabis plants do not suffer from “wet feet” – a cute name for a dangerous issue, when the roots sit in wet soil for too long. This can result in fungal disease, mildew, and poor absorption of nutrients. 

Another nasty side effect of permanently wet soil is that it does not allow oxygen to reach the roots – good potting soils should be “fluffy” and airy, because plants need to have both water and oxygen available for their roots to carry out aerobic and anaerobic processes. The wrong soil type will retain water, become compacted, and compromise your plants’ health and potential.

Basically, cannabis plants should never sit in a puddle of water or soggy soil. After you’ve checked that your pots have proper drainage holes and plenty of space for excess water to run out, it is time to ensure you have the kind of soil that will allow excess water to drain properly instead of retaining it when it’s not needed.

A well-balanced soil mix is called loam: soil with the right mix of clay, silt, and sand plus plenty of organic matter. Soils with too much clay or silt in them will tend to become waterlogged and compacted, and that is the last thing you want to happen to your cannabis plants. If you notice the soil does not dry within a few hours after watering, or your plants start looking yellowish, it’s time to look into amending the soil with sand, perlite, or even better: organic components that improve drainage and soil structure, like coco coir or ground coconut husks.

You don’t want to disturb root systems without need, but most growers usually start seeds in a small pot and then upgrade them to bigger pots once they’re established. If you do this too, use this opportunity to review the state of the soil around the roots, and mix in more organic matter and a little coarse material if it looks like it is not draining as well as it should.

Water retention

If all a good soil needed was good drainage, we’d all plant in a coarse, sandy soil and call it a day. But of course it’s not that simple: plants need air about their roots, but they also need water in the right amounts. A soil that dries too quickly will leave your plants thirsty too soon before their next watering – or it will have you watering them much more often than you should. 

It is very easy to know if your plants are thirsty, because they will tell you in no uncertain terms: droopy, sad leaves are the clearest sign that a plant needs water, and it needs it now. Fortunately, a little occasional dehydration can be reversed 100% with a good watering, with no lasting effects. Having done this, it’s time for you to sit down and figure out why your plants got so thirsty in the first place.     

There are many reasons why plants need different amounts of water, and it is your responsibility as a grower to take all of them into account. For example, small plants in the early stages only need water 2-3 times a week, while big plants will need water almost daily to support vigorous growth and budding. The temperature and hours of light will also affect evaporation rates, which will influence how often and how abundantly you need to water. But if you have done your homework, are watering appropriately, and your plants are still thirsty, it’s time to check the soil composition and structure for a solution.

Organic matter tends to be the solution for drainage and water retention issues alike. Soil too soggy? Add compost. Soil too dry? Add compost. It may sound simplistic, but it isn’t – organic matter is not a simple thing, it is a living, breathing mix of many elements that interact with each other to create a balanced biome around your plants’ roots.

If your plants sit in the kind of soil that lacks adequate water retention capabilities, try mixing in some peat moss – a beloved soil amendment that improves the structure of sandy soils, helps retain nutrients and keeps moisture in place without becoming waterlogged. A good, mature compost will also do the job of improving water retention – some refer to compost as black gold, for all its wonderful properties when it comes to growing cannabis. We make two types of aged compost in our state-licensed facilities, and either of them will be sure to help you with insufficient drainage or poor water retention issues. 

Soil acidity

Time for a quick chemistry refresher: pH is basically (pun intended) a measure of how acidic or basic a solution is. The scale goes from 0 to 14 – a lower number means more acidic, higher indicates a basic solution, and 7 is neutral, like water. 

Cannabis soil should have a pH of around 6, which is ever so slightly acidic. The acidity of the soil affects the tiny chemical reactions that happen at the root level and directly influences the availability and absorption of essential nutrients. For example, very basic soils (with a pH higher than 7.5) block the absorption of vital nutrients like boron, copper, iron, manganese, and zinc. A very acidic soil, on the other hand (and by very acidic we mean 5.5 or less) will keep your cannabis plants from getting the calcium, magnesium and phosphoric acid they require.

Amending the pH of the soil is not as simple as fixing its water retention, but it is possible. Check out the other blogs we have written about soil amendments, and become good friends with your soil pH tester.

Nutrition

Having sorted water retention and acidity, all that remains is ensuring your soils have all the nutrients your plants will need. A properly mixed soil will not require fertilizers and may result in more productive, healthier plants. 

If you want to mix your own cannabis potting soil, try to include as many of the following specialty ingredients as possible:

  • Peat moss
  • Composted forest material
  • Perlite
  • Coco fiber
  • Ground Pumice
  • Worm castings
  • Feather meal
  • Bat guano
  • Non-GMO soybean meal
  • Fishbone meal
  • Greensand
  • Volcanic rock dust
  • Kelp meal

We carry Roots Organics growing medium proudly, and what we just shared with you is quite literally the list of ingredients in their Formula 707 potting mix, which reads like a recipe for the perfect cannabis potting soil. There are other very good ready-made potting mixes, such as Fox Farm and Black Gold, but we do favor Aurora Innovations soils, nutrients, and amendments for their commitment to organic, sustainably sourced ingredients and their scientific approach to formulating their products.

We wrote an entire blog post specifically on soil nutrients and amendments for optimal cannabis growth, feel free to read it if you want to learn more about microelements, amino acids, and other advanced soil topics.

Whether you buy your cannabis potting soil ready-made or make it yourself, you can’t go wrong if you keep an eye on these four elements: drainage, water retention, soil acidity, and nutrition. Start with a good loam as a base, add compost and other amendments as needed, and watch your plants thrive. We wish you the best in your growing endeavours! Remember that our specialist advisors are always happy to answer your questions and help you achieve successful crops.