Soils and Amendments for Cannabis

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What types of soils and amendments to grow cannabis? How to choose?

Hi there and welcome (back) to Free the Tree! In this piece we’re going to cover soils and amendments you can use to grow Cannabis.
We will go over their characteristics, advantages and disadvantage.

Hopefully you’ll get the knowledge you’re looking for, if not don’t hesitate to ask our growers 😉 Here’s what were’re going to cover, you can click to go straight to what you want to check out!

Soils and Amendments for Cannabis - Table of content

Propagation Cubes & Mixes

Soil Mixes (coming soon)

Compost (coming soon)

Can you re-use soil?

Grow Medium Problems (coming soon)

Soil for Cannabis

You may have heard this thousands of times, but one of the most important things is the soil you choose and how you treat it.
Three basic factors contribute to a good soil for Cannabis

3 factors for a good soil to grow cannabis:

Texture: The soil shouldn’t be too compact and this for 3 reasons. Roots need to be able to grow easily. They need an oxygen flow, which isn’t the case when its too compact, and last but not least drainage! If your soil is compact water won’t be able to flow.
pH: pH level of the soil is critical since the absorption of nutrients by the roots depends on the pH.
Aim for a pH between 6.5 and 6.8 you should be good.
Nutrient Content: Last but not least, nutrients.. Obviously, your soil must have nutrients, whether this is achieved by compost, companion plants and micro-organisms, or bought already enriched.

Now that we’ve gone over this, let’s go in depth the different soils and amendments to grow cannabis

Types of soils and their Textures

There are globally 3 types of soils, clay or adobe soil, sandy soils and loom soil that we’ll cover.

Clay or Adobe Soils
Sandy Soils
Loom Soils
How to check Soil Texture Quality

Clay soils (aka adobe soils)

Adobe soil is made up of very small, flat mineral particles. When these particles get wet, they pack tightly together which will slow or stop root penetration and water drainage.

With this type of soil marijuana roots won’t be able to breath because of the very little (or no) space left for oxygen to passe through.
When watering the irrigation of these types of soil is very slow and once irrigated drainage is very slow which and drown the roots.

For these reasons, clay soils clay to grow cannabis

Sandy soils

Sandy soils are composed of large particles that allow a good supply of oxygen and drainage.
The main downside the sandy soils would be that there’s too much drainage, the water retention is so low that you’ll have to water very frequently.

Marijuana roots should develop without a problem in sandy soil since it has good air and water holding capabilities.

Loamy Soils

Loam soil is a mix of clay, silt and sand, it is ideal to grow cannabis in.The different sizes of the particles allow a large combination of pore spaces, meaning that it drains well, retains nutrients and moisture as well as allows the roots to grow nicely.

How to check soil texture quality for cannabis?

In order to see if the texture of the soil is good to grow you weeds pick up a handful of moist (not wet or soggy) soil and gently squeeze it.
The soil should barely stay together and have a little of a sponge-like effect when you slowly open up your hand.

For indoor soil make sure you have a rotation and you have soil amendments. The best thing is to have micro-organic life and companion planting, even in your indoor soil, remember all we’re doing is replicating nature.

Before we go into each element that you can use as soils and amendments to grow your cannabis babies, we though it would be important to quickly review 2 important aspects, soil temperature & pH.

If you want to go in depth on these subjects we added links to each article.

Best soil temperature for cannabis

The temperature of the soil is an important factor often overlooked. Remember, two thirds of what’s going on is below the soil.
At different temperatures, roots won’t behave the same way and chemical activity will differ.

Raising the soil temperatures speeds the chemical process and can hasten nutrient uptake. The idea range, for most chemical activity, is between 18-21°C (65-70°F).

How can cold soil impact growth

Cold soil slows water and nutrient uptake and stifles growth.
Since water uptake is slowed, growers often overwater when the soil or grow room is too cold, which further slows growth.

In order to raise the soil temperature you can use soil-heating cables or place the pots on a heating pad.
Both of these work very well, especially when lights go off.

What happens if soil is too hot

When the temperature of the soil climbs above 24°C (75°F) roots will dehydrate, higher temperatures they can actually cook!
Regular (daily) watering is necessary at these temperatures to keep the roots cool.

Heating up the pot is actually pretty easy, if your black container is exposed to sun light all day, or the source of the light is too close to a small pot, it’ll heat up the outside layers of the soil, which most of the feeder roots are located.

Once destroyed it takes a week or two for the roots to grow back.

Soil pH levels to grow Cannabis

As you may know pH levels are measured from 0 to 14, 0 to 6.9 being acidic, 7 is neutral (water) and from 7.1 to 14 the solution is considered Alkine

Something important to note, for every point change in pH the acidity or alkalinity increases (or decreases) by 10 folds, meaning that a solution at 5.5 is ten times more acidic than a solution pH’d at 6.5.

