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Letters to Rev – Soil Structure for Growing Cannabis

Letters to Rev – Soil Structure for Growing Cannabis

Soil Structure for Growing Cannabis

When it comes to soil structure for growing cannabis, we can all easily see some of the big differences between growing in the ground or in containers. Today’s article checks out a few of your issues of late I have read about, regarding soil structure in general.

Alright then earthlings, let’s rock and/or roll with today’s “Letters” to Rev…

Soil Structure, Question #1: Pumice?

FROM: John Q.

“Hey ya Rev. I don’t have much space to work with beyond my actual gardening spaces indoors. I’m basically bankrupt on storage space. I can keep one aeration amendment on hand in a fairly large size. I have kind of landed on pumice. Before I pull the trigger on this, I just wanted to get your input. Thanks, Rev, I can’t wait to get the new book!”

Rev’s Answer to Q1

Yo John. Thanks a ton for the support man, writing a book is pretty insane, workwise; LoL! Here’s the dealio, according to me… If I were you, knowing what I know, I would use perlite as my main amendment to aerate container soil, for a few reasons. That being said, I love-love-love pumice, and I use it along with perlite, and vermiculite too. Pumice, along with all its other outstanding qualities, also inhibits fungal growth—which is nice for cannabis—to a point.

Pumice is Awesome for Soil Structure and Especially Bacterial Soil Life
Pumice is Awesome for Soil Structure and Especially Bacterial Soil Life

Cannabis prefers bacterial dominant soil types, but there needs to be a balance between fungi and bacteria. I feel that using too much pumice in a soil mix may have negative effects on some of the good fungi in the mix. Like, say, the mycorrhizal fungus. I do use approximately ½ cup of pumice per about 4-cubic feet of soil (24-gallons) when I recycle my soil. If I were you, I would find a way to store a little pumice and more perlite. But perlite would be my sole choice.

Having a little vermiculite around is also a good thing for special situations, like sprouting, and planting freshly rooted clones. I feel ya regarding your space issues, I have grown in many different environments over the decades and a few of those were extremely space challenged. Perlite is also so lightweight, I used to just use a piece of duct tape to hang a 4-cubic foot bag from rafters. Pumice has more weight to it, it’s way heavier, and if it gets wet, it’s super heavy.

Soil Structure, Question #2: Using Coir Heavy Soil?

FROM: Tom M.

“I have started using a very coconut coir heavy soil mix as my living soil. Everything started out fantastic, but now I have problems left and right. Of course, these problems begin once I am three weeks into flowering. My plants all have a yellowish look on the top several nodes of leaves. I thought this was a pH issue so I used a little pH down and things became much worse. Please help Rev! Thank you.”

Rev’s Answer to Q2

I mean no insult when I say that you sound like a new grower. There are a lot of “rules” and physical laws you need to have a bit of a grip on; when growing indoors especially. Allow me to bring you up to speed on a couple of serious things I believe you have overlooked here…

First and foremost, if you are ever using coir as part or most of your soil mix, the coir MUST BE FULLY RINSED/FLUSHED! This is not a rule you can just dismiss. Coir carries a ton of great mineral salts, and uber tons of potassium is one of them. Unfortunately, these levels are so high as to be actually toxic to the plants. Yup… So, if you didn’t flush your coir, you’re boned and that’s it. Most growers that use coir in their mixes in greater amounts also use bottled organic nutrient liquids. The super-low pH of the organic nutrients counters the coir’s ability to raise the pH way up with super-salts. But they still rinse the coir first.

In Containers Perlite is Usually the Best Choice for Soil Structure Enhancement
In Containers Perlite is Usually the Best Choice for Soil Structure Enhancement

Another aspect here is how coir starts out as an excellent anti-compaction/aerating amendment for your containers’ soil. However, within a few weeks the coir is broken down and no longer has its anti-compaction qualities, and actually the coir sort of fuses together during decomp making it the opposite of an aerating amendment. Without another soil structure enhancing amendment anaerobic conditions are primed to happen, and this is what I would say is likely also happening to you.

Soil Structure, Question #3: Can I Make Just Vermiculite Work?

FROM: Johnny B.

“Greetings Rev. I have a situation where I can source vermiculite up the ass, but I have no options for perlite. I can’t really do online stuff so I have to rely on local suppliers. The vermiculite is agricultural grade and I can get 4 cu. ft. bags for a good deal. Can I just use vermiculite as my only amendment to help with aeration in my containers? Thanks for everything you do Rev.”

See Also

Rev’s Answer to Q3

Hey Johnny, greetings. The short answer is, yup, you sure can. But this will cause you to have to pay EXTRA attention to your watering habits very closely, you dig? Because as you likely know vermiculite can hold a shit-ton of water. More than soil can per volume, like 30x more at least. So, there’s that. You will be watering less often with some vermiculite in your soil. I would recommend a target of about 10% to 15% of your overall soil mix being vermiculite.

Vermiculite is a Fine Soil Structure Enhancement That Absorbs a Ton of Water
Vermiculite is a Fine Soil Structure Enhancement That Absorbs a Ton of Water

Vermiculite is actually very awesome stuffs. It (like pumice and perlite) has a pH of just about neutral 7.0 which is very bacterial friendly indeed. The vermiculite stores not only a lot of extra water but stores nutrients as well (like pumice). Vermiculite is super light like perlite, but unlike perlite which holds a little water on its exterior in nooks and crannies, vermiculite absorbs water. Once waterlogged vermiculite will be fairly heavy—duh—while perlite stays light.


Well, my esteemed homeskillets, that’s about it for today. I just want to say, regarding coir as a soil amendment. I no longer use it, and while it is fine in special situations you don’t really ever need it all. If you have some kind of desire to use it don’t even think about using it if it isn’t fully rinsed or flushed. Several years ago, you could still buy rinsed coir that had been left out in the rain for days before drying and packaging. Otherwise, you need to flush it out yourself. Also please, never ever use pH-Up or pH-down products on living soil. Ugh.

These days my main aeration and anti-compaction amendment is perlite, but there are also small levels of vermiculite and pumice as well. Perlite and pumice recycle extremely well and so less is added over time. Vermiculite kind of fully breaks down more rapidly than the other two, but it still hangs in for a while first—like 2 recycles for vermiculite, while perlite and pumice stay very well formed for many recycles.

Candyman Skunk Seeds—Say What
Candyman Skunk Seeds—Say What?

Back to working on the new book for me, whew. Swing by Kingdom Organic Seeds  and grab some of the latest: La Smush, Black SEA, and Candyman Skunk! Candyman Skunk will be available within a week or so of this publishing date. Get a copy of my latest TLO 2nd Edition book and learn the ways of the all-natural container grower Grasshopper. L8r G8rs…

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