Everything You Need to Mix Your Own Soil Medium

What a few basic ingredients can do for your plants

TLDR: A mix of FoxFarm Happy Frog with vermiculite and peat will make for very happy plants!

Which soil to use in a sea of options?

There are a million ways to bake a cake. The same is true for growing, especially when it comes to growing medium and amendments. There is no perfect composition of soil, and everyone has their own techniques and personal touches to make their flower stand out, so this is more of a base of information where you can start experimenting with your own recipes and coming up with something you like. 

The biggest takeaway from this article shouldn’t be the ingredients or the proportions, but rather the fact that soil isn’t something that has to come pre-mixed!

Can you use store-bought potting soil?

We’ll often get questions about whether Miracle-Gro type pre-fertilized soil can be used for cannabonsai. While technically it will support general plant growth, because it’s a relatively low grade medium enriched with fertilizers, it often creates problems as the needs of your plant shift throughout its lifecycle. Not to mention, for the poorly labeled products, you can’t be sure of the ingredients unless you mix the isolated ingredients yourself!

Not all premixed soils are bad

We don’t recommend commercial potting soils from big box retailers, but there are specialized soils such those made by Fox Farms or Gaia Green and many others that have great quality and rich ingredient sources.

Basic starter recipe

The recipe we use is a good starting point to being experiment and iterating from. This is by no means “the perfect soil recipe” or even necessarily the best soil to use, but rather something we’ve come to use over the years that works. I’ve never seen plants respond to a soil like I have with this mixture already, and we’ve got more experimenting to do. 

TLDR

Vermiculite and peat in a 50/50 combination has great water retention and cation exchange capacity making for a great medium, just add 1.4g/L of dolomitic lime to balance the pH of the peat and 0.35g/L of gypsum and you’re good to go. Amendments can be added and experimented with on top of this basic medium.

(You can also swap out peat for coco coir if its more available, as it is more sustainable because it is produced from renewable coconut husks)

Paired with our LED bar light, the combo has produced the frostiest, densest and overall happiest flowers so far. It’s as if the plants get pumped full of electricity and every leaf and growth tip stands tall and rigid. DM us anything that would be worth adding to this and we may send some freebies your way! (Note: we’re not sponsored by another company and there are no promotional affiliate links below.)

Mixing your own soil!

We use a variation of this on our houseplants and any time we take in a struggling plant and repot it in this, it responds immediately and leaves becomes extremely turgid and deep green. For the bonsai, they’re super happy and less likely to have pH or lockout issues later on.

What *is* soil?
Loosely paraphrasing, soil is the material that results from the interaction between life, climate, geology, topography, and time.

Not just rock dust
The soil on Earth today is not just the result of mechanical weathering of rock, but the degradation of rock by living organisms, and the accumulation of that organic matter over time. During the Devonian period (420MYA to 359MYA) early plant life spread dramatically onto dry land, developing roots, branching structures, woody tissue, seed bearing organs, and ultimately greening the planet by producing massive forests by the end of the period. Primitive sharks swam the seas as plants grew to cover the earth’s land surface, while our ancestors were just walking out of the sea onto dry land — all the while, plants were making soil [W].  

Soil represents the decomposed matter of hundreds of millions of years of life, the meeting point between the earth and the sun.

Land, then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants, and animals.

Aldo Leopold, A Sand County Almanac, 1949

All of our food comes from it. It feeds every plant, every animal, every bird, it grows our bread, our flowers, perfumes, luxuries, delicacies.

Ninety five percent of our food comes from the soil.

Maria-Helena Semedo, FAO Deputy Director General of Natural Resources

How much soil is there?
Soil is far thinner a layer than the atmosphere, ranging between just 2 meters and 50 meters in depth [1]. 

It’s said that the Earth’s atmosphere compared to the size of the Earth  is as thin as the skin of an apple is to the whole apple, representing just 0.75% of the size of the apple.

Compared to the Earth’s radius, the thin soil layer depth of 2 meters would equate to just 0.03% of the Earths radius, equating to 90 nanometers on the apple, about the same length as an organelle within the cell of the skin.

Why make your own ‘soil’?

