Diatomite, zeolite, pumice - do we need to soak them?

[GavinG]

A number of us use water-absorbent crushed rocks like diatomite or pumice in our mixes. I've been wondering whether they actually absorb this water with our usual overhead watering, or do they need to be soaked or immersed to absorb water effectively.

I tested pumice, scoria, diatomite (Mt Sylvia), zeolite and crushed brick, with decomposed granite as a non-absorbent grit for comparison.

A standard volume of the rock (a 4" pot full) was weighed dry, then 5 minutes after a "normal" gentle watering (until water ran from the pot), then weighed again after the rock was soaked in a bath for 5 minutes, then drained for another 5 minutes. The results show how much water was held after watering, and then how much additional water was held after soaking.

Both Diatomite (27%) and Zeolite (22%) held considerably more water after soaking - if your mix contains a lot of these materials, consider soaking shallow pots on extreme heat days.

Pumice and scoria both held around 15% more after soaking, so the technique may be useful.

Crushed brick and decomposed granite held very little extra after soaking.

What startled me was that diatomite held much more water than any of the others - 30-60% more, in fact. I wasn't aware that it was so much more effective. Pumice and crushed brick came next, scoria wasn't all that good, and zeolite, at least in this test, held water no better than decomposed granite, even after soaking.

I am fairly sure that particle size had little to do with it - they were all fairly even, and the diatomite was if anything, larger, and therefore should have been less effective.

I don't particularly like using pumice and Mt Sylvia diatomite, as they are both quite white, but unfortunately they do absorb water well. Hopefully Maidenwell will come back into production.

Gavin

[Phoenix238]

Diatomite is also used as an industrial spill-absorbant, if that helps explain its effectiveness

[SteveW]

Great research Gavin. The bit about diatomite holding 60% more water is intriguing: can you provide figures on how much water each ingredient held, when watered, when soaked, and, say after a week of normal watering? The last part of the question is to find out if daily watering results in a 'soaked or 'just watered' level of water retention.

[Scott Roxburgh]

So you measured the dry substrate, watered it, then measured again. Then used the same substrate for the soaking test?

How long was each overhead watering? How many times was the test repeated?

[kcpoole]

Interesting questions in Replies and interesting thought originally

I find that Newly potted trees seem to have good capilary action thru the Substrate, and and get good "wetness" throughout the pot. As it settles in the pot after a few months and the roots fill up the spaces, the centre of the pot seems to be a little less wet?
I Also noted a similer affect when I used to use organic mixes, so do not relly worry too much.

Ps I only ever top water by Sprinkler or Watering can.

Ken

[Graeme]

Another interesting experiment to conduct is to weight the pots after watering in the morning and then weigh again at the end of the day, or the next morning. There was a lot of discussion about the water conductivity of Zeolite many years ago, when I first started using it in my potting mediums. It was commonly believed Zeolite absorbed a large amount of water, but then 'locked' it up, thereby making it unusable to the plant. I remember conducting this experiment in my nursery for about a week and each day returned similar results. The Zeolite absorbed the water, causing the pot to weigh more and then when re weighed some 10 or 12 hours later returned the same weight, near enough, to the original dry weight. From that I reckoned Zeolite released the water into the potting medium for the plants use. Have never done the same experiment with any of the other common grits used in pot culture though. I would be interested in doing it with Diatomite, I must admit.

[lackhand]

I've been wanting to do a test like this myself so thanks for saving me some work!

My mix is about 80% diatomite with some organic material. I only top water, and my plants seem pretty happy in the super hot Phoenix dessert. I do have to water twice a day when the temps are around 120 F but I'm prett sure that would be the case with any medium in this heat. I buy my diatomite at the auto parts store - it's labeled for oil spill absorption and about $9 for 40 pounds of it.

One thing I really like about diatomite is that the color changes when completely dry, so it's pretty easy to see how badly more water is needed in the afternoon.

[GavinG]

Graeme: from my little test, it seemed that Zeolite didn't absorb much water at all - it just kept water on the surface like the decomposed granite.

Scott: Soaked straight after the watering test, to see how much additional water was held with soaking. Each overhead watering was until water ran from the 4" pot - about 3 seconds. The sample size was 1 - if you want to replicate the test, go for it.

SteveW: The numbers can get eye-glazing after a while, but here goes a summary. 
Diatomite held 26% more water than crushed brick, 35% more than pumice, 57% more than scoria, 66% more than decomposed granite, and 70% more than zeolite, after overhead watering. Similar results after soak, except that crushed brick, dec. granite and zeolite absorbed very little more after the soak, so the percentages increased. I can send you a pdf if you're feeling masochistic.

