watering & fertilizing for mineral soil mix

[ohugal]

Hi Everyone,

For those who grow (some of) their cacti in an all mineral soil mix, what is your watering and/or fertilizing regime?
Do you add fertilizer to each watering and in what strength? What type of fertilizer do you use?
I did not immediately find a (centralized) topic dedicated to this subject. If such a topic or post exists, my apologies!

[abhikjha]

Hi Everyone,

For those who grow (some of) their cacti in an all mineral soil mix, what is your watering and/or fertilizing regime?
Do you add fertilizer to each watering and in what strength? What type of fertilizer do you use?
I did not immediately find a (centralized) topic dedicated to this subject. If such a topic or post exists, my apologies!

Oh this is Steve's expert domain. I am sure he will give a detailed response with links for detailed thread.

My two cents:

1. I am growing all my plants in pure mineral mix ( almost equal mix of pumice 2-4 mm and lava rock).
2. Watering frequency depends on the climate. However as a rule of thumb, if sunlight is available for plants, once in every 2 weeks. If not, then no water as they don't need it. In rainy season, we don't get much sunlight so I avoid watering (maybe once in a month so that they dont wither away).
3. Every watering, I am using (started very recently) NPK of 7 6 11 with micronutrients. One important thing to look out for is Ca to Mg ratio of atleast 4. Mine is 3 but that's ok.
4. Ph should be around 5.5 to 6.

Cheers
Abhik

[ohugal]

Hi Everyone,

For those who grow (some of) their cacti in an all mineral soil mix, what is your watering and/or fertilizing regime?
Do you add fertilizer to each watering and in what strength? What type of fertilizer do you use?
I did not immediately find a (centralized) topic dedicated to this subject. If such a topic or post exists, my apologies!

Oh this is Steve's expert domain. I am sure he will give a detailed response with links for detailed thread.

My two cents:

1. I am growing all my plants in pure mineral mix ( almost equal mix of pumice 2-4 mm and lava rock).
2. Watering frequency depends on the climate. However as a rule of thumb, if sunlight is available for plants, once in every 2 weeks. If not, then no water as they don't need it. In rainy season, we don't get much sunlight so I avoid watering (maybe once in a month so that they dont wither away).
3. Every watering, I am using (started very recently) NPK of 7 6 11 with micronutrients. One important thing to look out for is Ca to Mg ratio of atleast 4. Mine is 3 but that's ok.
4. Ph should be around 5.5 to 6.

Cheers
Abhik

Thanks for breaking the ice here. Do you fertilize at the recommended strength or do you dilute? If so, at what ratio? I'm not really great at calculating all the numbers as some of the other forum members, but am trying to get a feel of what other people are up to.

[Steve Johnson]

I'm not really great at calculating all the numbers as some of the other forum members, but am trying to get a feel of what other people are up to.

Knowing the numbers is important, but unfortunately a lot of people have been conditioned from birth to believe that even basic math is too hard. Of course if teachers can't teach it well, we do have a problem. I'm not a teacher by profession, so let's find out if my non-professional teaching skills are any good at all...

The basic rule of thumb -- P lower than N, K higher than both. Using N as a constant of 1, the "ideal" ratio for cacti is 1:0.4:1.5. The NPK numbers reported on fertilizer labels need some explanation because many growers (especially hobbyists) assume that the P and K numbers represent the actual amount of Phosphorus and Potassium going to their plants. This is true in Australia and the UK, but if you live anywhere else, you'll need to look closely at the label. If it shows the true element values of P and K, excellent. If not, here's how to calculate them:

• %P reported as Phosphorus pentoxide (P2O5) x 0.436 = % of elemental P.

• %K reported as Potash (K2O) x 0.83 = % of elemental K.

My fert of choice is Dyna Gro All Pro 7-7-7. The first number (7%) is the true amount of Nitrogen available to my cacti. 7% P2O5 x 0.436 = 3.05, and 7% K2O x 0.83 = 5.81, so the true elemental NPK is 7-3.05-5.81. That gives us the amounts, but we need to break them down into a ratio to determine if ferts have a good or not-so-good NPK balance. Solving for the P and K sides of the ratio is easy -- in this case it's:

• 3.05/7 = 0.44. 5.81/7 = 0.83.

