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Hello,

Once again I find myself making a 'please help my poor cactus' post.

I wintered my ferocactus inside and a few months ago I noticed an orange spot on it. I read that an orange spot can be sun damage after water - and it had briefly been outside after watering - and so I've just been monitoring it. Over the last few weeks its got significantly worse, purple bits around the orange, and other spots on completely different parts of the cactus. I did move it outside into the sun during this time, although rain has since forced it back in.

All spots are firm to the touch and just feel like cactus skin. Does anyone know what this is? Or how to treat it?

Guess: fungal rust

These are usually caused by cold humid conditions. They do not occur when the plant is kept in a warm (min 10C) in a dry atmosphere over winter in my experience. We live in a Mediterranean climate, just the opposite of what Mexican plants like. The ones from the north and US do not mind at all. The ones from the south can be affected. ''Seasoning'' them really well seems to help. That is - low to zero N and high K in autumn and plenty of light with a gradual drying out. Calcium is extremely important as well. I use limestone and/or gypsum depending on the species. The purple colouring is anthocyanin pigment and is used by the plant for protection.
One more thing, Ferrocacti seem to like a rather poor soil. That is - mainly mineral. Yours looks a bit too rich in organics. Now is the right time to re-pot (into a dry mix with no watering for a while) Having said all that, your plant is not in such a bad condition.

Ah, thanks Mike. So likely not fungus then?

I haven't previously had this issue in winter and I've had this cactus for a few years now. That said it was a cooler and wetter start to winter this year, which would be when the problem started. I do not fertilise at all in autumn, I tend to give a cacti/succulent fertiliser in spring (which I have just done), I'll see what I can do next year for K in autumn. The water here is hard enough that I don't think a calcium deficiency is actually possible, although I can make sure I'm watering with the tap water rather than rain water for this cactus.

So in your opinion, what steps should I take at this stage to stop it spreading?

MikeInOz wrote: ↑Fri Sep 15, 2023 6:49 am ''Seasoning'' them really well seems to help. That is - low to zero N and high K in autumn and plenty of light with a gradual drying out. Calcium is extremely important as well. I use limestone and/or gypsum depending on the species.

Hi Mike

I remember you saying about the "seasoning" before. Its fall here now. Straight K2SO4 at what concentration K? How much Calcium in ppm?

jerrytheplater wrote: ↑Fri Sep 15, 2023 6:04 pm

MikeInOz wrote: ↑Fri Sep 15, 2023 6:49 am ''Seasoning'' them really well seems to help. That is - low to zero N and high K in autumn and plenty of light with a gradual drying out. Calcium is extremely important as well. I use limestone and/or gypsum depending on the species.

Hi Mike

I remember you saying about the "seasoning" before. Its fall here now. Straight K2SO4 at what concentration K? How much Calcium in ppm?

I have exactly the same questions. Mike, the growing season here in SoCal ends around mid-October, so I'll give the collection its last deep watering of the year in a couple of weeks. I haven't considered "seasoning" my cacti before, but I'd like to do it this time (straight K2SO4, no General Hydroponics fertilizers). Please give me the correct ppm for K, Ca, and Mg, then I'll do the calculations for my watering solution. The tap water here does provide Ca and Mg, although I don't know the concentrations. The Ca and Mg ppm numbers I'm looking for will apply to diluting my TPS CalMag (4.5% Ca, 1.1% Mg).

Two additional questions -- what is the benefit of "seasoning" rather than fertilizing when cacti are about to end their growing season? Does the benefit apply to mature cacti or just seedlings?

jerrytheplater wrote: ↑Fri Sep 15, 2023 6:04 pm

Hi Mike

I remember you saying about the "seasoning" before. Its fall here now. Straight K2SO4 at what concentration K? How much Calcium in ppm?

Hi Jerry
K2SO4 at 1 gram/Lt would be considered ''full strength'' so it would depend on how much you water between applications. If you are drying things off, one application at full strength should be acceptable or 2 applications at 0.5 grams/Lt with watering in between. No other fertilizers after that.
The Calcium should be included in the mix as gypsum or limestone or both. Applied as a liquid you would end up giving N as well. (CaNO3) Calcium chloride is a possibility but I don't like to use it.

Steve Johnson wrote: ↑Fri Sep 15, 2023 9:49 pm

I have exactly the same questions. Mike, the growing season here in SoCal ends around mid-October, so I'll give the collection its last deep watering of the year in a couple of weeks. I haven't considered "seasoning" my cacti before, but I'd like to do it this time (straight K2SO4, no General Hydroponics fertilizers). Please give me the correct ppm for K, Ca, and Mg, then I'll do the calculations for my watering solution. The tap water here does provide Ca and Mg, although I don't know the concentrations. The Ca and Mg ppm numbers I'm looking for will apply to diluting my TPS CalMag (4.5% Ca, 1.1% Mg).

