Are you growing your cacti in a soilless mix?

[jerrytheplater]

The availability of the compounds in pumice or any substrate is key to them being nutrients or not. The plant roots will have to dissolve them to be counted as nutrients.

[ChaoticN]

I think Jerry is right, most of the nutrients in pumice are bound and not available to the plant. I’ve read that in the wild some cacti have microbes that help them decompose rock but in cultivation this typically does not happen.

[Steve Johnson]

I think Jerry is right, most of the nutrients in pumice are bound and not available to the plant. I’ve read that in the wild some cacti have microbes that help them decompose rock but in cultivation this typically does not happen.

Correct. Not sure if you know about it, but a Xerophilia article called The Stone Eaters has gained a lot of currency it doesn't deserve in the cactus community. Here is a review of the article from our horticulturalist-in-residence MikeInOz:

As for the stone eaters article, I don't think the author has given much consideration to the fact that cacti in the habitat are not confined to a pot but are free to form extensive root systems to find nutrients they require. In natural systems, as a nutrient is removed by the plant it will get replaced. It should be remembered that the almost limitless surface area surrounding the plants will gather many nutrients during rain and re-charge what has been taken by the plant roots. That's why plants in the habitat can continue to grow well (given enough water). Nutrients move around a lot in nature but they are sooner or later depleted in pots unless we add more of them. If you imagine a cactus growing out of crack in what seems to be solid rock, you need to take the whole hill or mountain into consideration and realize that when it rains, water rich in dissolved nutrients will be channeled into these cracks. So the plants end up seeing nutrients from far and wide and not just the rocks they are attached to.
Another way nutrients reach the plants .... After rain the soil water begins to evaporate from the surface. As it does it draws up more water via surface tension from further down and brings dissolved nutrients with it. The nutrients remain where the water turns into vapor and leaves the surface. None of this happens in a pot full of rocks.
Also, no mention is made of the possibility of nitrogen fixation by bacteria living around the roots. (However I did not read it intensively). The plants are getting their N from somewhere and it's not from solid rocks. Thunderstorms also play a part.

SDK1 knows a thing or two about horticultural science too, and this is his review:

That effing Stone Eaters article. Whatever it contributes to furthering our collective understanding of how cacti grow in situ, it's done just as much towards hindering our collective understanding of nutrient management for cacti in cultivation. Dweebs of the highest order, ones who just got their hands on their first Lophophora or Trichocereus, will come crawling out of the woodwork to quote the Stone Eaters as if it's Scripture on other cacti forums I've visited...That article just irritates me to no end.

I bring this up precisely because too many growers still take The Stone Eaters as Scripture, and I don't want our members to fall for it. Thanks to what Mike is telling us here, they'll know why they shouldn't.

[Nino_G]

It's good to read opposing opinions. I think "The Stone Eaters" gained so much following because it's so extreme in it's viewpoints, and that always intrigues people. It's quite obvious (to me, at least) that no cactus can survive and thrive in a long term if potted in the pot of rocks without adding required nutrients in the form of fertilizers. The Stone Eaters should have probably emphasize more the importance of the (micro) biological interactions existing in the wild soils which we still don't understand enough, as well as (as Mike pointed it out) exchange of the nutrients due to migration of water. Still, the article influenced me greatly since all of the books on cultivation of cacti that I've read until that point proposed using organic materials in relatively large proportions in "cactus soil mixtures" - from inert materials like peat and coco coir to various decomposed plant materials in the form of compost, and, often, different kinds of manures (cow, horse, sheep, chicken, even rabbit ). In my current experience - and in my climate - mineral substrates with regular fertilization (as I mentioned previously, I add fertilizers with every water, only greatly diluted) work very well and with almost no risk of root rot for sensitive species.
Since I've joined this forum I have learned a great deal more about required nutrients (thank you Steve and Mike), and I expect for my plants to grow even better in the future.
Best regards to all,
Nino

[MikeInOz]

Still, the article influenced me greatly since all of the books on cultivation of cacti that I've read until that point proposed using organic materials in relatively large proportions in "cactus soil mixtures" - from inert materials like peat and coco coir to various decomposed plant materials in the form of compost, and, often, different kinds of manures (cow, horse, sheep, chicken, even rabbit ). In my current experience - and in my climate - mineral substrates with regular fertilization (as I mentioned previously, I add fertilizers with every water, only greatly diluted) work very well and with almost no risk of root rot for sensitive species.

Nino

In theory, there should be absolutely no difference in the occurrence of root rot in 100% organic to 100% mineral. Organic material does NOT cause disease of any kind. Pathogenic organisms do. Pathogens do not ''come with'' organic material. They are everywhere including the computer screen you are looking at right now, all over your clothes and even in the dust in the air. All they need is food and the correct environment. So properly managed, there should be no difference. So what is the problem with organic? Well it breaks down and as it does, water holding capacity goes up and air-filled porosity goes down. That can benefit some (but not all) pathogens. It is the same as using very fine mineral components to the mix. Therefore, using organic materials will be more acceptable for... plants which can tolerate slower drying in the mix (eventually), small pots, watering frequency which is low. Organic materials will be less acceptable for plants which do not like slowly increasing water holding capacity, large pots and watering frequency which is high.
Some non stone-eating cacti....As you can see, the stone-eaters article is basically a load of nonsense.

