Purple and orange spots on ferocactus

[Ilvin]

I hear you with the calcium. The cactus might well be deficient even with the Ca in the water, its odd that there's none in the fertilizer but even after only a couple of years growing cacti I'm becoming skeptical that people making cacti/succulent products actually know what they're doing. The bark in the soil mix is not great either. I'll see what I can get here as a calcium supplement.

I've been hesitant to go too far into soil-lean mixes because forgetting the fertilizer would then mean killing the plants, but I've definitely noticed the poor root systems in plants grown in too-rich soil now. Again, staggering how many sellers grow cacti in killer mixes.

Its a liquid fertilizer. I fertilize twice a year, once right at the start of the growing season - late October early November dependent on weather. I then do a half-strength feed this time of year, which I have just done. The full strength dilution is 2 caps per 9 L water.
I'm interested to hear your opinion on this, because I don't like to regularly fertilize cacti because they're adapted to desert environments, but obviously living in pots and the soil to mineral ratio will make a huge difference to how much the plant requires. I do wonder if it ran out of nutrients over the winter, but I didn't water it much because I was forced to keep it mostly indoors so it didn't grow much either.

[Steve Johnson]

I hear you with the calcium. The cactus might well be deficient even with the Ca in the water, its odd that there's none in the fertilizer but even after only a couple of years growing cacti I'm becoming skeptical that people making cacti/succulent products actually know what they're doing. The bark in the soil mix is not great either. I'll see what I can get here as a calcium supplement.

Your skepticism is well-founded -- especially when it comes to fertilizers "formulated for cacti and succulents". The only one that gets it right is this Osmocote slow-release fert:

Osmocote_c&s_formulation.JPG (96.71 KiB) Viewed 2047 times

Good news for you is that you can buy it in Australia available through Bunnings here:

https://www.bunnings.com.au/scotts-osmo ... r_p0168579

Mike uses the Osmocote to very good effect, and he can guide you on how much you should add to your pots based on their volume. If I remember correctly, I believe one application lasts for 6 months, so the "set it and forget it" approach will make things easier for you. Only problem is that the Ca-Mg ratio should be 3-1 or 4-1, and the Osmocote is upside-down in that regard. You have 2 options for bringing the ratio rightside-up:

• Limestone granules for limestone growers, gypsum granules for everything else. If you're not sure whether any of your cacti are native to limestone soils, then as I said on 2/5, you won't go wrong with gypsum for all of your cacti. Once again, Mike should be able to guide you on how much you should add to your pots.

• A nitrogen-free liquid calcium supplement (your cacti are getting enough N from the fert). After doing a fair amount of research online, all I can find are CalMag fertilizers, so you may have to go with limestone and/or gypsum.

Your tap water contains Ca and Mg. If you go with the Osmocote slow-release, its Mg argues in favor of watering your cacti with rainwater or distilled water only.

Its a liquid fertilizer. I fertilize twice a year, once right at the start of the growing season - late October early November dependent on weather. I then do a half-strength feed this time of year, which I have just done. The full strength dilution is 2 caps per 9 L water.

If you decide that the Osmocote product isn't for you, your liquid fert is a good fallback option. Should that be the case, "2 caps" doesn't tell me anything -- I'll need to know the amount per capful in milliliters. From there I can calculate nutrient dosages in parts-per-million based on your dilution and determine if the ppm numbers per feeding will be correct. If they're not, we can adjust the dilution. You're fertilizing only twice in your growing season, not nearly enough for cacti growing under pot cultivation. Either go with the Osmocote slow-release or dilute the liquid concentrate and fertilize with every 3rd or 4th watering. When your growing season comes to an end and you give your cacti their last deep watering of the year, here's a tip I learned from Mike -- water with pure water and no fertilizer at all. If you can get a nitrogen-free CalMag supplement, add some to the water. A calcium boost "seasons" your cacti (also known as hardening off) as they prepare for dormancy.

[Ilvin]

I apologise, I was in a rush: cap is the least useful measurement - its 40mL in a cap. Oddly enough my fertiliser is the Osmocote's liquid version of the slow release, I wonder why they didn't include the calcium? In any case it would be easy to switch to the slow release, which I will do, and there is a nitrogen free gypsum product I can get hold of.

[Steve Johnson]

Oddly enough my fertiliser is the Osmocote's liquid version of the slow release, I wonder why they didn't include the calcium? In any case it would be easy to switch to the slow release, which I will do, and there is a nitrogen free gypsum product I can get hold of.

Very good. The photo of the slow-release I posted actually came from one of Mike's posts. Couldn't see the application rates there, but we can see them here:

OSL_label.jpg (90.21 KiB) Viewed 2004 times

15 cm = about 5 7/8" diameter -- seems like a pretty big pot, so you'll need to estimate how much you should apply to your smaller pots. Since the Osmocote is a slow-release fert, gypsum granules make sense because they'll be releasing Ca slowly too. Here's a source for you in Australia:

https://www.plantdoctor.com.au/gypsum-g ... mini-prill

Even though the product is aimed toward in-ground growing, application should be fine for your potted cacti. When you estimate the amount of Osmocote slow-release for each of your pots, I believe half the amount for the gypsum granules will be correct.