Long Term Potting Mix Recipe - Alternative to Gritty Mix

arcan(10a)August 12, 2014

Hi everyone,
I'm a new gardener from Perth, Western Australia whose been lurking on these forums for about a year now. I'm mostly interested in containerised edible gardening. I thought I'd share a recipe for a long term potting mix that I found recently.

I bought quite a few (probably a few too many) dwarf and semi-dwarf fruit trees last year which I put into smaller pots. I'm now ready to repot them in larger containers: some ~50cm diameter glazed ceramic pots and some half wine barrel planters.

But this raises the question - what's the best potting medium for long term use? I've had a look through some threads which outline Al Tapla's Gritty mix but tracking down the ingredients locally is difficult. Also given Perth's Mediterranean climate (hot dry summers, cool wet winters - USDA 10a) and water restrictions during summer which allow reticulation only twice a week I wonder if the gritty mix can hold enough water. I know myself well enough to know I won't regularly hand water.

I found an alternative suggested by local Perth horticultural consultant Peter Coppin. He has three short youtube videos. The first is on the size & type of pots (http://www.youtube.com/watch?v=Whp1Kh4R5zw), the second is about the soil/potting mix (http://www.youtube.com/watch?v=zzPc5DJuGJc) and the third shows planting & establishment of the new trees (http://www.youtube.com/watch?v=9XvmTLO_h9I)

Peter's potting mix -All parts are by volume (not wieght)

Basic Mix
1 part good quality potting mix for organic matter
1 part course sand (e.g. washed river sand, quartz sand, lawn blend) for drainage
2-5kg Bentonite clay per 100L (sodium or calcium bentonite) for water retention

Suggested Additives - per 100L Basic Mix
100-200g Low nitrogen rock dust
600g Zeolite (holds moisture and nutrients)

Optional Extras
1 part Perlite/Vermiculite per 4 parts basic mix to reduce weight
Expanded clay - fill bottom of pot 1 or 2 clay balls deep to hold water

Peter says that this mix should last 10 years if you use a half winebarrel sized container (100L). He suggests hiring a cement mixer to evenly mix all the ingedients. In Australia all the ingredients are redily available. You can buy clumping kitty litter which made from sodium bentonite from Coles. Zeolite you can get from Bunnings as Zeo-Chlor pool filter replacement. Peter uses Ecoprime Natural PK rock dust.

I'd be interested to get everyone's thoughts on this mix or if you've used something similar before. Also how it stacks up to the gritty mix.

Here is a link that might be useful: Peter Coppins Website

This post was edited by arcan on Thu, Aug 14, 14 at 21:46

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It's not so much the exact medium recipe that's important... what's important is understanding the CONCEPT behind using such a mix of ingredients. We have to fully grasp WHY such a mix might be wanted, and HOW it works.

The first order of business is understanding why a plant requires medium, or soil, in the first place.

1. The medium offers anchorage and support for the plant.
2. Medium is the catalyst for bringing moisture and nutrition to the plant.
3. Medium serves as the catalyst through which gases and fresh oxygen are exchanged.

The second order of business is understanding that it's next to impossible to match what Mother Nature does in a garden setting with what goes on under the surface of soil within the confines of a smaller container.

Mother Nature has her own army of insects, worms, nematodes, fungi, bacteria, and other microscopic and larger living things all working in harmony to maintain aeration of the soil and break down organic matter into usable food/nutrition for plants to feed on. There's no accurate way to match and maintain that with any true balance within the confined space of a pot. It simply can't be done, especially with regard to any lengthy time frame. Therefore, it makes more sense to approach container growing from a more inorganic aspect... and save the real organic approach for the garden.

It's in understanding why a faster draining, more aerated and inorganic medium is beneficial to container growing, and how it reacts when moisture is added that will be most helpful in choosing ingredients to mix your own mediums.

It's important to know that when we water, we're helping the plant in the exchange of accumulated gases for fresh oxygen. As we water, the motion allows fresh oxygen to enter the medium around the roots, and gases are forced out.

At the link below, Tapla explains in more detail why a medium of larger particled, consistent size that is free-draining and aerated helps in the optimal growth of healthy root systems... which in turn allows for healthy plants.

There's another article floating about the forum, also by Tapla, that explains how to maintain plants, especially trees, in pots for long periods of time. It gets into root pruning and re-potting, which are essential to continued root health within a container environment.

After having used my own version of the Gritty Mix and 511 mediums, specifically tailored to my individual climate, environment and micro-environment, plant type and pot size, location where it will be kept, etc... for several years, now... I can say that understanding the concepts has really helped me in the ability to maintain healthy potted plants.

