Components of substrate separate

[fanaticactus]

Now that I've decided that using actual pumice in my soil mix, I buy it on eBay even though the cost to ship is more than twice what I pay for the pumice itself. But it's worth it. My cacti are noticeably healthier and grow faster in the lighter mix. I use 60-70% pumice, 15-20% coir and the rest is a combination of commercial cactus mix (Miracle-Gro), coarse sand (about half soil and half sand) and a handful or two of small gravel when and even more porous soil is called for. The trouble is that the sand/soil sifts down through the gravel and pumice in the tub I mix it in, so that the last cacti seem to get more of the denser parts of the mix than the first ones (which probably don't get whatever benefit is provided by the bit of soil I use. Those of you who use a similar mix, do you experience this 'sifting-down' problem too? How does one compensate for it--or even prevent it from happening in the first place?

[Steve Johnson]

I'll pass along a little trick I learned from a veteran grower in San Diego. Right when you pot your plants, spray the mix just enough to get it a little moist and mix thoroughly. This will allow the ingredients to hang together better when you get the mix into the pot. Then it'll dry out quickly, and root rot shouldn't be a problem. I've done this successfully with those of my cacti that need some soil in the mix. However, IMO it's best if you do it only in the growing season.

[fanaticactus]

I'll pass along a little trick I learned from a veteran grower in San Diego. Right when you pot your plants, spray the mix just enough to get it a little moist and mix thoroughly. This will allow the ingredients to hang together better when you get the mix into the pot. Then it'll dry out quickly, and root rot shouldn't be a problem. I've done this successfully with those of my cacti that need some soil in the mix. However, IMO it's best if you do it only in the growing season.

Thanks, Steve. I had sort of been wondering about using slightly dampened soil, and I have tons of spray bottles. This sounds like a very plausible solution. Do you use a bit of 'soil' in your pots as opposed to simply pumice/sand/grit/coir? I just finished potting 21 cacti that arrived Friday from Miles. Some already had huge buds! I potted smaller than I usually do simply to avoid to possibility of having too much substrate stay damp too long--although the pumice has helped that problem immensely.

[Steve Johnson]

Do you use a bit of 'soil' in your pots as opposed to simply pumice/sand/grit/coir? I just finished potting 21 cacti that arrived Friday from Miles. Some already had huge buds! I potted smaller than I usually do simply to avoid to possibility of having too much substrate stay damp too long--although the pumice has helped that problem immensely.

I've been meaning to post a brief tutorial on mineral mixes, so now is as good a time as any...

Here's a photo of my washed pumice-DG mix, and I'm including a scale for reference so you can see the different grain sizes in the mix:



I recently ran out of DG, but I didn't want to go through the whole process of getting more, so I bought a bag of crushed granite poultry grit on eBay. Here's the mix now with both types of granite and pumice:



Granite is granite -- nothing magical about DG, and the crushed granite will work just as well. I grow the vast majority of my cacti in soil-less mineral mix. For the few that need some soil added in, I use a big bag of cactus mix I got from the California Cactus Center about 5 years ago. Here's a sample:



I pull out the big rocks and chunks of organic material (tree bark and twigs), then the soil is ready to go in. (Okay, maybe not the best, but a far sight better than the crap that passes for potting soil these days.) My recipes as follows:

Eriosyce senilis and Tephrocactus -- 50% straight pumice and 50% soil
Discocactus and Melocactus -- 80% pumice-granite aggregate and 20% soil

I repotted a Melo matanzanus just today, using the spray-down technique to moisten the mix a little. Works really well, and the mix will maintain its integrity during the course of regular waterings. Unless you live in a dry climate, I'm not sure if you need any soil at all for your cacti, although that's a determination you'll have to make based on your experience and what you're growing.

[fanaticactus]

Do you use a bit of 'soil' in your pots as opposed to simply pumice/sand/grit/coir? I just finished potting 21 cacti that arrived Friday from Miles. Some already had huge buds! I potted smaller than I usually do simply to avoid to possibility of having too much substrate stay damp too long--although the pumice has helped that problem immensely.

I've been meaning to post a brief tutorial on mineral mixes, so now is as good a time as any...

