Top Tips on Organic Grow Mediums (and Additives)


Choosing the right stuff in which to grow your plants determines the quality of the cannabis you’ll harvest. Knowing the differences between different organic mediums and additives can help strengthen your ability to create craft-quality buds.

Strictly Roots

When selecting the right medium, you should not only consider NPK (nitrogen, phosphorus and potassium) ratios, but also the medium’s ability to hold water and provide aeration to allow proper capillary action between wet and dry periods. Below is a list of all the various grow mediums that can be bought from any grow shop, along with my top tips so that you can customize your organic grow medium.


When selecting a good soil, you’ll need to spend an extra bit of money and buy a brand that offers quality. If you’ve ever used cheap soil from a corner hardware store, you’ve noticed that its texture has a loose-dirt consistency. These cheap soils have no ability to retain moisture well, and their wicking action is almost nonexistent.

My top tip here is to invest with a well-known company that has good credentials and industry support. Companies such as Biobizz and Plagron offer some excellent soil-based products, each supplemented with perlite and peat moss. You’ll want something that has a strong NPK ratio and is jam-packed full of trace elements as well.


Using coco is one of the best experiences you can have as an organic grower. Coco can be mixed into a custom organic mixture or it can be used in hydroponics, as it is classed as an inert medium, meaning it contains almost zero micro minerals or trace elements. The coco from Canna, however, is supplemented with trichoderma, a potent biocontrol agent that wards off harmful fungi.

A good tip is to use a mixture of 50 percent coco and 50 percent perlite. Along with this, then you can add a substrate of organic material to provide nutrients to your plants. This will also provide your plants with a sufficient number of air pockets, and the lightness of this mixture is perhaps the softest you can get with a fine blend. I also add to my medium a mixture of 50 percent worm castings and compost to provide much-needed humates. Growing with coco will encourage your roots, and your plants, to grow to their maximum potential.


Perlite is a small white popcorn-like substance used to create spacing in the grow medium. It also has the ability to retain moisture as well as drying out again. This cheap and simple medium has no nutritional value, so it’s perfect to mix at a 50 percent ratio with soil, worm castings or coco, and some growers have even performed hydroponic grows using only perlite.

My top tip here is to make sure that if you are top feeding, cover the top layer of the medium with coco or soil. The reason for this is that algae will begin to grow on the white perlite after an organic feeding. The perlite will take a dark green color and the top of the surface will become a slimy dark green. You can deter the algae by simply breaking up the top of the medium.


Vermiculite holds water so that dry periods won’t happen frequently between feedings. This is useful, but you also want to push for some dry periods when growing. Adding vermiculite to a mixture of soil and perlite provides improved drainage too, but adding too much can keep the medium too wet over long periods of time, so my top tip here is to use it inside of propagators to keep moisture levels high when cloning. If you add a layer of vermiculite to the base of the cloning tray, it will keep the humidity inside nice and high.

Clay Balls (Grow Rocks)

Used in hydroponic dripper systems and inside grow mediums to create air chambers, these simple but effective additives are used for commercial gardening and landscaping. Whenever you buy clay balls from a grow shop or a garden center, you’ll need to thoroughly wash them until all the sandy-colored brown dust is completely gone. You don’t want any dust on them when you’re preparing your system.

A good tip is to add a layer of clay balls, at least an inch high, at the base of your pots. This will allow extra drainage and ensure that there’s enough fresh air in your pots. When you feed, you’ll see water pouring out of the bottom few inches of the clay balls, which causes oxygen to replace the falling liquid through water displacement. Additionally, you can do the same on the top of the medium by using an inch-high layer of clay balls to prevent algae buildup and to allow nutrients to be evenly dispersed when top feeding.

Diatomaceous Earth

This product is easily identifiable by its bright chalk-white stone characteristic. Often rebranded as a bug-killing powder, diatomaceous earth is naturally high in silicon, which deters insects that come into contact with it as well as strengthens leaf tissue against insect attacks.