As you can see in the image on the right all nutrients are available between a pH of 6.3 and 7, the real sweat spot being between 6.5 and 6.8.
At these levels marijuana can properly absorb and process all the available nutrients efficiently.

What happens when pH is too low?

When the pH of the soil is too acidic, acid salts chemically bind nutrients and the roots are unable to absorb them.
You’ll notice that your plants have deficiencies, especially in Nitrogen and Phosphorous, even though your soil has enough of those nutrients available.

What happens when pH is too high?

When the soil is too alkaline will render the soil unavailable, especially the micro nutrients like Iron, Manganese, Iron Copper and Zinc which are just as vital as the macro-nutrients.
With alkaline solutions you’ll also see a Toxic salt build up that limits water intake by the roots which will quickly become a problem.

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Now that we’ve gone over the global aspects of soils, let’s look into each component you can use for you own growing soil.

Remember, a good soil is a mix of soil(s) and amendments.

Soil Amendments for cannabis

Soil amendments increase the soil air, water and nutrient retaining abilities.
They fall into 2 categories, mineral and organic

Mineral soil amendments
Organic soil amendments

Mineral soil amendments for Cannabis

Mineral soil amendments are near neutral on the pH scale and contain few, if any, nutrients.
Mineral Amendments decompose through weather (watering) and erosion.
Here are the main mineral amendments we’ll cover here

Perlite
Pumice
Hydroclay
Vermiculite
Fine Dolomite Lime

Perlite

Perlite is a sand or volcanic glass expanded by heat. It hold water and nutrients on its irregular surfaces. Perlite works especially well to aerate soil.

This is a good medium to increase drainage during the vegetative and flowering stage and helps avoid fertilizer salt build-up.

Versatile perlite is available in 3 main forms Fine, Medium and Coarse. Most growers prefer the coarse grade for their soil since it best airs it out.

Perlite should make up at the most one third of the soil mix to keep it from floating and stratifying.

Pumice

Pumice is another lightweight volcanic rock which, as Perlite, holds water, nutrients and air within it’s many catacomb-like hols.
It’s a good amendment for aerating out soil and and retaining moisture evenly.

Just like perlite, pumice floats and shouldn’t make up more than 1/3rd or the mix in order to avoid any issues.

Hydroclay (aka clay pebbles)

Hydro clay pebbles are widely used within the indoor growing community in order to improve the drainage and air within the soil. This is what we use in all our soil for potted plants.

The large expanded clay pellets expedite drainage and hold air within the growing medium, improving root development and nutrient intake.

As well as being good for the plants it’s also pretty cheap, making it perfect.

Vermiculite

Vermiculite is mica processed and expanded by heat.
Just like the previous amendments, Vermiculite holds water, nutrients and air within its fiber and gives body to fast draining soils.

Fine vermiculite holds too much water for cuttings but does well when mixed with a fast-draining medium.
This amendments holds more water than perlite or pumice.

For this reason it’s generally used in hydroponic wick systems as it holds and wicks a large amount of moisture. You can find vermiculite in 3 grades, fine, medium and coarse.

Use fine vermiculite for cloning mixes, otherwise coarse is the best choice for an amendment.

Fine dolomite lime

Fine dolomite is widely used although some issues have been known to show, here’s a good article by smilinggarndner that covers the subject well.

Dolomite lime is generally used to level the pH of the soil since it’s neutral, stabilizing the pH “safely”.
It is composed of Magnesium and Calcium and especially popular among growers will acidic raining water.

The main issue that can show up is in the case where your soil is alright high in Magnesium and Calcium, adding Dolomite might get those level too high.

Make sure you mix dolomite well! Improperly mixed dolomite will stratify forming a “cake” or layer that will burn roots and repel water.

Dolomite does not prevent toxic-salt accumulation cause by impure water and fertilizer buildup. Regular leaching helps flush away toxic salts.

Image credit to dolomite.com.au

Hydrated lime

Hydrated lime is solely composed of Calcium and, as the “hydrated” in the name suggests, is highly water-soluble. Many growers mix 1/4 hydrated lime to 3/4 dolomite lime in their soil. The hydrated lime will be available immediately whereas the dolomite lime will act as a longer term buffer.

Do not use more than 12cl of Hydrated lime per cubic fool (30L). It will release so fast that it can toxify the soil and stunt, even kill, plants.

Fast-acting Hydrated lime alters the pH of the solution quickly.

How to add Hydrated Lime to the watering mix?

Prepare warm water
Add the Hydrated Lime
Mix thoroughly
Test the pH level of the solution
Apply at each watering for fast results

How fast does Hydrated Lime leave the soil?

At a regular watering rate Hydrated Lime will exit the soil in about 2 weeks. If you’ve added too much, flush your soil in order to make it leave faster.

