Fertilizers explained (2-part presentation) -- with 6/2023 update

[MikeInOz]

Did you happen to ask him about phosphorous binding with calcium (in all forms)? It's hard for me to ignore the clear organic chemistry basis: calcium (acetate, carbonate, etc) have all been shown to bond with phosphorous making each into a more slow release form the soil needs to break down.

You should ignore it. In horticulture, there only seems to be a small problem with some clays (but not all) making some of the P unavailable. The answer in that case is to simply increase the P. In soilless mixes there are no problems that I know of with P becoming unavailable. In fact it is highly mobile ( can be easily leached - usually as Calcium phosphate I believe) and therefore needs to be reduced where there is no soil in the mix and there is a danger of P toxicity if too much is applied to the wrong plant. That is the difference between fertilizing crops in the ground with those in potting mix. Here in AU for example, there are soils with quite reasonable P levels but most of it is tied up and unavailable so soluble P (like superphosphate) needs to be applied. That said, you will not have any problems with P availability, so yes you can completely disregard that concern.

[hoopgod32]

You should ignore it. In horticulture, there only seems to be a small problem with some clays (but not all) making some of the P unavailable. The answer in that case is to simply increase the P. In soilless mixes there are no problems that I know of with P becoming unavailable. In fact it is highly mobile ( can be easily leached - usually as Calcium phosphate I believe) and therefore needs to be reduced where there is no soil in the mix and there is a danger of P toxicity if too much is applied to the wrong plant. That is the difference between fertilizing crops in the ground with those in potting mix. Here in AU for example, there are soils with quite reasonable P levels but most of it is tied up and unavailable so soluble P (like superphosphate) needs to be applied. That said, you will not have any problems with P availability, so yes you can completely disregard that concern.

Are you saying calcium phosphate will slowly become available in soil and that is enough to supply cacti with? All of my cacti have a soil component. I'll follow your lead and mostly disregard for now, but I do hope to one day see a clear chemistry explanation of how phosphate and calcium interact in a soluble fertilizer solution that includes acidified hard water.

[Steve Johnson]

You should ignore it. In horticulture, there only seems to be a small problem with some clays (but not all) making some of the P unavailable. The answer in that case is to simply increase the P. In soilless mixes there are no problems that I know of with P becoming unavailable. In fact it is highly mobile ( can be easily leached - usually as Calcium phosphate I believe) and therefore needs to be reduced where there is no soil in the mix and there is a danger of P toxicity if too much is applied to the wrong plant. That is the difference between fertilizing crops in the ground with those in potting mix. Here in AU for example, there are soils with quite reasonable P levels but most of it is tied up and unavailable so soluble P (like superphosphate) needs to be applied. That said, you will not have any problems with P availability, so yes you can completely disregard that concern.

Are you saying calcium phosphate will slowly become available in soil and that is enough to supply cacti with? All of my cacti have a soil component. I'll follow your lead and mostly disregard for now, but I do hope to one day see a clear chemistry explanation of how phosphate and calcium interact in a soluble fertilizer solution that includes acidified hard water.

A couple of points here...

Although I'm in the minority (not just on the forum, but in the cactus community at large), I'm by no means the only one using a pure soilless mineral mix -- and I'll emphasize the word soilless. This is the 60% pumice-40% mineral mix I use to grow 64 of my 68 cacti:

Pumice-DG_rinsed04242016_forum.jpg (201.76 KiB) Viewed 485 times

The mix provides wide-open aeration, and cactus roots thrive in it. The benefits are:

• Growth of more extensive root systems compared to soil-based mixes. I've used both types of mix ever since I discovered the hobby in 1971, and the mineral mix I started using in 2012 was a real game-changer.

• Remarkably hostile environment for plant pathogens. Yeah, I know it's also a hostile environment for beneficial bacteria, but desert cacti grow perfectly well without it when they're under pot cultivation.

• Cacti are repotted only when they outgrow their pots and need bigger pots. A problem, but it's a nice problem to have!

This brings up my 2nd point -- acidification is required for growers who need to rely on hard water. This is the only way to remove the carbonate side from calcium carbonate (and calcium magnesium carbonate if it's also in your water). Since 5% white vinegar and citric acid are the only acidifiers which are safe for home use, the resulting phosphate and calcium interactions may be different in soil-based vs. soilless mixes. What those differences are and what they mean for growers I don't know, but I have to wonder if this is a matter of academic curiosity, not practical application.

One more thing I'll emphasize -- whether a soilless mix can or can't be used depends on the species. Examples from my collection are Tephrocactus inermis and papyracanthus, and Eriosyce senilis. I grow them in a 50% pumice-50% soil mix, they get the same feeding and watering as the rest of my cacti -- given the generally excellent growth I see across the board, I have a feeling that phosphate-calcium interactions are pretty much the same regardless of whether the mix is soil-based or soilless.

Cacti need the least amount of P. Remember this ratio?