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[MikeInOz]

double post....

[ChaoticN]

Very cool to see a ball type cactus climbing a tree! Thank you for posting that Mike. That’s Also very interesting regarding the organic information.

[Nino_G]

Mike, you should write "The Tree Eaters" article

[mmcavall]

I would like to share my experience. Much of it is similar to Nino's last post. I was strongly influenced by the "Stone Eaters" and by experienced growers of the forum.
I was fascinated by the idea of growing cacti without soil, and by the slow growth, compact body and strong spination that soilless mix can bring.
I started experiencing many kinds of mixes.
I tried several different mixes according to the genus. For most south American cacti I always added some soil (except for Chilean species). But for north american cacti, the mix was completely soilless.
For some time I was proud of the compact habit of my cacti.
.But after 4 or 5 years I noticed that my cacti were starving , even with fertilization (I use diluted fertilizer in almost all waterings during growing season).
Some of them were yellowing from the base. So I assumed they were lacking nitrogen. I believe that without sometingh (clay particles for example) to grab and release nutrients, it became too difficult for the cacti to find food...thats what was happening...they were starving.

I decided to change my mix and now I am not doing it soilless anymore.

My mix is composed of top soil, coarse sand, burned rice rusk and gravel. This is for all cacti but I can play with percentages according to the genus.

Plants are all recovering but remains compact. I dont use organic matter, coco coir, etc..I avoid high nitrogen content.
Well thats it. I tryed to do soilless and it worked only for some years...
Today I am more confident in my mix because I know it stores the nutrients that I give by fertirrigation. On the other hand, the mix is poor in organic matter and nitrogen, so the cacti dont grow excessively. For now I am satisfied. Lets see what the next couple of years will tell.

[keith]

I decided to change my mix and now I am not doing it soilless anymore."

I agree didn't work for me either cactus growers need to experiment around see what works for them

[Steve Johnson]

There are reasons why a soilless mix won't work for some growers -- I understand that. The specifics of knowing why the approach doesn't work for those growers would be helpful. A couple of things I'd be looking at:

• Species with very thin fibrous roots that can't be supported in a soilless mix because its aeration is too "open". I discovered that problem when I tried growing my Tephrocactus articulatus inermis and papyracanthus in a semi-hydroponic 60% pumice/40% granite gravel mix in early 2012. When I moved them to a 50% pumice/50% soil mix the following year, they started growing and they've been growing remarkably well ever since. My experience is limited to growing only 66 species, so Tephrocactus is the only example I can point to. I'm sure there are other species that really need to have soil in the mix, and I think this has to be a matter of trial and error. (On a side note, my 2 Eriosyce senilis love being in the pumice/soil mix too. They might do perfectly well in the semi-hydroponic mix, but as the old saying goes -- "if it ain't broke, don't fix it".)

• Fertilizer. I've been successfully growing 64 North and South American species in the semi-hydroponic mix. The only downside is needing to fertilize every time I water in the growing season. Actually not much of a downside since their roots do incredibly well given its wide-open aeration. However, I did run into a problem I wasn't aware of until MikeInOz educated me on the difference between good and not-so-good fertilizers for cacti:
  • Excessive P reduces root growth, stem growth, and flowering over time.
  A basic "rule of thumb" once I discovered the problem -- P substantially lower then N, K higher than both. And we can express this as a ratio -- N = 1, P = 0.25-0.35, K = 1.1-1.7. After 10 years of too much P and not enough K, the improvements with lower P and higher K in 2023 were pretty amazing. Another downside to soilless mixes is the fact that cacti have to get all of their nutrients from the fertilizer. In other words, it's not just about N, P, and K -- calcium is an important secondary major nutrient, with magnesium and sulfur being important minor nutrients. Once again, not much of a downside with the right fert. Here are the details on the fert regimen I started following last spring:
  
  https://www.cactiguide.com/forum/viewto ... 62#p404462
  
  It could be that cacti growing in soilless mixes are particularly vulnerable to improper NPK balance and/or nutrient deficiencies which are addressed by having soil in the mix. If that's the case for cacti struggling in a soilless mix, growers should take a good look at their ferts before they give up on going soilless.

But after 4 or 5 years I noticed that my cacti were starving , even with fertilization (I use diluted fertilizer in almost all waterings during growing season).
Some of them were yellowing from the base. So I assumed they were lacking nitrogen. I believe that without sometingh (clay particles for example) to grab and release nutrients, it became too difficult for the cacti to find food...thats what was happening...they were starving.

The yellowing you described indicates chlorosis, and that's a sulfur deficiency. If your cacti are getting a good amount of S through the topsoil and/or burned rice husk in your current mix, problem solved. Were your cacti starving in the soilless mix you tried? Not sure about that, although there are variables in your situation I'm not able to account for. All I can say is -- if your cacti are doing better in your current mix, go for it.