Here's a photo of my washed pumice-DG mix, and I'm including a scale for reference so you can see the different grain sizes in the mix:

I recently ran out of DG, but I didn't want to go through the whole process of getting more, so I bought a bag of crushed granite poultry grit on eBay. Here'e the mix now with both types of granite and pumice:

Granite is granite -- nothing magical about DG, and the crushed granite will work just as well. I grow the vast majority of my cacti in soil-less mineral mix. For the few that need some soil added in, I use a big bag of cactus mix I got from the California Cactus Center about 5 years ago.

I pull out the big rocks and chunks of organic material (tree bark and twigs), then the soil is ready to go in. (Okay, maybe not the best, but a far sight better than the crap that passes for potting soil these days.) My recipes as follows:

Eriosyce senilis and Tephrocactus -- 50% straight pumice and 50% soil
Discocactus and Melocactus -- 80% pumice-granite aggregate and 20% soil

I repotted a Melo matanzanus just today, using the spray-down technique to moisten the mix a little. Works really well, and the mix will maintain its integrity during the course of regular waterings. Unless you live in a dry climate, I'm not sure if you need any soil at all for your cacti, although that's a determination you'll have to make based on your experience and what you're growing.

Thanks for this nice post and the photos about the mix you use for your collection, Steve. I keep vacillating between using a purely mineral ('stone') mix and one with a little soil & coir. I use plastic pots because they're cheaper and easier to get than clay, and I found that in the greenhouse even the non-porous plastic pots dry out quickly. I suppose if I used DG (in the form of your typical automotive store oil absorber) it would be easier and less expensive than coir--although one order of that goes a long way. I was concerned that it might hold moisture a bit too long, but maybe not. I have bought poultry grit (now that I'm in farm country), but I can only get it in small bags at $8 a shot. I can't imagine the postage you have to pay for larger bags on eBay; the postage on pumice, which is lighter, is more than double the cost of the pumice itself. Anyway, you've made me reconsider not even using bagged soil at all except for the jungle type, which I don't recall you have anyway. BTW, all of my orchid cacti have buds this year--one is showing buds for the first time. I guess they've finally reached the blooming age/size.

[keith]

I went to lowes and they sell medium sized bags of red lava rock for less than 4 bucks. The lava rock bag is full of lava sand and tiny rocks of lava which for most people would mean its not that great but I shift it through 1/4" screen and use the small lava rocks and sand in place of pumice. The big ones 1/2" to 1" left over, the ones you are paying for, I'm not sure what I'll do with them? Maybe put them in my barbecue ? Top dressing for around plants in the garden ? smash them down with a hammer? Really a crappy product with so many fines in it but hey its Lowes king of crappy products and I want the fines. You probably get about 1/2 cubic foot of small lava rocks and sand out of a 3 cubic foot bag ?

Mix it in with my compost 50% lava rock 50% sandy soil. I used to use Pumice works well but expensive around here if you can find it. So I use the small lava rocks seem to work OK.

A few cactus I add organic to the mix like Echinocereus ridigimuss and Turbinicarpus ysebella. not too much I don't like scaria fly bad for seedlings.

[keith]

opps my sizes were way off 0.5 cubic foot bag. 3 cubic feet 1 yard way too much !!

http://www.lowes.com/pd_672933-11106-R1 ... =lava+rock" onclick="window.open(this.href);return false;

[greenknight]

A cubic yard is 27 cubic feet, about as much as a full-size pickup truck can haul - but 3 cubic feet of gravel would be too heavy to pick up, too.

The larger lava rock would make good top dressing, which is what it's sold for. It's used instead of bark mulch because it doesn't attract carpenter ants like bark does.

[fanaticactus]

I'll pass along a little trick I learned from a veteran grower in San Diego. Right when you pot your plants, spray the mix just enough to get it a little moist and mix thoroughly. This will allow the ingredients to hang together better when you get the mix into the pot. Then it'll dry out quickly, and root rot shouldn't be a problem. I've done this successfully with those of my cacti that need some soil in the mix. However, IMO it's best if you do it only in the growing season.