Tips when using hydrated lime

You can use it as a grow room fungicide.
Sprinkle it on the floor and around the room, it’ll kill fungus on contact
Do not use quicklime as it is toxic to plants.

Organic soil amendments for Cannabis

There are many different organic amendments available in order to add nutrients and improve the texture, drainage and over-all quality of your medium.

Soil amendments contain carbon and break down through bacterial activity, slowly yielding humus as an end product.
Humus is a soft, spongy material that binds very small soil particles together improving the soil texture.

If you use any of these amendments, make sure they are thoroughly decomposed (a least a year) and they are releasing Nitrogen rather than stealing it from the soil. A dark, rich color is a good sign of fertility.

We’ll cover the main organic amendments, which are:

Garden compost
Manure
Peat
Sphagnum peat moss
Hypnum peat moss
Coconut Fiber (coir)

Garden Compost

Garden compost and leaf mold are usually rich in organic nutrients and beneficial organisms that speed up nutrient uptake, but be careful! They can be full of harmful pests and diseases, so be careful.

Compost gardens are a known breeding ground for cutworms and beetle larvae, and just one cutworm means certain death to the defenseless Marijuana plant.

Garden compost is best used in outdoor cultures, bringing it indoors has a high risk.

Beware: Actively composting organic soil amendments require nitrogen to carry on bacterial decomposition. If they don’t contain at least 1.5% nitrogen, the organic amendment will get it from the soil, robbing the roots from this vital nitrogen.

Manure

Barnyard manure is a great fertilizer, but again for outdoor gardens.
It often contains high (or toxic) levels of salts and large numbers of weed seeds and fungus spores that disrupt an indoor culture.

If you’re using manure indoors, purchase a bag that guarantees the content.
There are many different types of manure you can find, whether it’s cow, horse, rabbit, etc; They each have their specificities so make sure you get the one best for your plants.

Tips on using manure in soil

When mixing manure as an amendment don’t add more than 10/15% of the total medium in order to avoid salt build up or over-fertilization.
The nutrient content of the manure is hard to know in advance since it depends on the diet of the animal, so make tests before rolling it out within your whole garden.

Peat moss

Peat is a term used to describe partially decomposed vegetation. The most common peat is made of sphagnum and coconut..
These materials are used to amend soil and can be used as a growing medium. Peat moss is very dry and difficult to wet the first time (unless bought wet).

Tips on using peat moss in soil

When you’re adding peat moss as a soil amendment cut your workload by dry-mixing all the components before wetting it, then use a wetting agent (such as biodegradable soap) to hydrate it all.

Peat tends to break down and should only be used for one crop.

Sphagnum peat moss

Sphagnum Peat Moss is a light brown and the most common peat found at any commercial nursery or indoor growing store.
This bulky peat gives soil body and retains water well, absorbing from 15 to 30 times its own weight.

Essentially it contains no nutrients of its own and has a very acidic pH, ranging between 3 to 5.
While it decomposes pH could continue to drop which can be deadly to Cannabis plants. You’ll need to counter this by adding an alkaline amendment such as dolomite lime.

pH levels of Sphagnum Peat Moss

The pH of Sphagnum Peat Moss ranges from 3 to 5. It’s important to add alkaline solutions within the medium in order to counter-balance this extremely acidic amendment.

Hypnum peat moss

Hypnum peat moss is more decomposed and darker in color, with a pH from 5.0 to 7.0. This peat moss is less common and contains some nutrients. Hypnum peat is a good soil amendment even though it cannot hold as much water as Sphagnum Moss.

Coconut fiber (aka Coir)

Coconut fiber, also called palm peat, coco peat, cocos, kokos and coir is made of coconut pith, the fibery part just under the heavy husk.

This pith is soaked in water for up to 9 months to remove salts, natural resins and gums in a process called “retting”. After this, they beat the straw-brown coir to extract the husk.

Advantages of using Coconut fiber in soil

Coir is biodegradable and a good medium for the flowering stage. It holds lots of water while maintaining structure.
It’s a durable, rot-resistant and good insulator. Cheap, easy to control and holding lots of air, Coir is a great choice as a Soil Amendment.

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Potting Soil

Potting soil fresh out of the bag often does the job and checks all the boxes.
With good texture, water retention and drainage as well a stable pH between 6 and 7 it’s a safe choice.
You can also choose different levels of nutrients within the soil, making it a very good choice for new growers.
Generally organic nutrients are used and the whole range of nutrients is available to the plant, there are many high quality soils out there, just ask your local nursery or soil shop 🙂

Additional information about potting soil

Quality Soil is extremely important, if you’re trying to cut corners to reduce price, don’t do it on the soil. Cheap, discount soil may seem attractive, but down the road you’re plant will get deficiencies.

Deficiencies means that you’ll have to buy nutrients, meaning that in the end it will cost you more and may strongly impact the total yield of your harvest.

