We shouldn't make any assumptions about what our members do or don't know, so for those of you who don't know about CEC, we'll dig into it (pun intended)...
From Ohio State University Extension (
https://ohioline.osu.edu/factsheet/anr-81):
- "Cation exchange capacity (CEC) is a fundamental soil property used to predict plant nutrient availability and retention in the soil. It is the potential of available nutrient supply, not a direct measurement of available nutrients. Soil CEC typically increases as clay content and organic matter increase because cation exchange occurs on surfaces of clay minerals, organic matter, and roots. Soils in Ohio can encompass a wide CEC range, but typically fall somewhere between 5 to 25 meq/100 g soil (Table 1). Values over 25 meq/100 g soil are found with heavy clay soils, organic, or muck soils."
![CEC_table.jpg](./download/file.php?id=99207&sid=43b6e658fef38c8f8a32ad3913881e10)
- CEC_table.jpg (24.18 KiB) Viewed 596 times
![CEC_explanation.jpg](./download/file.php?id=99208&sid=43b6e658fef38c8f8a32ad3913881e10)
- CEC_explanation.jpg (24.48 KiB) Viewed 596 times
- "Calculating CEC from a Soil Test
"CEC is reported as milliequivalents per 100 grams of soil (meq/100g), or charge per weight of soil. Milliequivalents are used instead of weight because charge is more useful when talking about ion exchange.
"So, how do we take the concentration of nutrients in a soil test (ppm) and convert to charges (meq/100g soil)? Soil testing laboratories often provide these values already on the soil test report, or will provide them upon request."
Not sure that we need to go so far as to pay for a soil test done by a test lab, but that's your choice (last time I checked, soil tests here in the US run into the hundreds of dollars).
I used to think that a lot of commercial potting soils are garbage because they're
way too rich in organic matter for cacti. Mike corrected me (and by extension our other members) when he said this:
MikeInOz wrote: ↑Sun Jan 28, 2024 11:33 pmOrganic material does NOT cause disease of any kind. Pathogenic organisms do. Pathogens do not ''come with'' organic material. They are everywhere including the computer screen you are looking at right now, all over your clothes and even in the dust in the air. All they need is food and the correct environment...So what is the problem with organic? Well it breaks down and as it does, water holding capacity goes up and air-filled porosity goes down. That can benefit some (but not all) pathogens. It is the same as using very fine mineral components to the mix. Therefore, using organic materials will be more acceptable for... plants which can tolerate slower drying in the mix (eventually), small pots, watering frequency which is low. Organic materials will be less acceptable for plants which do not like slowly increasing water holding capacity, large pots and watering frequency which is high.
For soil-based mixes, it appears that organic soils are actually the best fit with the mineral component in the mix -- and if that's the case, maybe commercial potting soils aren't so bad after all. The trick will be to find the correct proportions of mineral gravel and soil. That's up to individual growers, their local climates, and what they're growing. The only problem I see with soil-based mixes -- water-holding capacity increases to unacceptable levels if the mix isn't periodically changed out with fresh mix. How often? That I can't tell you, but from what I've heard over the years, most growers repot their cacti with fresh mix every 2-3 years.
Question for you, Mike -- if growers are using soil-based mixes, would a 1:1 ammonium-to-nitrate ratio be okay, or would you recommend tilting more in favor of ammonium N in the fertilizer? I also have another one that makes me question my belief that a 1:2 ammonium-to-nitrate ratio is suitable for a hydroponic mix like the pumice and granite gravel mix I'm using:
MikeInOz wrote: ↑Sun Jan 08, 2023 1:16 am
Steve Johnson wrote: ↑Sat Jan 07, 2023 10:13 pmAnother is that Nitrate ions tend to be preferred in hotter conditions.
The other way round I believe. A level of ammonium which is completely safe during the bright and warm summer can be quite toxic in winter due to the lack of photosynthate production leading to ammonium toxicity. Traditionally, nitrate is seen as a '' low temp, low light winter'' fertilizer of preference in greenhouse production.
The growing season here in SoCal ends around the middle of October, so I stop watering before then. I also give the collection its last deep watering of the year with plenty of Ca and no fertilizer (that's a good tip I got from you -- thanks!). While overnight lows cool off down to the 40s (Fahrenheit) starting around late October/early November, there are plenty of hot days even into December. If I supplement the fert with a touch of urea to give me a 1:1 ratio, would that be better or do you think that ammonium toxicity is still a risk? If you say the touch of urea is helpful, I know how to do it by adding it to my ammonium sulfate-potassium sulfate stock solution.