Thanks, Steve. I had sort of been wondering about using slightly dampened soil, and I have tons of spray bottles. This sounds like a very plausible solution. Do you use a bit of 'soil' in your pots as opposed to simply pumice/sand/grit/coir? I just finished potting 21 cacti that arrived Friday from Miles. Some already had huge buds! I potted smaller than I usually do simply to avoid to possibility of having too much substrate stay damp too long--although the pumice has helped that problem immensely.

Just to let you know I've tried the 'dampening' method to keep the soil integrated. It works well. I've started to use Daiv's mix for jungle-type cacti: 25% each of orchid bark, potting soil (I use a richer one for African violets), coir and pumice. Nothing has been growing in it long enough for me to judge its effectiveness, but it looks, feels and smells perfect for Schlums, Epis, Rhipsalis, Hatiora, etc.

For desert cacti I have a problem with 'topsoil'. Anything I found in a commercial bag labeled 'topsoil' was too heavy and had too much debris in it. So I've invented my own. My topsoil mix consists of 6 parts commercial cactus soil, 3 parts gritty sand, 2 parts of that AZ soil we found caked too much to use by itself (remember?), 1 part poultry grit and a large handful of earthworm castings. Mixed together it seems like a nice texture sandy soil. So the full desert mix is also Daiv's: 60% pumice, 20% coir and 20% of my special topsoil mix described above. It's quite porous, so water soaks right through quickly. I figure whatever small amount of organic matter is in there is OK.

One thing I forgot to ask you. What is the ratio of pumice to DG you use? Do you always use DG or do you sometimes substitute poultry grit? I assume they're not the same since DG (litter or Oil-Dri) holds some small amount of water, whereas grit does not. Does it make sense to use all three together (pumice, DG and grit)?

Our weather finally warmed up. Officially it was 75 today, but here in the sun my weather station showed 88. Things are really starting to take off now. The forecast is for mid-to-upper 70s for the next week and just a few showers.

[Steve Johnson]

What is the ratio of pumice to DG you use?

My pumice-to-DG ratio is about 3-2. Since I keep my collection outdoors year-round, I have to be mindful of wind problems -- truth be told, the only reason I use granite at all is to make the pots heavier than they'd be with pumice alone. If you have your plants in a greenhouse, wind obviously won't be a problem, and IMO your desert cacti should do quite well with straight pumice.

[fanaticactus]

Even though I have a greenhouse with a good exhaust fan, the temperature on those hot days can rise into the 90s inside. So I will routinely keep a number of the larger, more well-established cacti outside in the fresh air, which I know benefits many of them and reduces the likelihood of mealy bug infestations. They seem to thrive more in still, warm environments. Only if I know when a very rainy spell, a strong storm front or a cool spell is going to arrive do I take those cacti back into the GH. So, following your reasoning, it's probably a good idea I continue with some grit or DG in the pots anyway; it can get quite windy up here in the valley even on a nice clear day just before a different pressure area comes in.

[Steve Johnson]

Okay, since you do have a practical use for granite as a component in your mix, I'll describe my decomposed granite a little further...

When I decided to go soil-less in 2012 (thanks to Darryl Craig of CoronaCactus -- some of the best advice I ever got!), I found a source for DG in Pasadena, CA that quarries it locally. About 60% is "play sand" -- completely useless, so I had to sift it out. The other 40% is granite gravel, and that's what went into my pumice-DG mix. As I said last Sunday, I recently ran out of DG, but I didn't want to go through the whole process of getting more (all that sifting is labor-intensive!). Functionally there's no difference between DG gravel and crushed gravel, so my reason for going with crushed gravel poultry grit now is just to make things easier.

Granite has a porosity of less than 1%, so it contributes nothing to moisture retention in a mineral mix. If anything, a pumice-granite mix may dry out more quickly than pumice alone. I haven't tested this yet, but I'll run one and do a side-by-side comparison to see if the difference is significant. If it is, then whether or not a soil component should be included will depend on the humidity of one's climate. You know how methodical I am, so after I run the test, you can have some confidence in the accuracy of my results. I'll let you know what they are, but in the meantime please don't let this keep you from doing whatever you have in mind.