Once again using N as a constant of 1, that gives us an NPK ratio of 1:0.44:0.83. The N-to-P balance is pretty much ideal, although K is out of balance because it's too low. I dilute 1/2 teaspoon of the 7-7-7 per gallon of water, and to correct for the K imbalance, I supplement the watering solution with 5 fluid ounces of 0-0-52 Potassium sulfate stock solution (7 grams per gallon of distilled water for that).

When fert labels have the same number for N and P, but they're reporting P as P2O5, you'll have the right balance between N and P. For example, if you find a 10-10-10 or a 20-20-20, the N-to-P ratio is the same, and the amount indicates the proper dilution so that we're not over-fertilizing -- obviously you'd want to dilute a lot more with 20-20-20 than you would with something like my 7-7-7. K can be brought into balance with N and P by using Potassium sulfate as a supplement, but determining the right amount does get kinda complicated. Wouldn't it be nice if we found a fert with the proper balance of N, P, and K? Multiplying the %N by 1.857, I'll throw out a bunch of numbers: 3-3-5, 4-4-7, 5-5-9, 6-6-11, 7-7-13, 8-8-15, 9-9-16, and 10-10-18. If you can find a fert with any of these NPK values, the proper N-to-elemental K ratio won't be exact, but close enough. (I would consider Abhik's 7-6-11 to be acceptable.) Then it would simply be a matter of figuring out the proper dilution of the fert you select. Note to Abhik -- the correct dilution for you should be .65 mL of your 7-6-11 fert per L of water.

It's not just about NPK, and here's what to aim for:



Calcium, Magnesium, and Sulfur are important minor nutrients for cacti. Calcium-to-Magnesium ratio of 4:1 is perfect, although Abhik's 3:1 rato is also acceptable. The trace amounts of S in the 7-7-7 don't cut it, and my cacti had been Sulfur-deficient without even knowing it until I learned a lot more about ferts from MikeInOz last year. That deficiency was corrected by Potassium sulfate supplementation. We can't underestimate the importance of micronutrients either, so I would recommend avoiding any and all ferts that don't have them unless you include a micronutrient supplement.

If you have a well-balanced fert with all of the minor and micronutrients, use it regardless of whether you're growing cacti in a soil-based or soil-less mineral mix. How often you fertilize depends on the cation-exchange capacity (CEC) of the mix. For soil-based mixes, the soil has good CEC -- when the mix dries out, any nutrients not taken up by the roots are retained in the soil for release to the roots in the next watering. That's the upside. The downside is if you fertilize too often and fertilizer salts build up in the soil. To prevent this from being a problem, flush your pots with pure rainwater or distilled water at least once in each growing season. Of the 68 cacti I have in my collection, I'm growing 65 in a soil-less pumice and granite gravel mix -- both ingredients have zero CEC, so I have to fertilize every time I water. Mike recommended that I add 10% zeolite to give the mix some CEC, and since I need to repot everything anyway, I'll implement his recommendation when I do the repotting early next spring. For the 3 cacti growing in a 50/50 pumice and soil mix -- yeah, I should've been flushing their pots. I'll have to start making a practice of doing it.

[abhikjha]

I'm not really great at calculating all the numbers as some of the other forum members, but am trying to get a feel of what other people are up to.

Knowing the numbers is important, but unfortunately a lot of people have been conditioned from birth to believe that even basic math is too hard. Of course if teachers can't teach it well, we do have a problem. I'm not a teacher by profession, so let's find out if my non-professional teaching skills are any good at all...

The basic rule of thumb -- P lower than N, K higher than both. Using N as a constant of 1, the "ideal" ratio for cacti is 1:0.4:1.5. The NPK numbers reported on fertilizer labels need some explanation because many growers (especially hobbyists) assume that the P and K numbers represent the actual amount of Phosphorus and Potassium going to their plants. This is true in Australia and the UK, but if you live anywhere else, you'll need to look closely at the label. If it shows the true element values of P and K, excellent. If not, here's how to calculate them:

• %P reported as Phosphorus pentoxide (P2O5) x 0.436 = % of elemental P.

• %K reported as Potash (K2O) x 0.83 = % of elemental K.