Two additional questions -- what is the benefit of "seasoning" rather than fertilizing when cacti are about to end their growing season? Does the benefit apply to mature cacti or just seedlings?
[/quote]

Applying K will tend to counter the effects of N and strengthen cell walls. 0.5 to 1 gram per litre will do. Mg is not necessary. When I mentioned the Ca. I meant that it should always be always available to the plant throughout the growing season. If it is, there should be no need to add more. Remember that these suggestions are no miracle, just good general practice in my opinion.
Rather than applying straight K, omitting N (if you can) for the last few feeds would work just as well or perhaps even better. I can't do that because I'm using the osmocote so I give a couple of feeds of K at that time.

Ilvin wrote: ↑Fri Sep 15, 2023 9:27 am Ah, thanks Mike. So likely not fungus then?

I haven't previously had this issue in winter and I've had this cactus for a few years now. That said it was a cooler and wetter start to winter this year, which would be when the problem started. I do not fertilise at all in autumn, I tend to give a cacti/succulent fertiliser in spring (which I have just done), I'll see what I can do next year for K in autumn. The water here is hard enough that I don't think a calcium deficiency is actually possible, although I can make sure I'm watering with the tap water rather than rain water for this cactus.

So in your opinion, what steps should I take at this stage to stop it spreading?

Yeah WA is known for it's high Ca water. No need to add more. In you case it would be prudent to keep an eye on the pH of the mix after a while to make sure it doesn't creep up too high. 5.5 to 6.5 is about right. If you use rain water exclusively, you will need to add Ca in some way.
I don't think you will see it spreading no that the warm weather is here. Just treat it mean in autumn and keep it in a warmish place.

MikeInOz wrote: ↑Sat Sep 16, 2023 3:39 amRather than applying straight K, omitting N (if you can) for the last few feeds would work just as well or perhaps even better. I can't do that because I'm using the osmocote so I give a couple of feeds of K at that time.

Okay, now I understand why you do the K-only feeds at the end of your growing season. As I said earlier, I'll give my cacti their last deep watering of the year in a couple of weeks, and per your no-N recommendation, I'll just make it water without the General Hydroponics ferts. And since I acidify my tap water, I'll keep my pH meter at the ready to be sure that the water going to the plants will have a pH no lower than 6.0. Speaking of...

Don't know if this would be useful to you, but perhaps for Ilvin and others who water their cacti with tap water. Here's the problem -- presence of calcium bicarbonate in the water. This can be detected with a digital pen-style pH meter, and they're not that expensive. If your pH meter shows that your water is higher than 7.0, there's calcium bicarbonate in the water. The 2 acidifers safe for home use are 5% white vinegar and citric acid. For growers with large collections, citric acid is the better way to go because it's more concentrated than vinegar. The byproducts of acidification are:

• Calcium acetate from vinegar.

• Calcium citrate from citric acid.

Calcium acetate and calcium citrate are highly soluble, making calcium available to the plant. If there really is no such thing as too much calcium, acidifying tap water could be a nice little benefit. With that said, all pH meters are not equal, and pH meters you can't calibrate should be avoided. Meters with 1-point calibration are good, meters with 2-point calibration are better. For example, this is the one I recently purchased from Amazon:

https://www.amazon.com/Poniie-PH2022Plu ... 776&sr=8-1

Superior to the Milwaukee Instruments pH 600 I had been using for years.

There's just one caution -- know the pH of your potting medium. If it's acidic, do not acidify tap water.

Ilvin, if you'd like to acidify your tap water and you need some guidance, send me a PM. If you're already doing it, great!

The tap water here is for sure >pH 7. I think its around pH 8 and given the amount of limestone in the soil around Perth, I'd also put a fair bet on calcium bicarbonate being the cause.

Rather than pH testing, I've been mixing about 50/50 tap and rain water for cacti/succulent watering. Its a somewhat unscientific way of doing it, but my hope had been to provide the calcium cacti need without changing the soil pH too much. On that note, I'm not aware of the soil pH so I'd be wary of actively acidifying.

As always, I follow these topics with great interest. I wonder if we could establish some guidelines?
As I understood it, in the last watering(s) of the growing season you leave out any fertilizer and focus on (adding) calcium to strenghten the cell walls. But not magnesium, correct? Because magnesium aids in chlorophyll production.
I can imagine substrate composition is an important factor. If you have limestone in your mix, watering with rain water is probably enough to dissolve the calcium and make it available to the plant.
But what if you're growing in 60% pumice and 40% DG/quartz grit mix? Do we then water with acidified tap water to provide calcium?
I grow in 50% pumice, 40% quartz grit and 10% loam (high in clay, not in sand). So, I would probably be good with watering with rain water to dissolve the calcium present in the clay?
Ilvin, do you know what type of soil the plant is in? What type soil do you repot your other cacti into?

ohugal wrote: ↑Sun Sep 17, 2023 9:17 am As always, I follow these topics with great interest. I wonder if we could establish some guidelines?
As I understood it, in the last watering(s) of the growing season you leave out any fertilizer and focus on (adding) calcium to strenghten the cell walls. But not magnesium, correct? Because magnesium aids in chlorophyll production.
I can imagine substrate composition is an important factor. If you have limestone in your mix, watering with rain water is probably enough to dissolve the calcium and make it available to the plant.
But what if you're growing in 60% pumice and 40% DG/quartz grit mix? Do we then water with acidified tap water to provide calcium?
I grow in 50% pumice, 40% quartz grit and 10% loam (high in clay, not in sand). So, I would probably be good with watering with rain water to dissolve the calcium present in the clay?
Ilvin, do you know what type of soil the plant is in? What type soil do you repot your other cacti into?