• N 1:P 0.25-0.35:K 1.1-1.7

If P exceeds the upper threshold, reduced root growth, reduced stem growth, and reduced flowering will eventually become a problem. No problems when it's in the optimal range. There could also be a problem if P is below the lower threshold, and I found that flowering is a good benchmark to know if it is. With that in mind, start here...

https://www.cactiguide.com/forum/viewto ... 52#p401552

...then go through the posts up to and including my post on 8/19/23:

https://www.cactiguide.com/forum/viewto ... 99#p402699

I get what Mike said about calcium phosphate leaching in soilless mixes -- P loss is a matter of least concern unless cacti aren't getting enough to grow and flower as they should. Calcium loss could be a matter of more concern -- a good way to address it is by adding a small amount of gypsum granules to your mix. And unlike limestone, gypsum doesn't change the pH of the potting medium.

[MikeInOz]

Are you saying calcium phosphate will slowly become available in soil and that is enough to supply cacti with? All of my cacti have a soil component. I'll follow your lead and mostly disregard for now, but I do hope to one day see a clear chemistry explanation of how phosphate and calcium interact in a soluble fertilizer solution that includes acidified hard water.

I think you may be over thinking it. Calcium Phosphate probably falls under what we call water insoluble but citrate soluble. That means it is only slowly available to plants. Then there in citrate insoluble which is not available. If you acidify your fertilizer solution to pH 6 or so most if not all the P will be available regardless of water hardness. Most of the P in soluble fertilizers are in the form of ammonium or potassium phosphates. Most of my cacti have some soil in the mix and the only P they get is from the osmocote. They don't seem to need any more. Also the soil will form a ''bank'' of P over time.
https://incitecpivotfertilisers.com.au/ ... itopic.pdf

[Steve Johnson]

Before we make assumptions about calcium acetate as a phosphate binder, we'll go through some of the ingredients in the General Hydroponics FloraBloom 0-5-4 I use. They are:

• Magnesium phosphate -- Mg3(PO4)2

• Phosphoric acid -- H3PO4

• Potassium phosphate -- K3PO4

The chemical formula for acetic acid is CH3COOH, and as we already know, calcium carbonate is CaCO3. Now we'll plug everything into ChemicalAid's chemical equation balancer:

Equation_balancer.jpg (32.05 KiB) Viewed 435 times

If calcium acetate is a phosphate binder, I don't see it here. Could be that binding takes place with phosphates in soil, but not in a soilless mineral mix. Maybe someone with a better knowledge of chemistry can explain it to us.

[AirWreck]

Even if the K3PO4 remains unchanged in the above equation, it needs to be indicated on the right hand side of the equation. As it is not, I'd say the equation is incomplete and unbalanced. Something's not right about it. The red flag saying "some elements only exist on one side of the reaction" is also saying somethings not right about the equation.

[Steve Johnson]

Even if the K3PO4 remains unchanged in the above equation, it needs to be indicated on the right hand side of the equation. As it is not, I'd say the equation is incomplete and unbalanced. Something's not right about it. The red flag saying "some elements only exist on one side of the reaction" is also saying somethings not right about the equation.

Yeah, chemistry was never my strong suit. Are there any chemists in the house?

[jerrytheplater]

Steve, K3PO4 is pretty basic, a 1% solution has a pH of 11.8 courtesy of a Google search. (Trisodium Phosphate is Na3PO4, the cleaning agent). If that is used you'll have a very alkaline solution. But, maybe it is used to neutralize the Phosphoric acid? It would help to know the pH of the GH Flora Bloom. (Did I post that in the past?) Another Potassium Phosphate is Mono Potassium Phosphate KH2PO4. This is usually what is meant when the label says Potassium phosphate.

It looks like your calculator needs you to know what the results of the reaction are. You need to fill in both sides of the equation and then let it do the balancing.

[Steve Johnson]

I may be chasing down a rabbit hole here, so let's try this another way...

When hoopgod32 said that calcium acetate is an effective phosphate binder, I'll take his word for it. Question -- is white vinegar a good choice for acidifying hard water? When calcium acetate from the acidified water reacts with the magnesium phosphate, phosphoric acid, and potassium phosphate in my General Hydroponics 0-5-4, we get calcium phosphate. According to the Byju's website (https://byjus.com/chemistry/calcium-phosphate/):

• "Calcium phosphate appears as a white amorphous or crystalline powder that is odourless and tasteless. It is insoluble in ethanol, and acetic acid but soluble in dilute nitric acid and hydrochloric acid. It slightly dissolves in water. "

The "slightly" part doesn't exactly thrill me. Now we'll go over to Mike:

Calcium Phosphate probably falls under what we call water insoluble but citrate soluble. That means it is only slowly available to plants. Then there is citrate insoluble which is not available.

Slowly is better than not at all, and I think not at all is what happens with calcium phosphate produced from the calcium acetate-phosphate reaction. Unless someone says otherwise (and since we know that it's safe for home use), I'm leaning heavily in favor of citric acid as the only choice for acidifying hard water. If everyone here agrees, I'll make that my recommendation from now on.

[hoopgod32]

I may be chasing down a rabbit hole here, so let's try this another way...