On top of having low nutrient levels, discount soil may contain weed seeds (not the good ones haha), diseases, pests, hold water evenly or drain poorly. Invest in good, high quality soil

Can you re-use potting soil after a grow?

From our trials yes, but not too many times and it is not recommended as the micro-nutrients start running low and undesirable microorganisms, insects and fungi may start growing.

In order to reuse your soil you really need to take care of it and nurture it, as organically as possible
We’ve re-use partially some of our soil, adding fresh soil every round, and micro-nutrient deficiencies started showing up at the 3rd grow.

That said, we’ve now decided to start companion planting (yes even indoors) and rotating our soil, nurturing it with alfalfa, chamomile and nettle, this last grow that had Phosphorus, Potassium and calcium deficiency tough use that taking care of the soil is paramount.

How to nurture soil to be able to re-use it:

Take care of the soil:
– Don’t over-fertilize or use chemical nutrients.
– Flush the soil regularly to wash away the possible salts that have built up.
– Plant Companion Plants: Within the same pots place some friendly plants that will naturally provide nutrients to the soil. Chamomile, basil and chives are 3 good examples, on top of returning Nitrogen, Potassium and other nutrients they are pest repellents (such as the infamous spider mites)
Have a soil rotating system: Once used let your soil rest for a couple weeks or months.
During this time it’s good to nurture the soil, with some nettle and alfalfa manure for example.
Mix with fresh soil: After every grow mix your used soil with

Recap on Soil for Marijuana

Make sure it has good texture and drainage
Don’t go for discount bags of soil; Ultimately it will cost you more in nutrients and a lower yield (deficiencies)
Nurture your soil after each grow; Just like in the wild, nutrients need to go back into the soil

Mushroom Compost

Mushroom compost is an inexpensive potting soil and soil amendment that is full of organic goodies for your plants.
This compost is very fertile and packed with beneficial micro-organisms and could also foster anti-fungal and antibacterial properties. On top of all that, Mushroom compost is also loaded with bacteria that hasten nutrient uptake.

Since it have very high water holding ability and low drainage you should add some perlite to it in order to promote better drainage.

Image credit to thusrakedzarathustra

Where does Mushroom Compost Come from?

Mushroom compost is sterilized chemically to provide a clean medium for mushroom growth. After serving that purpose it is discarded and must be set aside for at least 2 years to allow the harmful chemicals to leach out.
After sitting there for several years this “mushroom compost” is extremely fertile and packed with very beneficial micro-organisms; that’s what we use 🙂

Recap on Mushroom Compost

Very good amendment or soil for marijuana
High in nutrients
Contains micro-organisms that helps nutrient uptake
Low in drainage, should be amended with perlite.

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Soilless Mix

Soilless mixes are very popular, cheap, lightweight and sterile growing mediums and commercial greenhouse growers have been using them for decades.

These mixes retain moisture and air while allowing strong root penetration and even growth. Furthermore, fertilizer concentration, moisture levels and pH are very easy to control with precision (sounds perfect doesn’t it?)

Image credit to urbangardencasual.com

What does Soilless’s contain?

Depending on the mix you buy than can contain either of the following:

Pumice
Vermiculite
Perlite
Sand
Peat Moss
Coconut Coir

These components can also be purchased separately so that you can get the desired consistency.

What are the strong points of Soilless Mixes

Good texture
Hold water
Good Drainage
Retain air
pH is balanced between 6 and 7
Fast-draining mixes leach out efficiently nutrients, strongly reducing any change of toxic build ups.

What are the weak points of Soilless Mixes

The main issue is that soil mixes don’t contain any nutrients at all. You will need to provide them all by keeping a precise nutrient schedule and keep a close eye on any nutrient deficiencies.

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Before we leave you, here’s some soils available when trying to root your cannabis clones

Propagation Cubes & Mixes

Propagation Cubes are pre-formed containers that make rooting cuttings, starting seedlings and transplanting easy. They also help encourage strong root systems.

These cubes are really awesome when germinating seeds or cuttings but you need to make sure you keep them evenly and constantly moist so that the roots don’t dry up.

What kinds of propagation soils are available?

Different options are available out there but really all of them have the same purpose and (generally) result.
Here’s what you can find:

Rockwool root cubes
Peat pellets
Oasis blocks

Alright folks that’s it for this one, hopefully you found what you were looking for
Until next time, be safe and grow easy 😉

About the author

plant-lover

Plant Lover is Free the Tree's founder and a passionate grower of plants. Mostly from the city, I work on my computer all day so about 4 years ago, on my off time, I started growing to have some more green in my life. I never imagined I would fall in love with botany and discover so much about this plant. Since then I keep learning everyday, meeting new awesome people of our community and which keeps me growing constantly You can always get in touch with me on our Instagram!