You're touching on a couple of important points, so here's the first...

This is something I posted on Mrs. Green's "Astrophytum help needed" thread on the Cultivation forum:

viewtopic.php?p=402602#p402602

The only difference between the hard spring water Mrs. Green uses to water her cacti and the hard tap water I use to water mine is the presence of calcium bicarbonate in the tap water. Calcium is the hardness and bicarbonate is the alkalinity. If growers water their cacti with tap water, the plants are getting plenty of calcium and magnesium in the water itself, so I'll be in good shape when I give my cacti their last deep watering of the year with acidified tap water and no fertilizers. "So, I would probably be good with watering with rain water to dissolve the calcium present in the clay?" Unfortunately I can't give you an answer because I don't know if the calcium would be available to your plants. When in doubt, I think you'll be better off with tap water. If you're acidifying, great.

Acidic potting media do the job of at least partially neutralizing bicarbonate alkalinity, so acidification may not be required at all. If the mix itself has a pH of 5.5-6.0, then no. If it has a pH between 6.5 and 7.0, a small amount of acidification may be needed unless the fertilizer lowers the tap water's pH -- this depends on the fert.

And now for point #2...

MikeInOz wrote: ↑Fri Sep 15, 2023 6:49 amOne more thing, Ferrocacti seem to like a rather poor soil. That is - mainly mineral. Yours looks a bit too rich in organics.

Completely agree. Looking at Ilvin's photos, it appears that the organic material is chipped/composted tree bark of some sort. It means that his mix is on the acidic side, so adding an acidifier to his water won't be a good idea. But he's also using the wrong mix for North American cacti like his Ferocactus. My recommendation -- a mix of 4 parts mineral gravel (pumice or perlite) and 1 part organics.

Ilvin, mixing 50/50 tap and rainwater isn't the best way to go -- choose one or the other. If you choose rainwater, your cacti will need a calcium source. Some species use limestone, others use gypsum, and Mike is the go-to guy on knowing what needs what. I don't have his level of expertise, so I supplement my fertilizers with a nitrogen-free CalMag supplement. Find out if you can get one -- nitrogen-free is important because your plants will be getting their N from your fertilizer.

If you choose tap water, your cacti will get their Ca and Mg right from the source. But you'll also be dealing with the bicarbonate problem. When you lean out your mix with a higher amount of mineral, that's also the time when you'll need to know its pH so you can figure out the right amount of acidifier (5% white vinegar or citric acid) you should add to your tap water. Here are the steps:

• First -- if you don't already have one, buy a digital pen-style pH meter for testing the water. They're not that expensive. Avoid pH meters you can't calibrate. Meters with 1-point calibration are good, meters with 2-point calibration are even better because they're more accurate. Be sure to follow the manufacturer's instructions.

• Test your tap water and write down its pH.

• Take a clean glass jar and fill it partway up with your mix of choice. Fill the jar all the way up with tap water, put a lid on it, and let it sit for a few hours. Test the the water in your jar and write down its pH -- this is to see if the water reacts to the mix. If the pH is the same, your mix is pH-neutral. If it's lower, your mix is acidic. If it's higher, your mix is alkaline.

Before we go any further, let me know about which option you choose -- either rainwater or tap water.

I think if you decide to go down the rabbit hole, knowing the acidity of your soil, water and fertilizing solution is important and practical. I did not yet test the pH of my mineral mix after watering with rain water. I flushed my pots 2 weeks ago with rain water. Since the pH of my soil is 6 and the pH of my tap water is 6,9, I'll probably just water them once more with tap water. Thanks again for the advice.

ohugal wrote: ↑Sun Sep 17, 2023 7:44 pm I think if you decide to go down the rabbit hole, knowing the acidity of your soil, water and fertilizing solution is important and practical. I did not yet test the pH of my mineral mix after watering with rain water. I flushed my pots 2 weeks ago with rain water. Since the pH of my soil is 6 and the pH of my tap water is 6,9, I'll probably just water them once more with tap water. Thanks again for the advice.

You're welcome, and I'm glad that you don't have any bicarbonate in your tap water.

All stages of pH testing are critically important -- potting medium, source water, reaction of the water to the potting medium, pH after adding fertilizer to the source water. At that point -- and only at that point, growers can determine whether they do or don't need to acidify tap water. If they do, the last stage is determining the correct amount of acidifier going into the watering solution. Oscar, you're doing everything right.