My fert of choice is Dyna Gro All Pro 7-7-7. The first number (7%) is the true amount of Nitrogen available to my cacti. 7% P2O5 x 0.436 = 3.05, and 7% K2O x 0.83 = 5.81, so the true elemental NPK is 7-3.05-5.81. That gives us the amounts, but we need to break them down into a ratio to determine if ferts have a good or not-so-good NPK balance. Solving for the P and K sides of the ratio is easy -- in this case it's:

• 3.05/7 = 0.44. 5.81/7 = 0.83.

Once again using N as a constant of 1, that gives us an NPK ratio of 1:0.44:0.83. The N-to-P balance is pretty much ideal, although K is out of balance because it's too low. I dilute 1/2 teaspoon of the 7-7-7 per gallon of water, and to correct for the K imbalance, I supplement the watering solution with 5 fluid ounces of 0-0-52 Potassium sulfate stock solution (7 grams per gallon of distilled water for that).

When fert labels have the same number for N and P, but they're reporting P as P2O5, you'll have the right balance between N and P. For example, if you find a 10-10-10 or a 20-20-20, the N-to-P ratio is the same, and the amount indicates the proper dilution so that we're not over-fertilizing -- obviously you'd want to dilute a lot more with 20-20-20 than you would with something like my 7-7-7. K can be brought into balance with N and P by using Potassium sulfate as a supplement, but determining the right amount does get kinda complicated. Wouldn't it be nice if we found a fert with the proper balance of N, P, and K? Multiplying the %N by 1.857, I'll throw out a bunch of numbers: 3-3-5, 4-4-7, 5-5-9, 6-6-11, 7-7-13, 8-8-15, 9-9-16, and 10-10-18. If you can find a fert with any of these NPK values, the proper N-to-elemental K ratio won't be exact, but close enough. (I would consider Abhik's 7-6-11 to be acceptable.) Then it would simply be a matter of figuring out the proper dilution of the fert you select. Note to Abhik -- the correct dilution for you should be .65 mL of your 7-6-11 fert per L of water.

It's not just about NPK, and here's what to aim for:



Calcium, Magnesium, and Sulfur are important minor nutrients for cacti. Calcium-to-Magnesium ratio of 4:1 is perfect, although Abhik's 3:1 rato is also acceptable. The trace amounts of S in the 7-7-7 don't cut it, and my cacti had been Sulfur-deficient without even knowing it until I learned a lot more about ferts from MikeInOz last year. That deficiency was corrected by Potassium sulfate supplementation. We can't underestimate the importance of micronutrients either, so I would recommend avoiding any and all ferts that don't have them unless you include a micronutrient supplement.

If you have a well-balanced fert with all of the minor and micronutrients, use it regardless of whether you're growing cacti in a soil-based or soil-less mineral mix. How often you fertilize depends on the cation-exchange capacity (CEC) of the mix. For soil-based mixes, the soil has good CEC -- when the mix dries out, any nutrients not taken up by the roots are retained in the soil for release to the roots in the next watering. That's the upside. The downside is if you fertilize too often and fertilizer salts build up in the soil. To prevent this from being a problem, flush your pots with pure rainwater or distilled water at least once in each growing season. Of the 68 cacti I have in my collection, I'm growing 65 in a soil-less pumice and granite gravel mix -- both ingredients have zero CEC, so I have to fertilize every time I water. Mike recommended that I add 10% zeolite to give the mix some CEC, and since I need to repot everything anyway, I'll implement his recommendation when I do the repotting early next spring. For the 3 cacti growing in a 50/50 pumice and soil mix -- yeah, I should've been flushing their pots. I'll have to start making a practice of doing it.

Awesome Steve! I think you would have been a terrific teacher if that would have been your profession.

Thanks for your notes for me - I am using 20 L bucket to mix my ferts. I am currently using 15ml in 20 L which translates to 0.75 ml per L. I will increase this to 21L so that I get around 0.65 ml per L.

Interesting thing to note on Zeolite for better CEC. We don't get such exotic stuffs here . I will continue with my pumice and lava rock mix for the time being.

[ohugal]

Thanks! I’m going to print this out and compare it to the fertilizers which are readily available to me. I currently have a fertilizer which does not meet your formula (7-17-35-3 + micronutrients). I don’t know the exact composition, but will look into it.