None of my pots hold too much moisture for long periods of time... they dry rather consistently, allowing for the roots to breathe. They no longer drown in a fine particled mix that can't support pockets of oxygen, and hold perched water tables... all of which is explained in the linked article below.

And by "a more inorganic approach", I mean using medium ingredients that will hold up well and don't break down quickly (or at all)... like fir bark, granite chips, turface, perlite, cherry stone, DE, lava rock, pumice, etc... and feeding plants using a liquid fertilizer and micro-nutrients, as opposed to some organic mixture that may not be in usable form for a plant's uptake.

Reading Tapla's articles is a great place to start... because understanding the concepts is the most important part.

A "green thumb" is nothing more than applied knowledge.

Happy Gardening!

Here is a link that might be useful: Container Soils - Water Movement & Retention XX

    Bookmark   August 12, 2014 at 4:35AM
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Hi Jodik thanks for your reply.

I have read those threads by tapla a few months back and I will read through them again; they put forward a strong case for inorganic container gardening.

What I like about the gritty mix is that it's so light weight due to all those big particles giving space for air. But I'm trying to strike a balance: enough drainage and enough water holding capability. No hand watering, no frequent repotting.

I do understand that good drainage is a must in order to prevent root rot and provide gas exchange. The question I have though is can gritty mix be modified so that it is suitable for twice weekly waterings in typical Perth summer temperatures of 35 C (95 F)?

I'm after what's great for the the gardener and good (not to say best) for the plant. I wonder whether Peter's mix might not suite the needs of a lazy gardener better than Al's?

    Bookmark   August 12, 2014 at 6:05AM
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I live in Adelaide and have been using gritty mix for nearly 3 years. It has no problems with water retention. Last summer was hotter than average with two days at 45C. Usually I was watering twice a week , rarely three times.
I either use Turface in the mix ( buy from Melbourne) or the Aussie made Kleensorb diatomite.

I don't want to sound too negative but make the following comments-
"good quality potting mix" -This is a misnomer as most are junk especially for long term plantings. If you can get the March 2013 edition of Burkes Backyard magazine there is some interesting reading.
Kitty litters are a minefield. Suggest you do a small test run first.
Not sure of pool zeolite is suitable for plants. I use a dedicated horticultural grade of zeolite in my other mixes.
Bonsai shops and clubs are often a good source of materials

    Bookmark   August 12, 2014 at 6:26AM
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Hi Alan,
It's great to get a response from someone who gardens in a similar climate - sounds like you had to endure a scorching summer last year! To be honest I was sceptical that the gritty mix would be suitable for our climates but now I'm intrigued. What does your version of the gritty mix consist of exactly? How does the gritty mix compared to other medium's that you've used? How often do you fertilise? Is it possible to use a controlled release fertilizer or do all the granules fall to the bottom?

And you don't have to tell me about the quality of potting mixes; I've definitely been burned in the past. I already have some Debco Raised bed mix though which I've found to be of fair quality so I think I'll still mix up a batch of Peter's mix but I might experiment with the gritty mix down the track to see how they compare.

As for the Coles brand clumping kitty litter - that was recommended to me at a GardenWise seminar by the Beyond gardens experts (www.beyondgardens.com.au). It's the cheapest available but other types have finer particles which are considered faster acting. The Zeo-chlor is also 100% zeolite and recommended at numerous places around the net.

Here is a link that might be useful: Debco Raised Bed Mix

This post was edited by arcan on Tue, Aug 12, 14 at 7:45

    Bookmark   August 12, 2014 at 7:29AM
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Hi Arcan
The plants I grow tend to need good drainage so they really appreciate a durable open mix like gritty compared to the bagged mixes I used before. The roots look so much healthier.
I have used Osmocote controlled release with success and also liquid fertiliser at recommended doses.
One trick I have learnt for plants in full sun is to have them in white or pale coloured pots -- heaps cooler than black pots.
I just watched Peter Coppins videos from your link, I agree with most of what he says. However the "drainage layer" at the base of the pot is not good - read what Tapla has to say about them .

1 Like    Bookmark   August 12, 2014 at 7:48AM
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Yes, Alan - that's why I didn't include that part of the video in my original post. I've included a link to an article that makes just this point about crocking.

I wonder if having a small layer of expanded clay would also cause this problem?

On another note: where do you source all the ingredients for the gritty mix? When I looked some months ago I found every ingredient but never in quite the right sizes.

Here is a link that might be useful: Rocks in pots? What a crock!

