Fertilizers explained (2-part presentation) -- with 6/2023 update

[MikeInOz]

Dear friends, my soil mix is composed by:

20% zeolite
40% lapillus
20% dolomite
5% coal
10% coconut fiber soil
5% fine gravel

For older plant (>30 years) i usually replace organic component with fine gravel.

Before Mike gives us an evaluation of your mix and what it means for determining the right amount of Mo, I just wanted to point out a few things...

Zeolite has good CEC (for members who don't know the term, "CEC" is cation-exchange capacity), so you won't need to fertilize with each watering as I do using my pumice-granite gravel mix. (Pumice has very low CEC and granite has none.) Once I show you the recipe, the nutrient dosages in parts-per-million for your watering solution will give us an idea on how often you should fertilize. Any recommendation I make will be nothing more than a guess, so I would defer to Mike if he has a better one. We don't have enough information for that yet, but we will soon.

5% coal -- do you mean charcoal? From Better Homes and Gardens Australia (https://www.bhg.com.au/charcoal-in-potted-plants):

• "Another great way to keep your plants healthy is by adding a layer of horticultural charcoal to the pot or planter.
  
  "This absorbs excess water from the roots of your plant and keeps the soil “sweet” by guarding against bacteria, fungus and rot. It also helps to eliminate odours, and is particularly effective in terrariums."

I can see that in nonxeric plants, but I don't know if it does anything given the fact that your mix is 90% mineral. Which brings me to the other 10%...

Coconut fiber = coir, and I believe you're using ground coir as your soil component. Not sure why you're using it in the mix for cacti that are less than 30 years old. It causes more problems than it solves, as Mike explains here...

viewtopic.php?p=398697&hilit=coir#p398697

...and here:

viewtopic.php?p=398705#p398705

If you need the 10% component for better water retention in the mix, try this instead:

viewtopic.php?p=398714&hilit=sphagnum#p398714

My recommendation at the moment:

• 20% zeolite and 40% lapillus have roughly the same water retention capability as the pumice in my 60% pumice-40% granite gravel mix, so you're fine there unless people who use both say otherwise.

• You and I share a similar Mediterranean climate -- get rid of the charcoal and dolomite (they're actually working against each other), and go with 25% coarse gravel. Mix it with 5% fine gravel -- remarkably close to the 40% granite gravel in my mix.

Because pumice retains a surprising amount of water, straight pumice in a humid climate takes too long to dry out. Unlike the high porosity of pumice, granite has a much lower porosity, so I use granite gravel as a "moderator" to let the mix dry out in a timely manner. Your gravel doesn't necessarily have to be granite as long as it's relatively nonporous. Cactus roots need something they can grab onto -- smooth aquarium pebbles won't do the job, so your gravel needs to have some roughness to it. I'm not convinced that you need a 10% component for additional water retention, although I don't know the climate in Rome well enough to say if you do or not. If you don't, the only reason for repotting is when cacti outgrow their pots and require bigger pots -- a nice problem to have!

I had to look up ''lapillus'' Looks like scoria to me. It holds a lot of water. I use it but too much can give you a false sense of how moist the mix is. It can appear dry on the surface and be quite wet lower down = root rot. Zeolite also hold water. Coal ( I presume charcoal) holds a little water. Fine gravel holds water. Coconut hold water. Dolomite ( I presume dolomite gravel) does not. All this equals a highly moisture retentive mix. It might be worth adding a non absorptive component like coarse quartz, or granite or some other volcanic stone to balance all the water retention for some of the desert species? I have not checked if any of the components have native molybdenum in them. I doubt they have much so probably treat it as a ''hydroponic'' medium. In fact, I presume anything other than actual soil has enough Mo (and most other trace elements) in it over the long term.

[Steve Johnson]

I had to look up ''lapillus'' Looks like scoria to me. It holds a lot of water. I use it but too much can give you a false sense of how moist the mix is. It can appear dry on the surface and be quite wet lower down = root rot. Zeolite also hold water. Coal ( I presume charcoal) holds a little water. Fine gravel holds water. Coconut hold water. Dolomite ( I presume dolomite gravel) does not. All this equals a highly moisture retentive mix. It might be worth adding a non absorptive component like coarse quartz, or granite or some other volcanic stone to balance all the water retention for some of the desert species? I have not checked if any of the components have native molybdenum in them. I doubt they have much so probably treat it as a ''hydroponic'' medium. In fact, I presume anything other than actual soil has enough Mo (and most other trace elements) in it over the long term.

