Gritty mix water retention - sanity check

alan.ozJanuary 27, 2012

I started using gritty mix here in Australia over a month ago after being introduced to it by fellow GW member pkozul.We use diatomite instead of Turface, although it looks like we now have a local supplier of Turface MVP.

So far I have over 30 shadehouse plants in the mix and all seem to be growing very well despite some hot summer days up to 105 F.

One thing that concerns me is when watering it seems most of the water passes right through.

So I decided to do a measurement test. I selected a pot that hadn't been watered for 4 days of 90F+ weather and was dry with the bamboo skewer test. I measured the volume of mix to be exactly 2000ml ( just over 1/2 gallon)ignoring the volume of the roots.I slowly poured 500ml of water into the mix and placed a bowl underneath to collect the water passing through.

After waiting 15 mins I measured the amount of water in the bowl - 450 mls. Hence the mix retained 50mls which is 2.5% of its volume, Does this number sound right? Seems very low.

Has the mix become water repellant?

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tapla (mid-MI z5b-6a)

Start with a given volume of DRY soil in a pot and a given volume of water in another vessel, such that when the pot is submerged gradually in the vessel the water level remains equal to the soil level. Allow the soil to absorb water for an hour, then lift & allow the water to drain back into the vessel. You can calculate the water retention after noting how much water is missing from the vessel.

Bark does become water repellent as it passes below about 30% water retention, so that probably played a part.

Report what you find. ;-)

Al

    Bookmark   January 27, 2012 at 4:32PM
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alan.oz

Thanks Al
I can do that test, give me a few days as I only watered my gritty mix pots yesterday.
I gather that the 2.5% water retention figure is too low, I need to find out what has gone wrong as my plan is to eventually have over 100 plants in gritty mix. Several plants that were dying a slow death in bagged potting soil are now growing with renewed vigour in gritty mix - this is even more impressive considering I might not have the gritty mix optimum.
Alan

    Bookmark   January 27, 2012 at 5:51PM
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Ohiofem(6a Ohio)

Alan:

I have to say your calculations sound about right. I've been using the gritty mix for all my houseplants since last summer. In my house, I have almost 40 houseplants in one and two gallon pots that I usually water about once a week in the winter. I pour one to two quarts of water into each one, and about 70-80% of the water runs out almost immediately. The media is not repelling water; it just doesn't hold very much.

At first when this happened, I was very concerned and took to soaking my pots as Al advised. In some cases, I actually emptied out the pot to figure out what was going on. I found that generally, if I had watered slowly taking care to water the entire surface, the potting mix was evenly damp.

Now, six to eight months after repotting in this mix, my plants seem to be thriving. So, I've stopped worrying.

There is one caveat: When my plants first went into the mix in the summer, we were having temperatures in the 90s. I do think it took at least a week of watering copiously for the media to "settle down" and my plants to stop showing "drought symptoms." At least that's what I told myself.

    Bookmark   January 27, 2012 at 6:11PM
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penfold2(4b, MN)

I think you'll come up with a much higher number if you follow Al's directions. I did some similar tests on individual ingredients, and even granite grit (the least water retentive ingredient in the gritty mix) still held 8% water.

More Soil Substrate Comparisons

    Bookmark   January 27, 2012 at 6:29PM
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greenman28 NorCal 7b/8a

Thanks for posting that link, Pen.
I was just referencing the granite in your test the other day....

Josh

    Bookmark   January 27, 2012 at 7:43PM
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alan.oz

Hi Pen
That's a very interesting comparison in your link.

Regarding my current mix, I'm quite sure it will retain more after dunking the pot in a bucket of water for an hour, I want it to retain more from a standard watering.
Alan

    Bookmark   January 27, 2012 at 8:20PM
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tapla (mid-MI z5b-6a)

How is it mixed - components & relative sizes & volumes?

Al

    Bookmark   January 27, 2012 at 8:36PM
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alan.oz

Hi Al
I used diatomite, pine bark and granite grit mixed 1:1:1.
I was very fussy with the sieving - nothing over 1/4" and nothing under 1/8". No dust.
I will do the diagnostic test you suggested late Sunday my time to let the mix in my pot dry out. The current 90F+ temps should take care of that.
Alan

    Bookmark   January 27, 2012 at 9:01PM
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meyermike_1micha(5)

Yes, Josh, what a coincidence..

I love that link and thanks Pen!

Mike

    Bookmark   January 27, 2012 at 9:29PM
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penfold2(4b, MN)

No problem. I just wish I had more results for Turface, Floor Dry, and bark as well. It might be useful when altering ratios or ingredients.

    Bookmark   January 27, 2012 at 9:48PM
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alan.oz

Al
Just to recap on the numbers.

Standard watering from top ,then drain - 2.5% retention.

Since then I did your suggested test.
Full immersion 1 hr, then drain - 6.25% retention.
Next week I hope to have Turface MVP, will make new gritty mix and check these numbers again.
Alan

    Bookmark   January 29, 2012 at 6:37AM
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tapla (mid-MI z5b-6a)

I think the average particle size is probably larger than what most of us end up with, and I think you'll find your difference therein. Turface has most particles under 1/8", and the grit we use is usually always smaller than 3/16. Why don't you try

4 diatomite
3 bark
2 grit

to increase water retention. You might even end up with no grit at all by the time you get it right. The grit's main function is to act in concert with the diatomite to adjust water retention when you vary the ratio. Adjustability is one of the soils attributes, you might as well use it. ;-)

Forgive the tardy reply - the houseplants forum has been giving me fits & I've had company for most of the weekend. ;-)

Al

    Bookmark   January 29, 2012 at 10:53PM
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alan.oz

Thanks for that Al.