[Steve Johnson]

Awesome Steve! I think you would have been a terrific teacher if that would have been your profession.??

The best teachers are the ones who make things understandable to their students. Sounds like I did a pretty good job, so thanks for your kind comment!

By the way -- Mike taught me well, and I give him endless amounts of credit for the learning I pass along to our members. The best explainer of ferts and the role of different nutrients in cactus functioning I've ever seen, so it's a blessing to have him on the forum.

Thanks for your notes for me - I am using 20 L bucket to mix my ferts. I am currently using 15ml in 20 L which translates to 0.75 ml per L. I will increase this to 21L so that I get around 0.65 ml per L.

Good news is that cacti aren't "precision instruments", so we do have a range of amounts indicating whether or not they're being over-fertilized. In your case, the range is 0.65-1.3 mL per L of water. Since you're on the lower end of that range, you're definitely not over-fertilizing if you stay with 0.75 mL per L. With that said, flushing your pots once a year wouldn't be a bad idea. Here's the procedure I got from one of my PMs with Mike:

I always recommended a good flushing of the pot with 2 times the volume of the pot of plain water - preferably at pH 5.5 after say 5 waterings with the fertilizer. Meaning, that if your pot has a capacity of 1 litre, pour through 2 litres of plain water about 1 hour apart (or even more if you want). This will help in bringing any build up of excess salts back down to a baseline level. You can fertilize again immediately after if you want.

The only difference -- he's growing his cacti in a mix containing soil, but I believe once a year in the growing season should be sufficient for soil-less mixes. Of course your growing season never ends, so you may want to do it twice a year. Flushing with pH 5.5 water? That was a revelation, and I'll have to make this an annual event starting next year. No need for rainwater (hard for me to store rainwater anyway, SoCal's "rainy" seasons are too unreliable) or distilled water
either -- I can acidify my tap water!

I currently have a fertilizer which does not meet your formula (7-17-35-3 + micronutrients). I don’t know the exact composition, but will look into it.

I'm glad we caught this, and a few things to consider here...

"The whole is greater than the sum of its parts" -- applying this basic principle, I came to understand ferts as nutrient systems. When all of the nutrients are in balance, they work together. Call it a synergistic effect (although I don't know if that's a scientifically accurate use of the term). When they're out of balance and/or certain nutrients are missing, cacti can still be grown, but problems will eventually get the better of them. And the more out of balance the nutrient systems are, the worse these problems become. Sorry, but your 7-17-35 is horribly out of balance, so you'll need to find a much better fert for your cacti. As I said before, it's not just about NPK -- if your fourth number (3) is Calcium, this is the first time I'm seeing it reported on the label. That's good, although I'd like to know what's in the "+ micronutrients". Product analysis coming from the manufacturer tells us a lot about whether or not we're using the right fert, so be sure to look at all of the details as you search for one more suitable to the needs of your cacti.

[ohugal]

The fourth number (3) is Magnesium.
The micronutrients are (in Dutch) Boor (B), Molybdeen (Mo), Koper (Cu), Mangaan (Mn), Zink (Zn). No indication on the percentages.
No further details and no manufacturer listed. I wasn't very critical when buying it I guess...
I've been using it for about 2 years and did not experience any side effects (yet), although I do repot my cacti when needed (meaning I don't postpone). I don't have a cactus which I have been giving a liquid fertilizer for over +2 years.
I will be definitely using your calculations on various fertilizers to see if they fit the bill.
I would really like to thank you for taking the time to answer my question at such length! Maybe I'll post some of my findings here on this topic and perhaps contact some of the members on the forum from Holland or Belgium.

[Steve Johnson]

The fourth number (3) is Magnesium.

That's odd, and no mention of Calcium? (Awfully high amount of Mg anyway.) Now you know two of the things to look for -- Ca 2% and Mg 0.5%, good balance, although both could be higher. Ca 3% and Mg 0.75% is better, Ca 4% and 1% Mg is best. Don't worry about it if the ferts you're looking at give you numbers somewhere in between -- as long as the Ca and Mg are reasonably well-balanced, you should be fine.

I've been using it for about 2 years and did not experience any side effects (yet), although I do repot my cacti when needed (meaning I don't postpone). I don't have a cactus which I have been giving a liquid fertilizer for over +2 years.

