-------------------------------------------------------------------------------------------------------------------------
Note: I posted "Fertilizers explained" on 1/21/23. If you haven't seen it yet, go there first...
viewtopic.php?t=47603
... then come back for the update.
-------------------------------------------------------------------------------------------------------------------------
This just in.
Steve Johnson wrote: ↑Sun Jan 22, 2023 6:53 amMy 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...
Actually, no -- thanks to more recent information coming from MikeInOz, the acceptable range for P is 0.25-0.35. Go any higher, and problems will slowly creep up on you over time. Ever since I started using the 7-7-7 in 2012, those problems were creeping up on me too, and I didn't notice until I saw most of my older cacti exhibiting these symptoms last year:
- Growth slower than normal for the species.
- Unnaturally pale skin color (Mo deficiency has something to do with it too).
- Inhibited flowering or no flowering at all.
When Dyna-Gro decided to discontinue the All-Pro 7-7-7, it turned out to be a blessing in disguise as I replaced it with General Hydroponics FloraMicro 5-0-1 and FloraBloom 0-5-4 liquid fertilizers. Only problem there -- P is still a little too high in relation to N if we're supplementing it only with Potassium sulfate.
Solution -- include Ammonium sulfate. (Truth be told, I could've done the same thing if Dyna-Gro was still selling the 7-7-7.) This is the recipe I'm using now:
What we're talking about here is a balancing act, so if your fert's NPK is out-of-balance, you can correct it with the addition of Ammonium sulfate and Potassium sulfate to strike the right balance in your watering solution. How much you'll need in an Ammonium sulfate-Potassium sulfate stock solution, and how much of the stock solution should go into the watering solution depends on your fert. The calculations involved aren't difficult, although if you're math-challenged and you can use some help, please feel free to PM me. Once I run the numbers, I'll get back to you with the right recipe and instructions.
Steve Johnson wrote: ↑Sun Jan 22, 2023 6:53 am...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!
They
were beyond my pay grade, but not anymore. The tricky part is knowing that dry fertilizers need to be diluted by mass (weight), not volume as we'll do with liquid ferts. Another tricky part -- calculating grams per gallon or grams per liter for the proper dilution. Here are the tricks:
A couple of things come to mind:
- Nitrogen is the benchmark indicator to follow, and you won't be overfeeding your cacti if you keep it below 90 ppm in your watering solution. If you prefer growing your cacti in the "hard" style, aim for 40 ppm. (Anything below 30 ppm is putting them on a starvation diet! The same applies to growers who think it's a good idea to feed their cacti only once or twice a year.)
- Compared to liquid fertilizers, dry ferts give you more precise control over how much you're diluting.
Given the choice between liquid and dry, I don't know if there's any particular advantage to choosing a dry fert. However, for growers who find that liquid ferts are too expensive (or they can't find ones that are suitable), dry is their only choice -- in which case, they'll be better off if they dilute by weight rather than volume.