- Note: If you're seeing this presentation for the first time, please read both parts, then go here for an update:
https://www.cactiguide.com/forum/viewto ... 10#p401210
After I joined the forum in November 2011, I wanted to learn about the different growing practices being followed by members with a level of experience I simply didn't have at the time. The subject of fertilizers came up fairly often, but I didn't find anyone who could properly explain all the ins and outs of ferts that are most suitable to the needs of our cacti. That changed when MikeInOz joined the forum in 2018, so endless amounts of credit go to him -- horticulturalist with a deep knowledge of cacti and succulents, and the best explainer of fertilizers we could ever hope to have. I learned a lot from him, and I'd like to "pay it forward" by passing the knowledge along for the benefit of other members who can use it to improve the growing quality of their plants.
How fertilizers are reported
The reporting is done through your fertilizer's label -- all fert manufacturers report N as the total amount of Nitrogen available to our plants. In Australia, ferts are reported in elemental P and K values. In other countries (Ireland and the UK, for example), the ferts report P as P2O5 and K as K2O along with their elemental values on the product label. If the elemental P and K values aren't on the label (as is the case with manufacturers reporting only P2O5 and K2O in the US), use the following calculations:
%P2O5 x 0.436 = %P
%K2O x 0.83 = %K
While the Oxygen in P2O5 and K2O helps to support the plant's overall heath, it has no nutrient value, so all we're concerned about is getting the right amounts of the elemental P and K available to our cacti and succulents.
NPK ratio
This ratio determines whether you have well-balanced or unbalanced proportions of N, P, and K in your fert. The ideal NPK ratio for cacti and succulents is 1:0.4:1.1-1.7. Contrary to what you may have heard or read elsewhere, high-Phosphorus ferts should be avoided -- when P is higher than N, poor root growth, reduced stem growth, and reduced flowering (or no flowering at all) will be the eventual consequence of the plant's longterm exposure to a high-P fert.
Evaluating the fertilizer's NPK ratio
My fert of choice is Dyna-Gro All-Pro 7-7-7, so we'll use it as an example. The guaranteed chemical analysis:
Available N = 7%
P2O5 = 7% x 0.436 = 3.052% elemental P available to the plant
K20 = 7% x 0.83 = 5.81% elemental K available to the plant
The true NPK value for the 7-7-7 is 7-3.052-5.81. Using N as a constant of 1, the NPK ratio is 1:0.44:0.83 -- the balance between N and P is pretty much ideal, but I had to bring the ratio of K up to the 1.1-1.7 range. If your fert strikes the right N-to-P balance, but you find yourself in a situation with K that's not as high as it should be, the next step is...
Addressing the K deficiency
I did that with a 0-0-52 Potassium sulfate (K2SO4) supplement, and you can too, although the amount required will depend on the fert. We'll discuss it in more detail a bit later.
Important minor nutrients
- Calcium and Magnesium. Okay, maybe not so minor -- an enduring myth (which I fell for until Mike busted it after he joined the forum) is that Calcium inhibits cactus growth. In fact, it's the exact opposite, and he's spot-on when he says that "there's no such thing as too much Calcium". However, Ca needs to be balanced with Mg. Theoretically the "ideal" Ca-Mg ratio is 4:1, although I'm not sure if we have to be all that strict about it. A more detailed look at Ca and Mg is beyond the scope of this presentation, but if you'd like to dig deeper, go here and start digging:
https://www.cactiguide.com/forum/viewto ... 54#p394954
Are your plants getting enough Ca and Mg? If not, what can you do about it? Both questions are worth asking, and you'll find the answer to question #2 when you follow the link.
- Sulfur. Although S is a minor nutrient, it's also important, and S deficiency shows up in the form of abnormally pale-looking cacti. If you're adding a K2SO4 supplement to address a K deficiency in your fert, the S is bonus.
