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Gritty mix water retention - sanity check
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Posted by alan.oz z10a (My Page) on Fri, Jan 27, 12 at 11:49
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? |
Follow-Up Postings:
RE: Gritty mix water retention - sanity check
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- Posted by tapla z5b-6a mid-MI (My Page) on
Fri, Jan 27, 12 at 16:32
| 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 |
RE: Gritty mix water retention - sanity check
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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 |
RE: Gritty mix water retention - sanity check
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| 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. |
RE: Gritty mix water retention - sanity check
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| 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 |
RE: Gritty mix water retention - sanity check
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Thanks for posting that link, Pen.
I was just referencing the granite in your test the other day....
Josh |
RE: Gritty mix water retention - sanity check
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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 |
RE: Gritty mix water retention - sanity check
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- Posted by tapla z5b-6a mid-MI (My Page) on
Fri, Jan 27, 12 at 20:36
| How is it mixed - components & relative sizes & volumes? Al |
RE: Gritty mix water retention - sanity check.
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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 |
RE: Gritty mix water retention - sanity check
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| Yes, Josh, what a coincidence.. I love that link and thanks Pen! Mike |
RE: Gritty mix water retention - sanity check
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| 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. |
RE: Gritty mix water retention - sanity check
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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 |
RE: Gritty mix water retention - sanity check
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- Posted by tapla z5b-6a mid-MI (My Page) on
Sun, Jan 29, 12 at 22:53
| 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 |
RE: Gritty mix water retention - sanity check
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| 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 |
RE: Gritty mix water retention - sanity check
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- Posted by tapla z5b-6a mid-MI (My Page) on
Mon, Jan 30, 12 at 18:21
| 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 |
RE: Gritty mix water retention - sanity check
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| 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. |
RE: Gritty mix water retention - sanity check
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- Posted by tapla z5b-6a mid-MI (My Page) on
Sat, Mar 31, 12 at 12:07
| 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 |
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