Growing Cannabis Outdoors: The Best Soil Mix

Published :
Categories : BlogCannabisGrowing Cannabis

Growing Cannabis Outdoors: The Best Soil Mix

Getting the soil mix just right can make all the difference between so-so herb and premium bud. Here‘s how to create the perfect soil mix for the outdoor season.

Good quality soil is the foundation upon which any successful outdoor (and indoor) cultivation is built upon. While it is possible to get by with store bought soil, it‘s impossible to get anywhere near the same quality the pros get. Soil quality will affect the taste, aroma and yield of your plants – when it comes to cannabis, the phrase “you are what you eat” has never been truer.

Not convinced? Well, let’s put it this way. Using additional plant feed and fertilisers is common practice when it comes to growing cannabis, but a skilful organic grower can get from seed to harvest without ever having to touch the stuff, as their soil already perfectly prepared - this does not only make for better quality bud, is it also a huge advantage for the outdoor grower who doesn‘t want to deal with nutrient deficiencies as time goes on.

Why top notch soil is so important for a successful outdoor grow

Why is high-grade soil so important? Most obviously, soil contains all of the nutrients that your cannabis will require to grow and thrive. If the soil does not contain the correct level of nutrients for cannabis, then the plants will develop with deficiencies – in the same way a human would be adversely affected by a diet lacking a certain element.

But nutritional content is not the only aspect of good soil. Soil, although commonly just dismissed as dirt, acts as a base for a complex and diverse micro-environment. Whilst providing your cannabis plant with all the nutrients it needs, the occupants of healthy soil - bacteria and fungus - will form a symbiotic relationship with the roots of your plants, bolstering its immune system and further increasing nutrient uptake efficiency. When this relationship is established and well maintained, it can result in some absolutely mammoth plants! This aspect of soil is often overlooked, but it‘s one of the little secrets that make an organic outdoor grow hugely successful.

The interaction between fungi and root systems is called mycorrhizae. This symbiotic interaction can be found throughout the natural world. Below the rich soil of old forests are hidden, vast and elaborate mycelial networks that even connect trees of different species. Fungi and bacteria evolved along with their host species, and over thousands of years, they formed a mutually beneficial relationship. It is possible to add in spore and bacteria mixes, such as Mycotrex and Bactrex into a lesser quality soil to awaken it, although this will still not be anywhere near as good for cannabis as a specially crafted and constructed soil.

Why won’t any old dirt do?

Rich and healthy soil is the result of thousands of years of breakdown and decomposition of plants, rocks and animal matter – leaving the minerals and nutrients contained within behind. It can take an untouched forest up to 1000 years just to make just 2.5 cm of topsoil – and if there are no plants or animals to give back to the soil, then it eventually turns to lifeless dust. On a side note - this is where conventional agriculture finds itself nowadays: soil has largely become an empty sponge that holds petrol based fertiliser. But even if you can find untouched and rich soil there is still no guarantee that it will be suitable for cannabis, as the right consistency and balance of nutrients is of paramount importance for cannabis to really thrive.

How to mix up the perfect soil

There are essentially two approaches when it comes to soil mixes. Either your start with a high quality, store bought organic soil as a base and then enrich it with various organic ingredients, or you go all the way back to the start and create your own compost. Let‘s look at the first approach.

The following organic soil mix has been made popular by Subcool, the founder of the TGA Genetics seed bank, who has been experimenting with organic mixes for over 20 years. While this is by far not the only working soil mix out there, this one has been tried and tested over years, and shown to consistently produce amazing results.

Subcool’s Super Soil is split into two separate layers. These are the base soil and the super soil. The base soil is high-quality store bought organic compost that acts as a nutritionally balanced top layer for a young cannabis plant to thrive in. The super soil acts as a highly concentrated nutritional layer at the bottom, far below the roots. By the time the roots begin to venture into the super soil, the cannabis will be developed enough to handle the large amounts of additional nutrients.

By having two layers of soil, the plants will have everything they need throughout both vegetation and flowering - with water being the only thing left to add.

Choosing a base soil

Firstly, the base soil must be selected. This will act as the top soil, and the base ingredient for the super soil. High-quality organic compost should be used for this, and it is important not to skimp on cost - the better quality, the better the final results will be. The central idea here is to start out with a rich compost that already packs a punch, but not so much as to overload young plants. Check out the ingredients before you buy; subcool describes his ideal compost as containing as many of the following ingredients:

Lignite, coco fibre, perlite, pumice, compost, peat moss, bone meal, bat guano, kelp meal, greensand, soybean meal, leonardite, k-mag, glacial rock dust, alfalfa meal, oyster shell flour, earthworm castings and mycorrhizae.

Most likely you won‘t find this exact composition in your garden center, but any combination of these elements will work well.

Creating the super soil

Once you have a base soil selected, it is time to create the super soil. These are just quantities to start out with, adjust as needed. For the super soil you will need:

4 large bags of base soil (high-quality organic potting soil with coco fibre and ideally, mycorrhizae)
5 to 10 kg of organic worm castings (worm poop)
1 kg of blood meal
1 kg of Bloom bat guano
1 kg of steamed bone meal
.75 kg of rock phosphate
1/8 cup Epson salts
1/4 of cup azomite (trace elements)
1/2 of cup sweet lime (dolomite)
1 tbsp of powdered humic acid

If your base soil has been heat treated or sterilized, it is a good idea to add in the previously mentioned Bactrex and Mycotrex mixes.

To create the super soil, mix up all of these ingredients:

1. To begin with, you will need a clear area to work in.

2. Either place down a tarpaulin or large plastic kid’s pool to do the mixing in. If you are not one for physical labour, then it is also possible to use a clean cement mixer.

3. Create a mound of base soil (a few bags) in the centre of the tarp/pool.

4. Spread the powdered nutrients onto the top of the mound.

5. Add another layer of base soil onto the mound.

6. Spread the bat guano onto the top of the mound.

7. Add another layer of base soil onto the mound.

8. Continue to add layers of ingredients followed by layers of base soil until everything is used up.

9. Now it's time to get dirty. Put on your boots and using a spade, mix everything together vigorously until it’s just one homogenized substrate. This might take up to 20 minutes of intense work, but it is a very important step – the soil will not be effective if it is not mixed thoroughly.

10. The soil mix is then spaded into large plastic garbage cans for storage.

11. Add 10 liters of water to each garbage can. This will make future mixing hard, but it activates the mycorrhizae within it and help dissolve all of the powders.

12. Now leave the garbage cans to cook in the sun for 30 days – careful they don’t get rained heavily on.

Note: Subcool advises that just before any of the soil in the garbage cans is used for growing, the cans should be tipped out and mixed one last time for good measure.

There you have it, you have now made a supremely rich soil perfect for nurturing your cannabis plants into full fruition.

How to plant your cannabis in the super soil

It is extremely important that you do not plant seeds, seedlings or clones directly into the super soil – it will burn them up. As mentioned before, your final planting pot will consist of two layers: a top layer with the base soil, and a bottom layer with the super soil mix. Fill the bottom of your final container to a third or half with the super soil. In the case of extremely nutrient hungry strains, fill it up to three quarters - however, this is only necessary if you know your strain is very hungry. Fill the remainder of the pot with your base organic compost. Transplant the plants into the base soil once they are at least 15cm tall. When transplanting a clone or seedling, make sure that it doesn’t come into contact with the super soil. Only when it is large and hungry should the roots reach it.

If done correctly, this soil mix will take care of all fertilization needs of your grow, with only water being required for the rest of the season.

Read How To Make Your Own Compost

Propagation