My tip is to break up the white stones and add a layer at the base of your medium. This will provide excellent drainage as well as allow the roots to grow around the stones. If you mix clay balls with diatomaceous earth, you have the best of both worlds in terms of drainage and silicate availability. Adding a layer of diatomaceous-earth rocks on the surface is also a great way to deter insects and regularly provide silicon with every feed.


This hydroponic medium has absolutely no nutritional value, which is why nutrients must be supplied in hydroponic growing systems that use rockwool. Many growers will root clones in rockwool and then later transport them to another system such as a dripper or nutrient-film-technique (NFT) system. Rockwool isn’t technically organic, and, typically, plants in rockwool are fed with synthetic nutrients, but since it’s possible to feed them organic nutrients, we included it on this list.

When growing with rockwool, especially in hydro, you can always stack a second rockwool cube on top of another. Of course, this is when working with 4- or 6-inch rockwool cubes, stacking them up to create 8- or 12-inch cubes of packed-out roots—a great way to grow huge yields. As long as the sides of the cubes are covered, the plant can root through the bottom cube and become even larger.

Peat Moss

Typically used by commercial farmers and found in bags of growing soil, peat moss is the decomposing organic remains of sphagnum moss. It’s actually a form of compost, as it’s in a state of humification, constantly leaching minerals into the grow medium. Peat moss has the ability to hold up to 20 times its own weight in water, so be careful not to overwater when using it. It’s great when growing outdoors and on a large scale where the ground may not be as moisture-rich as you might like it to be.


My earliest memory of growing up was when I’d dig a hole in the ground in the greenhouse, fill the hole with compost and place a seedling or clone in it. As its name states, this organic matter is in a constant state of decomposition. As compost breaks down, it releases all its micronutrients into the grow medium. Beneficial bacteria and plant enzymes play a major role in breaking down carbon-rich organic matter, which turns into a dark-brown or blackish color.

My tip here is to add 25 percent of a good compost source into your grow medium along with worm castings to provide a long-term supply of available nutrients that the soil will break down. If you follow my instructions for my personal organic mix, you’ll find that compost will also improve the texture and increase the lightness of your final medium even more.

Organic Additives

The list below shows which additives you should be conscious about adding to your medium. Once you’re happy with the overall consistency and feel of the grow medium, you’ll want to add these products in order to really supercharge your organic growing medium and bring your terpenes and resin production through the roof.

Bat Guano

This is one of the most underrated additives used in growing cannabis, but once you try it, you won’t look back. I often write about how bat guano makes the difference in terms of final flavor, and I can easily tell if a flower has been grown with or without it.

My top tip here is to add around 10 percent bat guano to the final mix. Bat guano in the grow medium will complement the microbial colony, making it rich with enzymes, and also enrich the medium with living humates to break down. Different bats produce different NPK ratios depending on their diet (if they eat mainly insects or fruits), so try a mix of variously sourced guanos. During flowering, top feed with a 24-hour aerated compost tea.

Seabird Guano

Seabird poop is perfect to mix in with your bat guano and offers some of the best nutrients for your plants due to the birds’ diet of fish. Rich in phosphorus and excellent for roots, this guano provides nitrogen, so it’s certainly a great additive to use during the vegetative stage.

Volcanic Rock Ash

Many grow shops sell different versions of volcanic rock. This fine black dust is very beneficial and it’s meant to be worked into the medium to improve drainage, air capacity and capillary action. It’s also teaming with trace elements. My top tip is to add this dark-black powder into your grow medium, focusing on the upper part. This way, new roots can grow into the volcanic dust and benefit from the trace elements early on.


This is one of the best additives and it costs literally nothing to obtain. You simply need to start a wood fire. Branches and logs work best, and you want to let the fire burn without allowing the wood to turn completely to ash. Under a microscope, charcoal has an incredibly intricate crystalline structure, so its ability to hold water is unmatched. It’s also a pure source of carbon.

My tip here is to break the charcoal into small pea-size pieces, then mix it into the grow medium as you’re preparing your final custom mix. Adding biochar will improve the drainage of the grow medium, enhance aeration and keep the beneficial bacteria well supplied with carbon.

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