Glad I remembered this...

Mike, back in August 2021 you mentioned that I might want to add some zeolite to my pumice-granite gravel mix. I asked you about that via PM, and you recommended a mix of 10% zeolite, 50% pumice, and 40% granite gravel, although I never followed through on the zeolite. My cacti are doing quite well without it, but since it sounds like Davide could benefit from a change in his mix, he may do a lot better if he can go with a 10% zeolite, 50% pumice, and 40% dolomite, granite or rough quartz gravel mix. Before I pepper you with more questions, here's one for Davide...

Are you able to buy horticultural pumice without having to go online? Reason I ask is that pumice tends to be on the expensive side when you go to places like Amazon, eBay, etc. Many bonsai nurseries in the US sell pumice, and since you do live in a major European city, I'd be shocked if you don't have a local bonsai nursery selling pumice. Look for it in the 3 mm-5 mm grain size range. I grow nothing but desert cacti, and if you do too, the mix we're discussing should be perfect. Aim for something like this:

Pumice-DG_rinsed04242016_forum.jpg (201.76 KiB) Viewed 4548 times

Just add 10% zeolite, and I think you'll be set.

[Fohat85]

5% coal -- do you mean charcoal? From Better Homes and Gardens Australia

yes exactly, I mean charcoal.

Coconut fiber = coir, and I believe you're using ground coir as your soil component. Not sure why you're using it in the mix for cacti that are less than 30 years old. It causes more problems than it solves, as Mike explains here...

viewtopic.php?p=398697&hilit=coir#p398697

...and here:

viewtopic.php?p=398705#p398705

If you need the 10% component for better water retention in the mix, try this instead:

viewtopic.php?p=398714&hilit=sphagnum#p398714

Well, I did not know about all these drawbacks. In the next soil mix I will replace this component.

If you don't, the only reason for repotting is when cacti outgrow their pots and require bigger pots -- a nice problem to have!

Actually my current situation is as follows !! For this reason i'm collecting precious info/tips to improve the cultivation

Are you able to buy horticultural pumice without having to go online? Reason I ask is that pumice tends to be on the expensive side when you go to places like Amazon, eBay, etc. Many bonsai nurseries in the US sell pumice, and since you do live in a major European city, I'd be shocked if you don't have a local bonsai nursery selling pumice.

Fortunately I live in a volcanic area where there are many quarries from which I can find a large variety of inorganic components such as lapillus, pumice, tuff and zeolite. In some places it is also possible to find marl (i'm not sure of the translation), which some cactus growers love using pure for growing Mexican plants. It appears like in pic.

[Steve Johnson]

Fortunately I live in a volcanic area where there are many quarries from which I can find a large variety of inorganic components such as lapillus, pumice, tuff and zeolite. In some places it is also possible to find marl (i'm not sure of the translation), which some cactus growers love using pure for growing Mexican plants. It appears like in pic.

Excellent! Pumice has been tested and proven by expert growers (me included, although I'm hardly an expert) -- stay with that, and avoid lapillus and tuff. Marl is another porous mineral -- while I have no direct experience with it, my gut instinct tells me that you should avoid it too. Zeolite you have access to, so you're set there. My recommendation from last night still stands:

• 10% zeolite, 50% pumice, and 40% dolomite, granite or rough quartz gravel mix.

A mix means just that -- mix all of the ingredients together so that you have a fairly even distribution of each ingredient in the mix (like the example photo I just showed you in my last post). There's one small exception I'm making now, and that's to put a layer of nonporous gravel at the bottom of the pot right before you install the plant for repotting. This is a good way to prevent the possibility of root rot that could take place if porous mineral on the bottom hides moisture before you water again.

Let me know if you'd like to use the mix I'm recommending. If you do, I'll ask Mike a couple of followup questions.

[MrXeric]

Lapilli is just naturally small grained scoria. I use scoria crushed to the size of pumice I buy (around 6mm or 1/4in.), probably around 10% of the total mix. It's heavier than pumice so I use less of it. I remember looking it up, but did not find anything conclusive on whether scoria is more moisture retentive than pumice so I just assumed they would be about the same.

Volcanic tuff is a sedimentary rock, 75% or more volcanic ash (stuff too small to be called lapilli) that has solidified into rock. Its porosity probably depends on the origin, but I would assume it's much less porous and water retentive than pumice (one source cites 36-37%, while pumice is often cited as 50-90%). Haven't checked if this is available to me.