When hoopgod32 said that calcium acetate is an effective phosphate binder, I'll take his word for it. Question -- is white vinegar a good choice for acidifying hard water? When calcium acetate from the acidified water reacts with the magnesium phosphate, phosphoric acid, and potassium phosphate in my General Hydroponics 0-5-4, we get calcium phosphate. According to the Byju's website (https://byjus.com/chemistry/calcium-phosphate/):

• "Calcium phosphate appears as a white amorphous or crystalline powder that is odourless and tasteless. It is insoluble in ethanol, and acetic acid but soluble in dilute nitric acid and hydrochloric acid. It slightly dissolves in water. "

The "slightly" part doesn't exactly thrill me. Now we'll go over to Mike:

Calcium Phosphate probably falls under what we call water insoluble but citrate soluble. That means it is only slowly available to plants. Then there is citrate insoluble which is not available.

Slowly is better than not at all, and I think not at all is what happens with calcium phosphate produced from the calcium acetate-phosphate reaction. Unless someone says otherwise (and since we know that it's safe for home use), I'm leaning heavily in favor of citric acid as the only choice for acidifying hard water. If everyone here agrees, I'll make that my recommendation from now on.

Just a quick note, I'm definitely by no means an expert on the chemistry front, I hope someone here is. I know this does seem like a bit of a rabbit hole, but we haven't really found a clear answer? At least no detailed explanation from what I'm able to source online. Pretty surprising.

Btw, I believe calcium in all forms is a phosphate binder. The more soluble (ie. calcium acetate), the more effective it is at binding into insoluble calcium phosphate. So, both carbonate and citrate bind as well just to a lesser degree. I'm honestly not sure if citric acid is a better choice. My knowledge caps out here, but it seems some level of bind is unavoidable if using hard water.

I did stumble on this literature discussing citric acid, calcium and phosphate.

[Steve Johnson]

I did stumble on this literature discussing citric acid, calcium and phosphate.

I'll see your study and raise you this study:

https://www.sciencedirect.com/science/a ... 9723023136

Both studies have absolutely no relevance to growing cacti in pots. All I can tell you is that a year of my lower-P fert regimen (plus TPS CalMag) and acidified water has led to substantial improvements in growth and flower production compared to where I was in 2012-2022. Trial and error plus close observation by the grower beats anything you'll find in the scientific literature. You have to be hands-on with this yourself over the course of more than a few years.

[hoopgod32]

I did stumble on this literature discussing citric acid, calcium and phosphate.

I'll see your study and raise you this study:

https://www.sciencedirect.com/science/a ... 9723023136

Both studies have absolutely no relevance to growing cacti in pots. All I can tell you is that a year of my lower-P fert regimen and acidified water has led to substantial improvements in growth and flower production compared to where I was in 2012-2022. Trial and error plus close observation by the grower beats anything you'll find in the scientific literature. You have to be hands-on with this yourself over the course of more than a few years.

Fair enough

[AirWreck]

Both studies have absolutely no relevance to growing cacti in pots. All I can tell you is that a year of my lower-P fert regimen (plus TPS CalMag) and acidified water has led to substantial improvements in growth and flower production compared to where I was in 2012-2022. Trial and error plus close observation by the grower beats anything you'll find in the scientific literature. You have to be hands-on with this yourself over the course of more than a few years.

Absolutely, the scientific literature can suggest ballpark figures to begin working with. However, the variables in each of our particular environments will differ. That will affect what chemical interactions will occur in your pots; for example, chemical reactions sometimes only occur at specific temperature ranges. One must do trial and error experimentation on our own to see what works with one's particulars. Frustrating and very trying but that's the way it goes. Take measurements and make notes.

Yeah, chemistry was never my strong suit. Are there any chemists in the house?

I'm not a chemist but I did take a couple chemistry courses that were required for my major of study in college. I made my best scores in those chemistry classes and was on top of the class (but that was 25 years ago and I'm not going to try to pretend to remember much). So, I do find all this chemistry talk to be very interesting as I don't get to flex my chemistry brain cells very much. I'll help where I can...

[Steve Johnson]

One must do trial and error experimentation on our own to see what works with one's particulars. Frustrating and very trying but that's the way it goes. Take measurements and make notes.

Don't mean to toot my own horn, but my powers of observation are keen, and I used to put some of them to work when I posted end-of-summer reviews with words and pictures on my Member Blogs thread. I have an end-of-summer 2023 photo archive, so I'll just need to put the words (lots and lots of words) together with the pictures. That is, if I can ever find the time to do it properly.

Absolutely, the scientific literature can suggest ballpark figures to begin working with.

Well, depends on the literature and whether it does or doesn't have practical applications for the grower. So you don't have to go through this thread from beginning to end, I pulled out a few posts you might want to investigate further:

https://www.cactiguide.com/forum/viewto ... 99#p398899

https://www.cactiguide.com/forum/viewto ... 14#p398914

This post from SDK1 is excellent and chock full of info:
https://www.cactiguide.com/forum/viewto ... 23#p398923
Pay special attention to what he said about Park Nobel.

https://www.cactiguide.com/forum/viewto ... 3&start=15

Sadly, there's not much in the way of scientific literature pertaining specifically to cacti, but SDK1 did us a huge favor for linking to the online sources of Park Nobel's work. I hope you'll get something good out of it.