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

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

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.

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

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

Josh

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

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

Al

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

Yes, Josh, what a coincidence..

I love that link and thanks Pen!

Mike

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.

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

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

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

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

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.

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

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.

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

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.

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

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.

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.

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??

Glad I found this thread. Explains my issue with gritty mix. All of mine are screened to 1/8 in

Hi all! I know its an old post but was wondering if anyone ended up checking / finding out what the water retention was for Turface ?

The % of water retention depends on particle size/ whether or not the product has been screened, and the ht of the soil column. It's listed as having a total porosity of 74%, with 39% capillary and 35% non capillary. I'm unsure of what determines capillary and non capillary, so I've contacted Profile Products for an explanation, which I'll share if/when I get a reply. In the overall scheme of things, it might prove more valuable to know that Turface MVP/ Allsport both holds a volume of water equal to its own weight; however, since each Turface particle is usually mixed into native mineral soils and entirely surrounded by soil particles, perched water is not taken into account with regard to the %s listed.

Al

Here is what I wrote and the reply I rec'd, in case you want to pursue your question further. Unfortunately, there is nothing useful (as it relates to your question) in the reply.

Me: Re: Turface MVP

The product is listed as having total porosity of 74%, with 39% capillary and 35% non capillary. Please explain the difference between 'capillary' and 'non capillary. I'm already familiar with the concept of adhesion + cohesion as the driving force behind capillarity.

Reply: On 10/31/22 8:16 AM, Derek Oosterhouse wrote:
Good morning Al,

Thank you for your interest in our Turface products.

Capillary refers to water holding pore space, and non-capillary would be non-water holding (or, air holding) pore space. Both components are beneficial for the purpose of drainage and healthy vegetation establishment.

Feel free to let me know if any additional questions.

Thanks,

Derek Oosterhouse
Regional / Market Development Manager
Midwest U.S. & Eastern Canada
Profile Products LLC
Tel: +1 616-581-6904
www.profileproducts.com

The primary reason it has no value is, the information is for an isolated Turface particle surrounded by mineral soil particles at field capacity. 'Field capacity' is a measure of how much water exists in the soil of fields or landscapes after having been watered/ rained on to the point of complete saturation, at the immediate point in time when the downward (gravitational) flow' of water has ceased. 'Container capacity' is a measure of how much water a containerized planting holds after having been fully saturated and at the immediate point in time when water has stopped draining from the container.

In a container filled with Turface MVP, there could not be a fixed % of air porosity owing to the presence of of a perched water table; this, because air porosity at container capacity will continue to increase as the height of the soil column (Turface in this case) increases.

Al

I have found that each ingredient used in a more gritty medium retains a different or slightly different amount of moisture, depending on various factors. Numbers are not my thing, so without bringing math or percentages into the discussion, I begin by soaking the pine bark fines, akadama, or any other moisture retentive ingredient I'm using overnight in a covered container before mixing my medium. I then get rid of any extra water that hasn't soaked in.

For all intents and purposes, the coarse perlite I use doesn't retain enough moisture to make note of, in my opinion. But beginning with moistened pine bark and akadama, I can ensure that the roots of any freshly potted plants will not dry out too quickly, but also won't have too much moisture to deal with.

When I water, especially when using the addition of fertilizer, I use deep saucers to catch the excess, then reuse that a second time to make sure the plant has been adequately watered. Once I'm sure the plant is sufficiently watered, I dump the saucers. I never want to leave them soaking when the process is done.

Using a more porous mix means watching moisture content more closely, and most likely watering more often than if one were to use what passes as potting soil sold in bags. But the extra effort is very worth it!

I continually read tags, instructions, and hear people say things like "water once a week" or things of a similar nature... when the truth is, I only water my plants as they need it. I only drink liquids when I'm thirsty... why would a plant be much different? :-)

There are many substitutions as ingredients go... turface, cherry stone, lava rock, perlite, pine bark, akadama, etc... some can retain moisture, others not... and at differing rates. I sort of tailor make my medium based on factors such as pot and plant size, plant type and needs, the climate or micro-environment it will be kept in, etc.

Happy Gardening!