I remember looking up marl because it seems the Italians are fond of using it. Here's where I first saw mention of it:https://www.ilfioretralespine.it/en/202 ... arl-cacti/. I don't know the contents of the article because the author sells them as a subscription , I did enjoy the pictures of their plants though. Anyway, marl is somewhere between limestone and clay in terms of both calcite and clay content, so it is an alkaline material. A quick search says porosity of marl is at an average of 1.6%. Probably good for the small Mexican species, but you'll really want to use rain or otherwise acidified water.

[Fohat85]

Let me know if you'd like to use the mix I'm recommending. If you do, I'll ask Mike a couple of followup questions.

Sure, i want try your "soil recipe". So there will be no organic component?

Lapilli is just naturally small grained scoria. I use scoria crushed to the size of pumice I buy (around 6mm or 1/4in.), probably around 10% of the total mix. It's heavier than pumice so I use less of it. I remember looking it up, but did not find anything conclusive on whether scoria is more moisture retentive than pumice so I just assumed they would be about the same.

Lapillus is usually used up to 70% in italian (maybe european?) cactus soil mix. However I would like to replace it following a new soil composition, as you do. Lapillus is probably used a lot in these areas due to its high availability and low cost. Instead, I noticed that in Asian countries they use another different type of soil based on elements of clay origin such as akadama and others.

[ohugal]

I went through the lapillus page on wikipedia and didn’t see anything on the water/air retention capabilities. Does anyone know? Perhaps there is a way to test it?

[Fohat85]

I went through the lapillus page on wikipedia and didn’t see anything on the water/air retention capabilities. Does anyone know? Perhaps there is a way to test it?

Here is described (second page).

[jerrytheplater]

I remember looking up marl because it seems the Italians are fond of using it. Here's where I first saw mention of it:https://www.ilfioretralespine.it/en/202 ... arl-cacti/. I don't know the contents of the article because the author sells them as a subscription , I did enjoy the pictures of their plants though. Anyway, marl is somewhere between limestone and clay in terms of both calcite and clay content, so it is an alkaline material. A quick search says porosity of marl is at an average of 1.6%. Probably good for the small Mexican species, but you'll really want to use rain or otherwise acidified water.

Here in New Jersey we have deposits of Greensand, which is also called Marl. It is a marine origin sedimentary deposit found in a band along the Delaware River in central New Jersey crossing over to the Sandy Hook area on the Atlantic Ocean border. The main mineral is Glauconite. It is loaded with trace minerals. I use it in my garden and in a clay mixture I make for use in aquariums where aquatic plants are being grown. It was used extensively in the southern NJ farm fields before the advent of chemical fertilizers in the early 1900's.

The particle size found in the bagged Greensand is really too fine to be used in a cactus substrate. It is not a powder, but a very fine sand particle.

Here is a review article on Greensand with a history of its use. It was published in 2002. https://njaes.rutgers.edu/pubs/publication.php?pid=E279

[keith]

Bagged Greensand they sell that at green thumb in CA .

[Steve Johnson]

Sure, i want try your "soil recipe". So there will be no organic component?

Nope -- a mix completely free of organic material will dramatically reduce the possibility of rot. To be more precise about it, the mix I'm using (and the one you're about to use) is a soil-less hydroponic mix. If I remember correctly, the relative humidity in Rome is 75% on average, and those are good conditions for a hydroponic mix.

I still need to ask Mike a couple of questions, but I'm running out of time at the moment. If you wouldn't mind waiting a little longer, I should have everything put together for you by the weekend.

[MikeInOz]

-- a mix completely free of organic material will dramatically reduce the possibility of rot.