What % water retention numbers should I be aiming for?

I have learnt so much in the last two months, even though my gritty mix may not be optimised, my plants growing in it are outperforming those in bagged soil.
Alan

    Bookmark   January 30, 2012 at 1:38PM
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tapla (mid-MI z5b-6a)

It's been soo long since I did any comparative tests that I don't remember. Prolly at least 10-12% seems right when I think of it.

Al

    Bookmark   January 30, 2012 at 6:21PM
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whitecap2

Interesting experiment. It suggests that Turface and similar materials need more time to absorb water than they get when water is poured on them from above. Common sense, really. We don't dump Floor Dry on a spill and expect it to work instantaneously. So, we can increase the water retention of these materials if we soak them in water for some interval of time. Alternatively, we can just water daily, in hot weather. A dubious trade off for the benefit of avoiding frequent repotting.

    Bookmark   March 30, 2012 at 11:45PM
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tapla (mid-MI z5b-6a)

Turface may need a little time to absorb liquids more viscous than water, but it absorbs water almost instantly.

Avoiding frequent repotting is a only one of the many secondary benefit that come along with the fact that the soil provides an environment which ensures the opportunity for excellent root health, the most significant benefit. In addition to less frequent repotting, repotting is much easier when plants are in the gritty mix, when compared to peat-based soils - another secondary plus.

Al

    Bookmark   March 31, 2012 at 12:07PM
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MrFrangi

Hi Alan,
I know this is an old thread but I would like to know what ratio diatomite:bark:granite you ended up with?
Two of my frangipanis need repotting and I was thinking of using the gritty mix. As Al suggested I might leave the granite out completely so I end up with a mix that can hold more water.

    Bookmark   August 1, 2012 at 9:30PM
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alan.oz

Hi Mr Frangi
I have continued with the 1:1:1 gritty mix.
Now have over 100 plants in it - all are loving it.

The Maidenwell diatomite is no longer available so I am now using Turface sourced from Sage Horticultural in Melbourne.

To improve water retention and save money I sieve the Turface on insect screen (1.6mm) . The bark and grit are on a 3.0mm screen.

Hope you frangipani (plumeria) do well

    Bookmark   August 2, 2012 at 7:11AM
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tomatotomata

I have a question: I use the bamboo skewer method of testing the moisture in my houseplants' gritty mix. I've never calculated water retention, but if the skewer is damp, can't I assume the plants have adequate access to moisture? They have only been in the mix for about a month, so I don't have long term results to note, but they seem to be doing fine.

    Bookmark   August 2, 2012 at 11:52AM
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tapla (mid-MI z5b-6a)

Yes - if the skewer is damp, and you have no reason to believe the soil will dry completely before your next opportunity to water, it's best to withhold water until the soil seems dry. The only exception I can think of concerns plants newly root pruned and repotted. If you root prune aggressively, most of the roots could be in the upper fraction of the soil, in which case you'll need to water a little more frequently until roots have established. A final thought, even if you DO water before the most opportune time, it's not like the gritty mix is going to shut you down. It's very forgiving because of its extremely high porosity at container capacity and lack of a tendency to hold perched water - so forgiving that watering on a schedule, as long as you're being reasonable, is rarely a problem. I do it all winter long. I have a very few plants in small pots that get watered daily, a very few more that get watered every 2 days, and the 125 or so other plants I maintain over winter get water every 3 days, whether they need it or not. That isn't to say anyone should emulate that practice, or that it's the best way to water, I just have so many plants that NOT watering on a schedule would be a logistical nightmare.

Al

    Bookmark   August 2, 2012 at 7:41PM
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MrFrangi

Thanks Alan for the update.
So you think the issues you were having were due to using diatomite instead of turface?
Haven't been able to find anyone selling either locally yet, so I might end up with bagged soil again.

    Bookmark   August 3, 2012 at 2:23AM
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edweather(Zone 5a/b Central NY)

Just my .02 worth. I used a 1/8" screen for my granite and floor dry when I made my first small batch, and the water retention was really poor. Will definitely use a 1/16" screen next time.

    Bookmark   August 3, 2012 at 9:20AM
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olympia_gardener(5)

This is a great thread. Thank you Alan for bring up this topic. I was in the process of move some of my plants into Gritty mix, but was stopped/slowed by the same concern Alan has. Water retention on my mix is about what Alan had expereinces, although I didn't track the data as closely as Alan did. but 2.5% retention sounded right. Most water just run right out of the pot, good sign of porus mix but I have to pour more water into the pot in order to cover the entire surface. Consequently have to have some larger enough container to catch the runoff water. Soaking the whole pot in a container is a solution, but if you have larger amount of plants, I don't exactly know how many plants I have, but if each soak 1 hour in the water, if I have 30 plants, I am sure I have more than that, I need about 1 day and half to soak all the plant. If only it has about 6-7% water retention after soaking, which means I need to water it more often so in another one or two days, I have to repeat the whole process again. The watering alone is going to be a part time job. Although,I can water them in paralle fashion, each has a larger container to catch the runoff but lift the plants, whole plant in a gritty mix is not light!, and empty the runoff containers will add extra work.

No doubt Al's mix promots plants' roots system. From the short term growth of my plants I had in Al's mix, I did see they are growing very well( I am a newib to the mix and don't have a long term growth expereinces to share yet) in the mix. But I would like to see a mix that can balance the porus structure for roots to grow healthy and retent enough water to cut down the water/fertilizer and maintenece time to once a week so growing plants still can be a pleasure not a job. Any suggestions??

    Bookmark   August 3, 2012 at 12:21PM
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