    Bookmark   August 12, 2014 at 8:08AM
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Arcan I suggest you go to my profile page and send me an email. We can then have a more detailed discussion about sourcing ingredients in Australia.

    Bookmark   August 12, 2014 at 8:34AM
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The grittier mediums allow for more frequent watering... in other words, allow for a wider margin of error, considering that most potted plants in general are killed by a heavy hand when it comes to watering.

I always water "as needed", and never on a specific schedule. I check individual pots for moisture content, only adding moisture where it's needed. The reason I do this is because each individual plant will use moisture at a slightly different rate, the medium within the pots drying at different times. It depends on the type of plant, the size of the plant, size of the pot, location of the pot, etc...

In my estimation, the grittier mediums are not really tailored for convenience... they're tailored for the health of plants. I'm not sure I would want to compromise when it came to my plants.

Good luck, though... I hope you can find what you're trying to achieve.

    Bookmark   August 13, 2014 at 8:16AM
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I understand where you're coming from Jodik. You must get great results using your fine-grained approach but as you say I want to try something more laidback.

I use adjustable drip irrigation so I can adjust the amount of water each of my pots get - I do this seasonally and I don't water at all in winter unless there's been a long dry patch.

In another thead Al suggested increasing the water retention of gritty mix by using 4 diatomite, 3 bark, 2 grit.

Now I just bought a 100L bag of coarse perlite and I found where to get diatomite and small pine bark. As diatomite holds more water than turface and perlite holds more water than grit, I figure a strait 1:1:1 mix of diatomite, perlite and pine would probably hold enough water. If not I could maybe add more diatomite.

    Bookmark   August 14, 2014 at 6:13AM
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Arcan : I have been experimenting with various combinations of soils and I agree that a 1-1-1 soil mix of pine bark, diatomaceous earth (DE), and perlite will hold a significant amount of moisture. In my very unscientific testing I've also used a 1-1 mix of pine bark and DE. That mix holds a lot of moisture, which would be good for very hot and dry areas.

Hope that helps.


    Bookmark   August 14, 2014 at 7:22AM
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Need2SeeGreen(10 (SoCal))

Interesting discussion.

I hope you will keep coming back here and not go offline for too much of the exchange, since discussion of moisture retention is very, very relevant to lots of us. I am in LA and it's a problem. We are having water restrictions too.

    Bookmark   August 14, 2014 at 9:01PM
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Well with my very limited experiment on this I will say that a mix of bark and either Turface or DE will hold a good amount of moisture. The more Turface/DE the greater moisture retention. I can say that my spruce tree seedlings growing in a bark/DE mix are doing very well this year however they use much less water than a tomato plant for example.

This does not mean that all types of plants will do well in this mix so I can't recommend it. As I mentioned before this is a very unscientific test on my part. If you are curious about trying a couple of plants in this mix feel free to experiment. It might help some plants in very hot and dry locations.



    Bookmark   August 15, 2014 at 7:22AM
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Thanks TYG - theory is one thing but its good to know that its seems to be working in practise too.

@Needs2SeeGreen - I mixed up a large amount of Peter's Mix maybe 500-600L on the weekend. It was very hard work even with a cement mixer (the sand weighs a ton); would have been impossible without one. So far I've potted up a double-grafted Mini Royal-Royal Lee cherry tree, a maltese blood orange and a meyer lemon. The cherry and lemon trees were root bound and needed a fair amount of root pruning.

I'm going away on holidays for for the next three weeks but I'll let you know how they're doing when I get back.

    Bookmark   August 19, 2014 at 4:08AM
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drew51 SE MI Z5b/6a

I experimented with DE this year and find it to be an extremely useful amendment. It really holds the water, and the pore size is big enough plants can utilize the stored water pulling it out themselves. Unlike turface which pore size is so small the roots cannot access it.
DE pore size is .36 microns turface is .045. Less energy is needed to release water from DE. Pore size is 8 times larger in DE! Turface can hold 95% of it's weight in water. DE can hold 145%,
The numbers speak for themselves. Source of info is the Axis company.

    Bookmark   August 19, 2014 at 7:21AM
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Some interesting comments here. I have been growing trees in large pots for nearly 30 years and have tried all the different suggestions. My videos were produced to show what I'm doing now.

Please be wary of information saying putting rocks/gravel in the bottom causes drainage problems. Yes, using just standard putting mixes (which are only short-term growing mediums) over a layer of rock or gravel, you may well get a 'perched water table'. But even with these crappy growing mediums, you must ask yourself "if I have a perched water table could that mean I'm watering too much?" - you don't have to be an Einstein to work that out.