This statement needs qualification Steve. I have seen cacti (rebutia and notocactus for example) growing in pure mineral soil lose their roots to rot and growing perfectly healthy in a 100% organic mix regardless of the amount of water given The reasons for root rot are not due to the use if organic material per se. This is another myth spread from one grower to the next without evidence. The cause of root rot are many but they don't include organic material.
Usually, rot is caused by lack of oxygen to the roots = asphyxiation = dead roots = attack by a variety of organisms, some of which are pathogens which can now gain entry into a weakened plant. That can occur in any mix. In fact well prepared organic mixes can have the capacity to protect plant roots from attack by pathogens through competition from beneficial bacteria.
Another cause of root rot can be lack of certain nutrients, particularly calcium and it's partner boron. That can lead to the death of root tips and subsequent attack by pathogens if they are present - and they are almost always present. The main reason organics have a bad reputation is that they are used in too fine a form (which holds too much water and hence have a lack of available O2) or they are immature and unbalanced as far as the bacterial population is concerned and/or they have not been treated for pathogens or insects which could do damage.
Using raw coconut dust or partly decomposed and unheated (as in composting or solarizing) material is a good example of the wrong thing to do. The main drawback of using organic material in cacti mixes is the decomposition and reduction of AFP in mixes used for cacti which do not need to be repotted for extended periods. But that seems to be only a minor issue because the vast majority of cacti are not watered as much as other plants and most are repotted before any problems with AFP arise. There are other benefits to using some organic material such as CEC and buffer capacity.
The big problem is getting high quality material. Almost none of the commercial stuff qualifies as good enough in my experience.
For those interested I have a picture of mature compost I make from garden shredings and use in my mixes here... viewtopic.php?t=45261&hilit=compost&start=15

[jerrytheplater]

Using raw coconut dust or partly decomposed and unheated (as in composting or solarizing) material is a good example of the wrong thing to do. The main drawback of using organic material in cacti mixes is the decomposition and reduction of AFP in mixes used for cacti which do not need to be repotted for extended periods. But that seems to be only a minor issue because the vast majority of cacti are not watered as much as other plants and most are repotted before any problems with AFP arise.

AFP? = Available Free Phosphate?

[Steve Johnson]

This statement needs qualification Steve. I have seen cacti (rebutia and notocactus for example) growing in pure mineral soil lose their roots to rot and growing perfectly healthy in a 100% organic mix regardless of the amount of water given.

I'll have to qualify what I just said. What I meant to say is that a soil-less mix completely free of organic material will dramatically reduce the possibility of rot. That means a hydroponic mix like mine:

Pumice-DG_rinsed04242016_forum.jpg (201.76 KiB) Viewed 4371 times

Not even a hint of soil there. The Rebutias and Sulcorebutias I have in my collection are S. rauschii (arrived in 2011), S. Callichroma longispina (arrived in 2012), R. heliosa (arrived in 2013), R. pygmaea (arrived in 2014), and S, heinzii (the latest arrival in 2018) -- all 5 of them are still going strong. Aeration in a soil-less mix is about as wide open as we can get, a hydroponic mineral mix being the most open of all, and the roots of my desert cacti thrive in it. The only exceptions involve 4 cacti which I discuss here:

viewtopic.php?p=399197#p399197

Mike, this bring up the first of two questions I'm posing tonight...

I realize that I have to fertilize every time I water because the pumice has low CEC (and the granite has none). I think the idea behind adding 10% zeolite to the mix as you recommended in 2021 is that I wouldn't have to fertilize as often given the zeolite's CEC "boost". When I switch from Dyna-Gro 7-7-7 to the General Hydroponics ferts and Ammonium sulfate-Potassium sulfate supplements in spring, the watering solution will contain 47 ppm N, 14 ppm P (P/N ratio = 0.3, much better than 0.44 from the 7-7-7), and 73 ppm K per feeding. If I add 10% zeolite to the mix, would you recommend that I alternate between water plus fert and water only at that dosage? The Ferty 3 I have in mind for Davide will give him a watering solution which contains 60 ppm N, 17.4 ppm P (P/N ratio = 0.29), and 91 ppm K. If he adds 10% zeolite to his mix too, how often should he be fertilizing whenever he waters?

Question #2 -- that Ferty 3 dosage will break the 60 ppm N down to 42 ppm NH4 and 18 ppm NO3 per feeding. Would you still recommend 0.03 ppm Mo in a 10% zeolite/50% pumice/40% nonporous gravel mix? With the answers to both of those questions, I'll be ready to give Davide the recipes and instructions for the Ferty 3, Potassium sulfate stock solution, and how much of each should go into his watering solution.

[MikeInOz]

Using raw coconut dust or partly decomposed and unheated (as in composting or solarizing) material is a good example of the wrong thing to do. The main drawback of using organic material in cacti mixes is the decomposition and reduction of AFP in mixes used for cacti which do not need to be repotted for extended periods. But that seems to be only a minor issue because the vast majority of cacti are not watered as much as other plants and most are repotted before any problems with AFP arise.

AFP? = Available Free Phosphate?

Air filled porosity. In other words, the space available that can hold air will reduce over time as organic particles become smaller and WHC (water holding capacity) will increase.