I spoke with a very experienced grower about 8 years ago, and he told me that the 5-year mark after starting or changing a fertilizer regimen is the time when cacti are either continuing to thrive or showing signs of "stalling out". You haven't gotten there yet, so it's good that you're investigating the fert situation. And I'm glad that I could help you here.

Maybe I'll post some of my findings here on this topic...

Please do!

...and perhaps contact some of the members on the forum from Holland or Belgium.

Knowledge-sharing is a major asset we have on the Internet, and with that in mind, I'll point you toward Sulco-Passion:

https://www.sulcopassion.be/

In case you don't already know about the website, Belgian collector Claude Bourleau specializes in Sulcorebutia, Weingartia, and Cintia. I've corresponded with him by email a few times, very friendly, and I think you would enjoy comparing notes with him on fertilizers. You might learn a thing or two from him or he might learn a thing or two from us -- that's what knowledge-sharing is all about!

[ohugal]

Many thanks again!
I think I will investigate some of the fertilizers i’ve come across using your formula and will post my findings at a certain point. This won’t be for right now as i need time to compile all the data. A list with suitable fertilizers (arranged by availability per country) would be nice. I’ll maybe already contact jangaudi en aiko from the forum here.

[ohugal]

Hi Everyone,

I've made a list of a few potential fertilizers. The criteria were:
1. 'Steve's ratio' of 1:0.4:1.5 (please see Steve's earlier post to how to calculate this)
2. Availability. I've focused on fertilizers which are available in Belgium, Holland or Germany. I must say almost 7/10 fertilizers do not meet 'Steve's ratio'. This includes 'professional' fertilizers used by nurseries such as a Haage or Uhlig Kakteen.

I'm also still enquiring about the micronutrients of most of the fertilizers, since they weren't mentioned in the descriptions.

Combiflor cactus fertilizer (liquid)
main nutrients: 3% N - 4% P2O5 - 4% K2O
main nutrients (elemental): 3% N – 1,7% P2O5 – 3,3% K2O
ratio: 1 – 0,6 – 1,1
trace elements: unknown
Calcium-to-Magnesium ratio: unknown


cactus fertilizer (Donau-Wald Kakteen) (liquid)
recommended dilution: 0,1% - max. 0,2%
main nutrients: 5% N - 8% P2O2 - 10% K2O
main nutrients (elemental): 5% N – 3,5% P2O2 – 8,3% K2O
ratio: 1 – 0,7 – 1,7
trace elements: 0,01% B - 0,002% Cu - 0,02% Fe - 0,01% Mn - 0,001% Mo - 0,002% Zn
Calcium-to-Magnesium ratio: unknown


COMPO Liquid Fertilizer for Tomatoes and Vegetables
main nutrients: 5% N - 5% P2O5 - 7% K2O
main nutrients (elemental): 5% N – 2,2% P2O5 – 5,8% K2O
ratio: 1 – 0,4 – 1,1
trace elements: unknown
Calcium-to-Magnesium ratio: unknown


Chrysal cactus- & succulent fertilizer (liquid)
main nutrients: 4% N - 7% P2O5 - 7% K2O
main nutrients (elemental): 4% N – 3% P2O5 – 5,8% K2O
ratio: 1 – 0,8 – 1,4
trace elements: unknown
Calcium-to-Magnesium ratio: unknown

[Steve Johnson]

This may be a long shot, but send an email to Dyna-Gro (info@dyna-gro.com) and find out if the company has a distributor in Europe. Let the person you contact know that you're looking specifically for the All-Pro 7-7-7 and/or Orchid-Pro 7-8-6. IMO Dyna-Gro is the "gold standard" -- either one of these ferts are quite acceptable, and if you get lucky, the only additional thing you would need to do is supplement your watering solution with the right amount of Potassium sulfate to bring the N and K into proper balance. In the meantime, I'm working on another option that may be possible for you.

I understand how frustrating it must be to spend so much time and find out that none of the ferts you're seeing fit the comprehensive nutrient profile we'll get with the Dyna-Gro ferts. Good news is that your cacti are about to go dormant soon (if they haven't already), so you have plenty of time between now and next spring to search for the fert that'll be as close to ideal as possible.