I realize how frustrating it is to find good commercial potting soils these days. But tempting as it may be to "engineer" custom soils, we're bound to run into problems with ingredients that won't play well together. Yes, I do remember the Arizona Earth Naturals soil I tested -- on its own the stuff holds moisture way too long, so I'm not sure if that property would be tempered by the other ingredients in your custom soil. Of course a good sandy loam would be wonderful, so simple and yet so elusive to find here in the US! Wish I could give you more possibilities to pursue, although because simplicity seems to work best, here's one I can give you right now. Use a piece of fiberglass window screen and sift your commercial cactus mix. Throw away the big chunks, then use the soil and fine bits of whatever gravel and organic materials are left. I don't think there'll be much of either, and my guess is that you'll end up with a perfectly acceptable soil for your desert cactus mix. Worth an experiment anyway, and a clear plastic bottle comes in handy when you run a wet-to-dry test.

[iann]

The good thing about pumice is that it retains air in the soil where granite doesn't. Damp soil without air encourages pathogens that cause rot. Decomposed granite especially produces a very dense soil that is not very forgiving. Saturated (meaning no air) with even a small amount of water and then completely dry in a heartbeat because there wasn't actually very much water there. Pumice holds a little more water but also maintains air pockets. Unless you do something like adding a really find sand that blocks up all the pores! That's one of the main reasons I worry about people mixing together too many ingredients. All the valuable things about your valuable pumice or sifted coarse gravel can be destroyed by mixing it with something that just clogs it up.

[greenknight]

My approach to cactus and succulent mix is to make up small amounts as needed in a heavyweight plastic bag, like an old potting mix bag. Put in the proportions of potting mix and pumice needed for what I'm potting and shake it up. The ingredients are already damp, so it stays mixed, but it's easy to shake it up again if need be anyway. Might not be good for someone with a really large collection, but if you need just small amounts it works well.

I'm fortunate to have available a locally-produced potting mix that doesn't contain any bark or wood - just worm castings, peat, and pumice - so I don't have to mess around picking out the chunks. I used to make a similar potting mix with my own compost, But I've gotten lazier.

I don't fiddle around with complicated mixes, I'm with ian on that.

[Steve Johnson]

I have some new info for the discussion -- you may find this rather interesting. And hopefully helpful too! Here's what I said on May 21:

Granite has a porosity of less than 1%, so it contributes nothing to moisture retention in a mineral mix. If anything, a pumice-granite mix may dry out more quickly than pumice alone. I haven't tested this yet, but I'll run one and do a side-by-side comparison to see if the difference is significant. If it is, then whether or not a soil component should be included will depend on the humidity of one's climate. You know how methodical I am, so after I run the test, you can have some confidence in the accuracy of my results.

I finally did this -- 3 test runs, same results with each test, so the following is accurate...

I began the test with 2 identical plastic pots and pretty much equal volumes of pumice-granite gravel mix and straight pumice:



Using a digital postal scale set to metric readout, I weighed each pot dry, then saturated. Here's what we get weighed in grams:

Pumice -- dry = 96, wet = 131, water weight = 35
Pumice and granite gravel -- dry = 138, wet = 176, water weight = 38

This surprised the heck out of me because it's the exact opposite of what I thought we would see given the first part of my quote. I won't even try to hazard a guess as to why, although maybe someone can explain the difference in water absorpotion. Not sure if it matters anyway, so we'll look at the time it takes when the media go from wet back to their completely dry state:

Pumice -- 6 days
Pumice and granite gravel -- 11 days

Not surprising, and it tells us that granite gravel contributes more to the mix than simply making the pots heavier. Also please bear in mind that the test pots don't have cactus roots there to soak up the water. I did random wet-to-dry weighing tests with some of my cacti a few years ago, so with their roots well in growth, I can tell you that my pots go from wet to bone dry in 6-8 days. But you may be in for 2 more surprises. First, drying-out times are roughly the same regardless of pot size (although to be perfectly honest, most of my pots are in the 2"-4" range, so I'm not sure if this would be true for pots much beyond 5"). Second, my pots don't dry out any faster in summer than they do during springtime. So long story short, the pumice and granite gravel I use is perfect as a soil-less mix, and IMO better than pumice alone.