You DON"T need water to drain out of pots - if that's happening then it's a total waste of the world's most precious resource. You want to be able to give potted plants just the amount of water they need to reasonably perform/produce. Period.

This is one of the reasons I put a layer of expanded clay in between. My trees are performing well with just 10-15mm of water per week, and this is in the middle of rainless 35-40 degree C weather with hot dry winds thrown in!

Cheers, Peter Coppin

    Bookmark   February 26, 2015 at 5:07AM
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Nil13 usda:9a sunset:21 LA,CA (Mount Wash.)

Here is a post from the Garden Professors thst shows whst happens when you have a layer of fine media over a layer of coarse media. Notice the water stops at the coarse layer and moves horizontally until the perched water table is created and then moves into the coarse material. If you put a coarse layer of gravel at the bottom of the pot amd you use a mix that can support a perched water table, the gravel layer will move the perched water table to a higher elevation in the pot than it would have been had the gravel not been there. Now if you have a study showing otherwise Peter, I would love to see it.
Container Planting: intuition vs reality

1 Like    Bookmark   February 26, 2015 at 7:57AM
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I'm not saying you don't get perched water tables when gravel is put underneath standard putting mixes (especially if there are insufficient drain holes or, worst still, none). I'm actually saying two different things:

1. We need to appraise the use of standard potting mixes as a long-term growing medium, because they aren't designed for it. That's why I've developed my own over three decades with the addition of both clay and zeolite being absolutely necessary to hold moisture.

2. Too much precious water is wasted on pot plants. As I said, my advanced trees do quite well on 10-15mm a week in a hot, windy area, where I know a lot of people who use 30-50mm! It's a matter of careful moisture monitoring, though that's often easier said than done. This is why I put a layer of expanded clay between the gravel and my soil mix - it absorbs excess moisture (when that occurs), and plant roots get down into that layer and can utilise it.

    Bookmark   February 27, 2015 at 5:21AM
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Nil13 usda:9a sunset:21 LA,CA (Mount Wash.)

How do you deal with salt accumulation?

1 Like    Bookmark   February 27, 2015 at 8:26AM
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tapla (mid-MI z5b-6a)

As in the link Nil used (good link - I 'borrowed' the picture), I think this is a case of someone's intuition telling them something that in reality doesn't happen.

Most of us are focused on making it EASY for others to consistently bring along healthy plant material because that is their focus. Once your priorities change and you're bound to those priorities, you self-limit your options. There is nothing wrong with focusing on saving water, but we all need to individually make the decision about the cost of our focus as it relates to the well being of our plants.

I see issues inherent in the plan described above, but can't offer a criticism as long as the focus is saving water. If the focus is on plant vitality, I see 2 substrate interfaces where perching is going to occur unless the expanded clay > .100 inch (.254mm) in which case there will only be perched water above 1 substrate interface.


    Bookmark   February 28, 2015 at 10:42AM
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this is J0 just found you.Ireallyreally realy like everything on your blog.Im From Tx.and I dont know about all that stuff that goes in your grit mix.I THINK I know but not sure.Can you explain those ingredeance for me.As I have been looking for good soil for containers.MG is no good any more.Please help.

    Bookmark   February 28, 2015 at 2:14PM
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tapla (mid-MI z5b-6a)

Jo - it's probably not a good idea to piggyback on Arcan's thread, but you can find a lot of info and get all your questions asked by clicking this embedded link and asking there.


1 Like    Bookmark   February 28, 2015 at 2:30PM
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I dont know how to do that.

    Bookmark   February 28, 2015 at 2:38PM
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i am sorry if I did something rong,

    Bookmark   February 28, 2015 at 2:39PM
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tapla (mid-MI z5b-6a)

Don't be disheartened and don't worry. All's well & I'm not scolding you. Everyone here wants to help you succeed. Normally, if you have questions, you would join in on a thread that is already discussing the topic you're interested in or start a new thread. If you click on the link I left in my post above, it will take you to a discussion already in progress on the topic you asked about, or for a wider variety of opinions, you could start your own thread. First, try clicking the link in my post above. It will take you to one of my threads. That's more polite than us starting a conversation in the middle of Arcan's thread. See you there!! ;-)


1 Like    Bookmark   February 28, 2015 at 3:45PM
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Nil, I've never had salt accumulation problems with the mix I now use, but to be honest I've never tested salt levels as I've not seen any visual symptoms to be concerned (maybe I should anyway).

But I don't use water-soluble fertilisers - my base fertiliser is a good rock (basalt) based product with a relatively low N%, and an organic liquid product is used if growth needs to be boosted.

    Bookmark   February 28, 2015 at 10:33PM
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