Lackhand: Phoenix sounds punishing in summer - are your trees under shade?

Gavin

[lackhand]

Lackhand: Phoenix sounds punishing in summer - are your trees under shade?

Yeah, they get shade most of the day. My patio is covered so they get some sun in the morning,and it's bright enough that they seem pretty happy. I've only lived here a year and a half - I'm from Colorado which is much cooler, and I'm still getting used to the weather here. I'm slowly rebuilding my collection too, because a lot of things don't grow real well here so I gave quite a few away when I moved. A few of them I brought didn't stand the heat well anyway. On the positive side, it has been fun to explore some new trees that wouldn't do well in the mountain winters where I'm from.

[lellul]

I bought some diatomite, zeolite and prunice and I'm ready to go but I'm unsure how to go about it. If it's an established tree, do you bare root the plant and re plant in the mixture ? Or is it better to start with a new plant so that it grows in it from the start ? Do you mix it in the soil ?

[kcpoole]

I bought some diatomite, zeolite and prunice and I'm ready to go but I'm unsure how to go about it. If it's an established tree, do you bare root the plant and re plant in the mixture ? Or is it better to start with a new plant so that it grows in it from the start ? Do you mix it in the soil ?

Hi there
I just repot a tree as normally would just replacing the new bonsai mix with the substrates. If I bare root the tree, i just repot up into substrate
No magic, just do what feels natural at the time.

Some members mix substrate into their normal soil, Some use it ( like me) exclusively.

Start with just a few trees and see what it performs liek for you. Have a search and you will find a few threads showing how I repot with Diatomite mix.

Ken

[Graeme]

Start with just a few trees and see what it performs liek for you. Have a search and you will find a few threads showing how I repot with Diatomite mix.

Ken

This is the best advice someone can offer any Bonsai enthusiast Ken. So many times we hear a new "idea" turn up in our Bonsai world (or any other world for that matter) and just because it's new, everyone rushes out and does whatever. Most times, for the majority, the venture ends in tears and heartache, leading to abuse of the originator. My favorite story of this type of behaviour relates to a Potting medium story.
I have told the story before but worth repeating again here. It is told that a leading Bonsai Authority named Dorothy made a lecture tour through South Africa many years ago, just after she started using the mix that became known as "Dot's mix". During the tour she informed all and sundry of the benefits of using this 'new' open mix and as the result one S.A. enthusiast went home and replanted his entire collection into "Dot's mix". A short time later the gentleman started his whole collection over again, because he lost every single one of his trees. Of course Dot took a bit of a hit with some of the South African Bonsai folks, which of course she shouldn't have. Dot's mix did work and still does for some people. For those people who live in hot climates and can't water every day (like our mate here) the mix just wont work.

As Ken said, if you hear of something new and you believe it will work for you then try it with 2 or 3 trees, not you whole collection and not for just one tree as well, it's also a good idea not to use your best trees for the trial either The trouble with single tree tials, we never know if it is the trial or something else that causes the reaction. The trouble with whole collection trials is as explained above. I also believe it is in our best interests to try new stuff as well. Some of the wild and weird stuff we hear that we tend to discard without the correct amount of research actually turns out to be beneficial to our hobby and if we discard the information to readily, we fail to move forward as we should. (Brett )

The bottom line is as quoted by Ken,

Start with just a few trees and see what it performs like for you.

[63pmp]

So you took some gravel, potted it than watered it some, let it drain for a bit, weighed it and then soaked it for 5 minutes. Drained it for 5 minutes before weighing it again.

How did you calculate the percentages?

Is it not possible that the larger % difference was simply because it takes time for zeolite and diatomite to become completely saturated, while the other materials require less time?

What size are the particles?

Paul

[bonzaidog]

Spot on Grahame...My mix has actually evolved to suit A: Tree Species B; pot size and C; work schedule. All based on correct water/air retention. Sometimes I was losing concentration at work on those really hot spells wondering what carnage may be occuring at home! It nearly became a "natural selection" process.....Dog.

[Bretts]

Hey Gavin, I brought this up when Grant did his soil tests. You can see this effect when you use clear pots. With diatomite and also akadama. Even if you water the pot very well after the water drains out the medium is still soaking up the water so the water table will go down. I believe a good second water will do the job but could not argue that soaking would not do an even better job.