[mmcavall]

Thank you Steve for the explanation
I agree on the Tephrocactus. They like sandy soil.

Here is my formulation: so maybe it should have less P:
N: 112 ppm
P (P2O5): 255 ppm
K (K2O): 242 ppm
Ca: 57 ppm
Mg: 16 ppm
S: 19,60 ppm
Plus micros.

There was another problem in the mix I didn't mention before..I was using pipi-cat (diatomite) in the mix and took long time to notice it was bad for the roots. Roots became fragile with time and disappeared in some cacti. I think I was using too much. Stoped now.

That is what I meant by starving and yellowing:

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[Steve Johnson]

Thank you Steve for the explanation

My pleasure, and I'd like to give you some additional explanations...

Here is my formulation: so maybe it should have less P:
N: 112 ppm
P (P2O5): 255 ppm
K (K2O): 242 ppm
Ca: 57 ppm
Mg: 16 ppm
S: 19,60 ppm
Plus micros.

If your ppm numbers are for P2O5 and K2O, we'll do a bit of math:

• 255 ppm P2O5 x 0.436 = 111 ppm P

• 242 ppm K2O x 0.83 = 201 ppm K

Remember that the oxygen in P2O5 and K2O supports the plant's health, but it has no nutrient value -- all we're concerned about are the amounts of elemental P and K available to the plant. No "maybe" about it -- if your ppm numbers are correct, P and K are way too high in relation to N. Even if my calculated numbers are correct, they're still too high. Using 112 ppm as your benchmark for N, P should be in the 28-39 ppm range, and K should be in the 123-190 ppm range. The Ca-Mg ratio is good. The amounts of Ca, Mg, and S are good too.

Just to compare notes, here are the nutrient dosages I give to my cacti per feeding:

N: 48 ppm
P: 14 ppm
K: 74 ppm
Ca: 33 ppm
Mg: 10 ppm
S: 33 ppm

What I found is that my cacti are able to achieve the kind of genetic growth potential that isn't possible with their counterparts fending for themselves in the wild. If growers prefer more of a "hard" style, aim for 20-30 ppm N per feeding -- the point is to make sure that the fertilizer's NPK ratio follows what I discussed in yesterday's post. Another problem I should mention is something called "nitrogen bloat" caused when N is above 90 ppm per feeding -- make a habit of it and cacti are constantly pushed beyond their genetic growth limits. I've seen growers who insist on blowing their cacti up like beach balls with too much N, and a local cactus nursery I won't mention by name used to do it all the time.

.There was another problem in the mix I didn't mention before..I was using pipi-cat (diatomite) in the mix and took long time to notice it was bad for the roots. Roots became fragile with time and disappeared in some cacti. I think I was using too much. Stoped now.

Diatomite (AKA diatomaceous earth) -- if it looks like soil (as in dirt), it acts like soil. This defeats the purpose of going soilless, and given what DE did to the roots, I'm not surprised by the fact that your cacti appeared to be starving. I understand why some growers add it to their potting media -- DE is supposed to be effective against root mealies and fungus gnats. But there are other ways to deal with those pests, and in my humble opinion DE causes more problems than it solves. If you'd like to give a soilless mix another shot, try it again without the DE. Regardless of whether you do or you'd like to stay with your current mix, a fertilizer regimen along the lines of what I'm doing will correct the problems you'll have with excessive P.

[MikeInOz]

Just for fun, I have taken a picture of the pages regarding soil in Borg's ''Cacti'' written in 1937
It is intended for use with porous terra cotta so it should be coarsened for plastic pots. It is as good now as it was then but local modifications will be necessary. If you use loam, it is important to understand that ''Good garden loam'' refers to a loam with a granular structure which holds it's structure even after many waterings. The Japanese ''akadama is a perfect example but similar material can be found in fields and gardens a few inches or feet below the surface. It should be dried and screened before use. The good thing about loam is that it can (usually) supply all the trace elements needed for the life of the mix so you could get away with just adding NPK. Have fun reading - if only for the beautiful classical language.

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s

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Summery at the end of the book..

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[MikeInOz]

Thank you Steve for the explanation
I agree on the Tephrocactus. They like sandy soil.

Here is my formulation: so maybe it should have less P:
N: 112 ppm
P (P2O5): 255 ppm
K (K2O): 242 ppm
Ca: 57 ppm
Mg: 16 ppm
S: 19,60 ppm
Plus micros.

There was another problem in the mix I didn't mention before..I was using pipi-cat (diatomite) in the mix and took long time to notice it was bad for the roots. Roots became fragile with time and disappeared in some cacti. I think I was using too much. Stoped now.

That is what I meant by starving and yellowing:

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This mix (if it's the same right through the pot) looks far to bony for cacti and especially green Mammillarias etc. It has zero cation exchange capacity and also looks like it would dry out too quickly. It can however be used as a base to which you can add some organic material, loam and finer sand until you think it feels right in your hand. That should make all the difference to the health of the plants with the watering adjusted accordingly.