Mixing your own basic soil medium has its own benefits that make it well worth the effort. First it’s sure to be free of pests and unwanted organisms, which allows you to decide which ones go in in the first place! Second, they transfer nutrients more efficiently, hold air and water in better proportion for longer, and flush through well when required. 

Insect/Mold/Pest Free
We get asked about what sources of soil can be used, and while soil from the earth itself grows some of largest and most productive cannabis plants, there are other options that we find better suited to indoor grows. When you make your own soil it’s far more likely to be free of insects, mold, mildew, and other uninvited guests than if you recycle old soil or use them from your back yard — some species can easily overpopulate in the right conditions, such as your grow box. A “clean” environment doesn’t mean that it’s void of life, as it helps to have all the right forms of life from fungi to bacteria in your soil. But when you’ve removed all barriers to growth, any species you’re not interested in cultivating can also grow rapidly, risking infection or just becoming a nuisance. In that sense it’s a bit more preferable to make your own soil from the ingredients than reusing soil you’ve got from other plants or your yard. It’s a preference really, since there are so many livesoil grows with entire ecosystems that produce great flower, but for us it’s a clean option for the place with lower risk of infection from mildew or insects.

Unfertilized
Cannabis has a specific set of needs, from pH to differing nutrient levels on growing vs. flowering, and when you grow in a soil that already has been loaded with generic fertilizer, those changing needs aren’t met and the plant won’t grow well, often developing nitrogen toxicity or nutrient lock out.

Porous
The mixture holds a lot of water thanks to its vermiculite but once those are saturated it flushes right through thanks to its large pores and gaps, which also help aerate the roots and soil. This means you can flush out your soil medium more easily than with silty clay-like soils.

Control of pH and water retention by shifting ingredients
Because vermiculite holds more water than peat, if you need to retain more water you can use slightly more vermiculite than peat. Peat itself is a bit acidic so you can adjust the pH back to ideal levels by mixing in the ratio of dolomitic lime to counter it.

High Cation Exchange
The two main ingredients, peat and vermiculite have high CEC or cation exchange capacity. In simplest terms it means the soil carries tons of ions for the plant to absorb and use as energy inside the cells for metabolism, transport and exchange. This means your plant is like it’s charged up with the soil supplying the roots with plenty of ions.

Cation exchange capacity (CEC) is a fundamental soil property used to predict plant nutrient availability and retention in the soil. It is the potential of available nutrient supply, not a direct measurement of available nutrients.

Ohio State University, College of Food, Agricultural, and Environmental Sciences

Micronutrients and microorganisms
You can add organic and more complex source of nutrients with tiny amounts of things like bloodmeal, guano, castings, etc. to “feed” the soil and its bacterial and fungal symbionts. It helps to have some organic matter, even if just a small amount, to feed those organisms. Peat and coir are both organic matter that will decay and feed those organisms, but these ingredients are good sources of a range of beneficial nutrients for both plant and microorganisms. Mycohrrizae and bacterial symbiosis helps a plant greatly by forming symbiotic relationships where they trade resources for their mutual health. These relationships are lost with more sterile mediums, so adding the best spores and bacterial innoculants can really help with establishing a great fungal network. In dry powder form the spores won’t activate until they’re around a plant and actively watered.

Cost
Probably one of the best things about mixing your own medium is that it costs a lot less than premixed mediums. You can buy four cubic feet of peat or vermiculite for the price of a bag of pre-mixed.

The Soil Recipe

50% Coir or Peat
50% Vermiculite
1.4g/L Dolomitic Lime
0.35g/L Gypsum
0.2g/L Diatomaceous Earth
0.1g/L Worm Castings, Bloodmeal, Bonemeal, or Guano etc.: very small amounts – not for fertilizing the soil – think small

Soil medium components
Since we’re making our own “soil”, or a type of soil-like medium, that achieves the goal of best supporting growth we should look at the components, why we use them, rough proportions and finally some basic nutrient options.

Peat/Coir
Forms the organic matter of the medium, being made of fibrous plant matter. Holds water well but less than vermiculite. Peat allows for adequate flushing, it avoids compaction and is supportive of aeration.