I agree with Ian -- if you go to the trouble of cleaning the fines out of your mineral gravel, the last thing you'll want to do is dump it back in the form of too much soil that'll clog up the works. Do you need any soil at all? The answer should be driven not so much by heat, but rather the humidity in your climate. My place is midway between the coast and downtown Los Angeles, so it's fair to say that I live in a semi-coastal microclimate. This screenshot shows a humidity curve that'll be typical for me throughout the growing season:



Once again, perfect for going soil-less. Before we continue, I highly recommend that you go onto the National Weather Service website and plug in your zip code. That'll bring up data being reported by your nearest weather station. Click through to your 7-day history, and the graph will show you what your climate is like. If your humidity curve is similar to mine, then your only reason for adding soil would be if you have cacti with very fine root systems that can't be supported well growing in a purely soil-less mix. (The only plants in my collection that really need a mineral-soil mix are the Tephros. Since your collection is larger than mine, you may have other species with the same basic requirement). However, if the humidity in your area tends to be lower, you'll need to use an across-the-board mix including soil for better moisture retention. Really hard to quantify, but in rough numbers here's an educated guess:

Humidity on average between 30% and 60% -- 4-to-1 mineral-soil mix
Humidity on average between 15% and 40% -- 3-to-1 mineral-soil mix
Humidity on average 15% at the high end of the curve (that's the desert!) -- 2-to-1 or 1-to-1 mineral-soil mix

Now would be a good time to correct you on a couple of things:

I use plastic pots because they're cheaper and easier to get than clay, and I found that in the greenhouse even the non-porous plastic pots dry out quickly. I suppose if I used DG (in the form of your typical automotive store oil absorber) it would be easier and less expensive than coir--although one order of that goes a long way. I was concerned that it might hold moisture a bit too long, but maybe not.

Nonporous pots are best, so good for you! First, products like NAPA 8822 are diatomaceous earth -- DE, not DG. I believe automotive oil absorbers are compressed DE, which may be okay, although I'd have 2 concerns about it -- not heavy enough to keep your pots from tipping over in the wind, and I don't know if its structure breaks down over time. Not a problem with pumice, and certainly not with granite gravel. If DE does break down, you'll have to repot with fresh mix whenever the DE turns to mush. Your best bet really is crushed granite poultry grit, so if you're not thrilled about initial cost, just remember that your pumice-granite gravel mix will last an incredibly long time.

Second, being a precision-oriented nutball, I'll take a digital scale over visual and hand-weighing impressions of moisture content as a mix dries out. Such being the case, you may find that any mixes you have in mind may not be going from wet to dry as quickly as you think (and we're talking about bone dry top-to-bottom). There's no reason to guess here, so fill a test pot with your minerals of choice (pumice and crushed granite or DE depending on which way you'll want to go). Weigh the mix dry, then wet, and make a note of how many days it'll take to go completely dry again. That'll give you a benchmark which determines whether or not you'll need to add soil for proper moisture retention. If you do, there's one more step to investigate...

I'm also in agreement with Ian and greenknight regarding "over-engineered" cactus mixes:

For desert cacti I have a problem with 'topsoil'. Anything I found in a commercial bag labeled 'topsoil' was too heavy and had too much debris in it. So I've invented my own. My topsoil mix consists of 6 parts commercial cactus soil, 3 parts gritty sand, 2 parts of that AZ soil we found caked too much to use by itself (remember?), 1 part poultry grit and a large handful of earthworm castings. Mixed together it seems like a nice texture sandy soil. So the full desert mix is also Daiv's: 60% pumice, 20% coir and 20% of my special topsoil mix described above. It's quite porous, so water soaks right through quickly. I figure whatever small amount of organic matter is in there is OK.

Keep it simple -- we already have 2 ingredients for your mineral base (the poultry grit counts as 1 if you go with that instead of DE), so do you really need the other 5? While I haven't worked with coir, that by itself could be sufficient for additional moisture retention. If not, then sift your topsoil with a piece of fiberglass window screen, dump out the big stuff and keep the soil and tiny bits, and mix it with your mineral gravel. And finally -- put it through the wet-to-dry test before you try the mix on your cacti. You already know that the "dampening" method works well to integrate the mix, so this is where a digital scale comes in handy. Be sure to use it so you'll know when your test mix is completely dry, and you'll be ready to run the wet-to-dry test.

This is as simple as we can make it. If the testing process seems complicated, at least you'll be able to bypass the trial and error (mostly error) "old school" growers had to go through.