The more I thought about what I've learned from Mike, the more I came to realize that fertilizers are nutrient systems, not just a bunch of unconnected parts. If your fert doesn't include micronutrients, it'll be missing some of the parts that keep the system functioning to optimal effect. The full spectrum of nutrients you see in the 7-7-7 is a "gold standard" you can apply when you evaluate your current fert. If yours doesn't have micronutrients on the label, it's time to look for a better one.
Dosage
Dosage = amount of fertilizer going into your watering solution, which in turn is taken up by the roots of your plants. How do we know if the NPK dosage in the watering solution is not enough, too much, or just right? For the mathematically inclined, we can get the answer in terms of parts-per-million. Once again, we'll use the Dyna-Gro All-Pro as an example -- I dilute 1/2 teaspoon per gallon of water. 1 gallon = 1536 half-teaspoons.
7% N = 70,000 ppm/1536 = 46 ppm
3.052% P = 30,052 ppm/1536 = 20 ppm
5.81% K = 50,810 ppm/1536 = 33 ppm
Cacti and succulents are adapted to living on "thin soup", so they will grow well with a steady supply of N and P at this dosage. But we still need to increase the K, so I'll give you the details on that in the next section. Before we get there, here's a table showing the proper dilutions for ferts with 5%, 7%, 10%, and 20% N in teaspoons per gallon and milliliters per liter. Note: The table applies only to liquid fertilizer concentrates. Unfortunately, dilutions using dry ferts are beyond my pay grade!
When you look at those ppm numbers, you may wonder if the N dosage is too high with the 5%, 10%, and 20% dilutions. Actually, it's quite acceptable -- anything below 90 ppm should be fine. With that said, cacti and succulents tolerate underfeeding a lot more than they do if they're overfed, so when in doubt, go with dilutions that are just a shade below what you see for 5%, 10%, and 20% N. (In my humble opinion, the dilution for 7% N is just right.)
Potassium sulfate
Thanks to Jerry Smith (AKA jerrytheplater) for helping me figure this out -- here are the instructions on how to supplement a 7-7-7 fert with Potassium sulfate:
- First create a stock solution by adding 7 grams of 0-0-52 K2SO4 powder to a 1 gallon jug of rainwater or distilled water (not tap water! RO water is acceptable). The powder doesn't dissolve instantly, so give it plenty of time to go into solution. Hint: If you can heat the jug in an oven at 150F for 1/2 hour, turn off the heat and let it sit in the oven for an hour, then take it out, you're set. Once the K2SO4 has gone into solution, it'll stay in solution.
- For the watering solution -- fill a 1-gallon jug with water halfway. Add 1/2 tsp. 7-7-7 and 5 fluid ounces of stock solution, then fill the jug up to the fill line. Cap it, shake well, and water your plants.
Nice thing is that cacti and succulents aren't precision instruments, so the 1.1-1.7 range on the K side of the ratio gives us a fair amount of "wiggle room". Although I use 0-0-52 K2SO4 for my stock solution, it doesn't matter if you prepare yours with 0-0-50 (the most common) or 0-0-53 (less common, but apparently you'll see it more often than 0-0-52) -- your plants won't know the difference. If your fert's K2O already places elemental K in the 1.1-1.7 range, you won't need a stock solution. However, if its percentage of K2O is lower than its percentage of N, you'll need to increase the amount of K2SO4 in your stock solution. For example -- in a 7-8-6 fert, you should prepare a 1-gallon stock solution with 8 grams of K2SO4. Another hint: Use a digital kitchen scale reading in grams to measure the amount of K2SO4 powder going into your stock solution. A certain amount of precision is helpful, and measuring spoons won't be precise enough for our purposes. For even more precision, use a scale measuring in 10ths or 100ths of a gram. (Scales with that kind of precision aren't expensive, and you can buy them online.) In case you need it, here are examples converting grams per gallon to grams per liter with stock solutions that supplement 7-7-7 and 7-8-6 ferts:
- For 7-7-7 -- 7 g./gal. = 1.85 g./L or round up to 1.9 g./L.