Vermiculite
A type of mineral more specifically referred to as Hydrated Laminar Magnesium Aluminium Iron Silicate, vermiculite flakes are mined and then baked to expand the layers between its mineral sheets. It holds a lot of water in between these layer as they act like little sponges. On the other hand, because of its large grain size it aerates well too.

Keep in mind that vermiculite is an expanded mineral so while it’s totally okay to handle you should wear a mask if indoors, preferably mixing soils outdoors. Old bags of vermiculite shouldn’t be used because they can contain trace amounts of asbestos! Newer bags of horticultural grade vermiculite produced in countries with good standards are asbestos-free and your safest bet. Vermiculite is a very common material in the horticultural industry and safety data sheets regarding composition are available online (perlite canada safety datasheet and uline’s safety datasheet).  

In addition to its water related properties, it is one of the best mediums at transferring cations to support internal transport and growth.

Dolomitic Lime
Derived from crushed limestone, powderized or granulized lime is used to balance the pH of the soil to ideal conditions and in this case also to counteract the acidity of the peat. Dolomitic lime contains magnesium and calcium carbonate, both of which are good for the plant’s health.

Gypsum (Calcium Sulphate)
Calcium is necessary for plants to grow well and helps plants move and allocate nutrients. Replenishing the calcium in the soil with a bit of gypsum helps them grow quickly and stay healthy.

Diatomaceous Earth
These little diatoms hold on to water way more than their weight, and because this stuff kills bugs I like to think it helps reduce risks of stuff like root mealie infestation which we had a huge problem with… and are a great source of soluble silica, which builds strong cell walls and helps with trichome development.

Additive options (small quantities)

Bloodmeal
Worm Castings
Bone meal / Fish bone meal
Rock phosphate
Glacial rock dust  
Mycorrhizae
Moon dust jk

Soil Mixture Behaviors

Particle size
While peat comes in a relatively homogenous shredded mix, vermiculite comes in different grain sizes. The larger grains are preferred as they allow the air to infiltrate the soil and provide for proper flushing. In bonsai practice, dust is often sifted out of the soil to allow for better drainage and breathing.

A good soil mixture will trap small pockets of air in between particles even when completely waterlogged, giving the plant access to both water and air to avoid cutting the roots off from air entirely.

Water retention: peat vs vermiculite
A good mixture will also trap pockets of water to preserve it for the plants to absorb over time. Plants don’t drink all at once so the water needs to be available for the plant to absorb slowly. After the fibres of the peat dry out, the thin layers in the vermiculite continue to hold water between them. When most of the medium dries there will still be clusters of moisture for the plant to draw from, so the soil doesn’t dry out all at once.

We did a quick experiment to demonstrate the difference in water retention between peat and vermiculite. We poured 100ml of water slowly through 100ml of peat and 100ml vermiculite and measured the amount of water that drained through into the beaker below.

A lot of the water retention is done by the vermiculite, holding 75% of the water that passed through it, whereas the peat only held on to about 25%.

Last thought

While there are clearly a lot of options for your soil, there are degrees of complexity to it and you don’t have to start with all of it at once. It’s a good idea to experiment with these ingredients, adding/removing some and seeing the results before moving on to the next, as well as testing varying amounts and proportions to see the impact on health and yield. You can use entirely organic nutrients and customise them to your preference. 

It does go to show though how much goes into soils; it takes a great deal of energy to produce these ingredients, from mining to farming, shipping etc. Plants, animals, bacteria and fungi together have created vast swathes of soil over hundreds of millions of years. Generating just 3 centimeters of soil over 1,000 years, it takes a tremendous amount of time to “grow” soil. 
Unfortunately, at the current rate the world will run out of useable soil in 60 years. There are initiatives working on this issue worth checking out here, and policies intended to preserve soil quality may provide some further insights. It’s possible we can even create systems that encourage the development of soil.

Let us know what types of soil you prefer! Happy growing!

  1. “[zone] A through C horizons range up to 50 m in depth.”  – Richter, D. D., & Markewitz, D. (1995). How Deep Is Soil? BioScience, 45(9), 600–609. doi:10.2307/1312764  https://www.jstor.org/stable/1312764 

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