Gypsum (CaSO4·2H2O) added to Promix HP for Sulfur?

I have read in several places that putting a bit of gypsum aka calcium sulfate dihydrate (about 10g/cuft) in the medium will provide the sulfur the plant will need through the grow.

What do you guys know about this, is it needed? Does Promix HP already have a source of sulfur? It has dolomitic and calcitic limestone in it for the Calcium and Magnesium, but for Sulfur?

Gypsum takes years to break down. It is also why you don't mix calcium nitrate and epsom together in a solution unless very dilute; calcium sulfate will form and flocculate out. I would get your sulfur another way like MgSO4 (epsom salt).

buyyouabeer said:

Gypsum takes years to break down. It is also why you don't mix calcium nitrate and epsom together in a solution unless very dilute; calcium sulfate will form and flocculate out. I would get your sulfur another way like MgSO4 (epsom salt).

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ok thank you, do you have any idea the concentration that should be added to the medium?

It is so soluble in water that you will rinse it out quickly. I add it every time I water using Jack's 321 formula. What nutrients are you planning on using?

I will probably be using the GH Floraseries that I got cheap off a friend:

I have only ever grown in coco, this is my first forey into anything else, I have just heard that promix hp is good and I was planning on making my own peat moss / perlite (or vermiculite) medium anyways. It was also on sale so I grabbed a bag.

Also I am growing in a 4'x5'x2' tent with a 600W HPS hood (dimmed to 400W though cuz it gets over 90F at 600W in the small tent). 230CFM exhaust, 403 CFM exhaust dedicated to the sealed hood.

buyyouabeer said:

Gypsum takes years to break down. It is also why you don't mix calcium nitrate and epsom together in a solution unless very dilute; calcium sulfate will form and flocculate out. I would get your sulfur another way like MgSO4 (epsom salt).

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No you are mistaken. Gypsum does not take years to become available to roots. A fraction of botth Ca and S is immediately available. CaSO4 is often an excellent additon to peat based substrate.

10 g to a cubic foot would be hardly noticeable. When your soil mix is assembled i suggest you send in a sample and obtain a chemical analysis. Dont guess measure.

The Floragrow and florabloom is derived from magnesium sulfate, and potassium sulfate (among other things) says 1.0% combined sulfur. Would these nutrients alone provide enough to just not bother adding something else to the Promix HP?

radiant Rudy said:

No you are mistaken. Gypsum does not take years to become available to roots. A fraction of botth Ca and S is immediately available. CaSO4 is often an excellent additon to peat based substrate.

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Well that's not what I read and remember from trying to do organic soil.

"It takes several months, and in extreme cases several years, for powdered gypsum to do its work."

buyyouabeer said:

Well that's not what I read and remember from trying to do organic soil.

"It takes several months, and in extreme cases several years, for powdered gypsum to do its work."

The 2 Types Of Gypsum And How Long It Takes Them To Work | How Does Your Garden Mow

Gypsum is a very versatile mineral that is used in a wide range of items from building materials to digestible healthcare products. It has also…

howdoesyourgardenmow.com

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"Gypsum takes years to break down." This is incorrect.

Gypsum is somewhat soluble. The effects of a gypsum application are rapidly detectable through a soil test, a tissue sample, a plant sap analysis and sometimes with a refractometer. Depending on substrate factors gypsum is often an excellent amedment supplying Ca, S and may displace Mg and K which is often at excess levels in compost amended soil mixes.

buyyouabeer said:

Well that's not what I read and remember from trying to do organic soil.

"It takes several months, and in extreme cases several years, for powdered gypsum to do its work."

The 2 Types Of Gypsum And How Long It Takes Them To Work | How Does Your Garden Mow

Gypsum is a very versatile mineral that is used in a wide range of items from building materials to digestible healthcare products. It has also…

howdoesyourgardenmow.com

Click to expand...

For the purpose of a clay breaker in sodic soils, definitely, I can get behind that.
But for the purpose of supplementing ca / s, it's actually much more readily available than lime. Apparently 200x more soluble.
Have to admit I mostly believe that statistic too. Use gypsum myself all the time.

buyyouabeer said:

Gypsum takes years to break down. It is also why you don't mix calcium nitrate and epsom together in a solution unless very dilute; calcium sulfate will form and flocculate out. I would get your sulfur another way like MgSO4 (epsom salt).

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Flocculation in soil is a good thing. I do see your point though... mightn't really be what you want using soiless media. And using soluble nutrients, Epsom salt would likely be a better choice to supplement sulphur.

Don't mean to nit pick your posts. Just felt it was important information to mention.
Haven't used gypsum regularly in soiless myself either, so won't dispute it's usefulness there.

Aussieaceae said:

Don't mean to nit pick your posts. Just felt it was important information to mention.

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Hey no worries I am here to discuss and learn about our favorite plant. I think I must have been remembering dolomite; my setup in coco based soil and strong LEDs (COBs and QBs) always leaves me struggling with Ca issues. I haven't used anything like that for years since going to hydro nutes doing Jacks 321. Thanks to you and Rudy for correcting me.

I will probably just leave the mix the way it is then. This will be my first time growing in anything different then coco, so it's a foreign world to me, and I just want to make sure the medium is prepared correctly before I plant, this Promix HP stuff turned out to be pretty expensive, so we're going to see how momma plant likes it.

radiant Rudy said:

No you are mistaken. Gypsum does not take years to become available to roots. A fraction of botth Ca and S is immediately available. CaSO4 is often an excellent additon to peat based substrate.

10 g to a cubic foot would be hardly noticeable. When your soil mix is assembled i suggest you send in a sample and obtain a chemical analysis. Dont guess measure.

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How well can one mix the soil to have even nutrient distribution? I doubt any two samples are alike. An average or an uncertainty value is needed.

savagemonk said:

How well can one mix the soil to have even nutrient distribution? I doubt any two samples are alike. An average or an uncertainty value is needed.

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Ya that is probably true about totally even distribution. Gather your soil submission by mixing a few different samples and taking a fraction of the mixture.