- For 7-8-6 -- 8 g./gal. = 2.11 g./L or round up to 2.2 g./L.
- 5 fl. oz./gal. or 39 mL/L.
The benefits of higher K are significant, and if your fert comes up short on the K, they make a good argument in favor of supplementing it with Potassium sulfate:
- Stronger, healthier roots.
- Better stem growth and stronger, more abundant spine formation in species that are sensitive to the effects of K deficiency. Hard to describe in so many words, but if your powers of observation are good, you'll notice significant "before and after" improvements when you see what your cacti look like after you start applying the supplemental K.
- Better resistance to pests and diseases. While it's not a 100% guarantee that you'll never see either problem, higher K greatly improves the odds that you won't have much trouble with both.
- More prolific flowering -- this debunks the "higher P" myth being promoted on other Websites. And sadly, there are "expert" growers who still believe it.
Frequency
Frequency = how often you should fertilize when you water. If you're growing cacti in a completely soil-free hydroponic medium (like the 60% pumice/40% granite gravel mix I use for 64 of my 68 cacti), you'll need to fertilize every time you water. If your mix contains a certain amount of soil, you won't have to fertilize as often. These guidelines are just a guess on my part:
- 10%-25% soil in the mix -- fertilize with every other watering.
- 40%-50% soil in the mix -- fertilize with every 3rd or 4th watering.
A note to beginners: Cacti should be fertilized only in the growing season. If you live in a tropical climate, you won't have that limitation because your growing season is year-round.
Succulents are a different kettle of fish (so to speak) -- I have no experience with them, so I'll leave the fertilizer routine up to the succulent people who grow them.
An avoidable problem
Nutrients not taken up by the plants are left behind as fertilizer salts in one's potting medium, and salt buildup can be a problem if it's allowed to go unattended. Maybe not this year, maybe not next year, but a few years from now, you may find that your cacti stall out and go into a slow decline. The best way to avoid this problem -- flush out your pots with rainwater or distilled water and nothing more once a year in the growing season. Here's the procedure:
- Water deeply until it's running down out of the pot like a faucet.
- Wait for an hour and flush again.
- Wait for another hour and flush for the 3rd time.
Liquid concentrates vs. slow-release fertilizers
Slow-release ferts have an obvious advantage over having to prepare a watering solution every time your cacti need to be fertilized. If you happen to live in Australia or New Zealand, Scotts Osmocote has a cactus and succulent formulation just for you. If you live anywhere else, sorry -- that formulation isn't being sold anywhere else either. But believe it or not, there are slow-release ferts for orchids and bromeliads which are suitable for cacti as well. Here's an example of one you can find on Amazon:
https://www.amazon.com/Fertilizer-13-3- ... 300&sr=8-5
If a slow-release fert's P is actually too low, add just a little bit of monopotassium phosphate to your watering solution.
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Although I left a detailed discussion of Calcium and Magnesium up to the link I gave you earlier, I'll touch on how I'm approaching the matter...
Going back to the Dyna-Gro 7-7-7's product analysis, you'll see that it contains 2% Ca and .5% Mg -- good balance, but before I learned about the importance of Calcium from Mike, I didn't realize that my cacti hadn't been getting enough. To correct the Ca and Mg deficiency, I doubled the amount with 1/4 tsp. of a Nitrogen-free CalMag supplement per gallon of watering solution.
After 2 years of the Potassium sulfate supplement and only a year of adding CalMag to the nutrient mix, the improvements have been amazing. The goal (and I may be on the way to achieving it) -- cacti able to reach their genetic growth potential with a full spectrum of well-balanced nutrients being delivered at a low but steady level. We're not quite done with this presentation, so stay tuned for Part 2 as I show you how to make the most out of the Nitrogen in your fertilizer -- and it's something you may not even know.