[4d3d3d3]

Another option are hydroponics fertilizers- one of the multi-part types. General Hydroponics for example has the Flora line. I use Flora Micro (NPK 5-0-1, reported as P2O5 and K2O) and Flora Bloom (NPK 0-5-4), equal volumes of each. If I use 2 ml of each per gallon of water, I end up with the following elemental ppm:

33 ppm N
13.4 ppm P
25.9 ppm K
33 ppm Ca
9.21 ppm Mg

This gives an elemental NPK ratio of 1:0.41:0.78, which is perfect for N and P, but a bit low on K. I still have to supplement with sulfate of potash, but that's almost always gonna be the case. The Ca:Mg ratio is 3.6 which is good. The benefit of using a multi part hydronics fertilizer is that they're a lot easier to find at least in my experience and they have a much longer shelf life than all-in-one liquid fertilizers. Also they're usually much cheaper. 8 oz of Dyna-Gro cost about $9, while 32 oz each of Flora Micro and Flora Bloom cost about $21 total. I don't know if you have the General Hydroponics brand up there but I'm sure you have something similar, their Flora line is a pretty standard formula.

[Steve Johnson]

Just a guess, but I think that hydroponics have been a real game-changer for many growers, the result being that fertilizer "technology" is light years ahead of where I was when I fell in love with cacti way back in 1970. I do agree that hydroponics fertilizers are a good option, and IMO superior to what people reach for when they go to their local nursery or big-box garden center. However...

The benefit of using a multi part hydronics fertilizer is that they're a lot easier to find at least in my experience and they have a much longer shelf life than all-in-one liquid fertilizers.

I'll have to disagree, and here's why:

http://www.cactiguide.com/forum/viewtop ... 25&t=46501

With that said, MikeInOz did point out what to look for if liquid ferts degrade and can no longer be used. As far as cost is concerned, what we're willing to buy depends on one's budget. Since it comes only by the gallon, the Dyna-Gro 7-7-7 is relatively expensive, but I can afford it. For people on a tight budget, the 7-8-6 is the next best thing. If your General Hydroponics ferts include the same micronutrients detailed in Dyna-Gro's product analysis, you are getting a better deal. I'll have to check them out in case they could be a better deal for me too. The only thing I'm concerned about is the fact that you have the same amount of N and Ca (even though your Ca-to-Mg ratio is good), seems like the Ca would be too high in relation to N. Mike is the go-to authority on these matters, so let's find out if he has anything to say about it.

[4d3d3d3]

Right, they don't exactly expire, but sometimes insoluble salts can crash out. With the multi-part ferts, Ca ions are kept in a separate bottle from phosphate and sulfate ions because they form insoluble salts together. Same for zinc ions, which form an insoluble salts with phosphate ions. So what the multi part ferts do is they put all the Ca, Zn, Fe, etc in a separate solution (the "micro" solution), away from anions that they might precipitate with. Once diluted into the irrigation water it's not really an issue any more, but in concentrated form it is and the easiest way to address it is the multi-part solutions.

I don't really know how, but somehow, all-in-one liquid ferts are able to combine everything into a concentrated solution without anything immediately precipitating out. They must be using some kind of chemistry voodoo, which probably explains the higher price (General Hydroponics also has an all-in-one solution very similar to Dyna Gro, it's called FloraNova and it's about the same price as Dyna Gro). They work well, but I did buy a bottle of Dyna Gro once that was already crashed out (maybe it froze in storage?). Multi part solutions have a virtually infinite shelf life. All-in-ones have a long shelf life too but if anything crashes out it's really hard to get it all back into solution.

The reason the Ca is so high in relation to the N is because it was optimized for hydroponics. Most peat-based potting mixes include limestone to balance the pH, whereas in hydroponics there's no limestone which supplies Ca and Mg. So it has to be higher than a typical fertilizer. I guess it depends on what you grow your cacti in. I grow most of my cacti in quartz grit, perlite- which I imagine have similar nutrient considerations as for hydroponics. But otherwise I don't know, that's a good point.

As a hydroponics fert it includes all the micronutrients. A quick glance at both labels shows some differences. Dyna-gro Bloom has 0.09% sulfur, GH Bloom 1%- which is a pretty big difference. Dyna-gro bloom 0.05% zinc, while GH micro only 0.015% zinc. Not sure how these differences stack up.