Questions on Correct Fertilization

[NYCorchidman]

Can someone please clarify the relationship between potting mix type, pH, fertilizer type and its bioavailability?

If there are any old but good threads that already dealt with this topic, I would love to be directed to that as well.

Thanks in advance!

[Cym Ladye]

Originally Posted by NYCorchidman

Can someone please clarify the relationship between potting mix type, pH, fertilizer type and its bioavailability?

If there are any old but good threads that already dealt with this topic, I would love to be directed to that as well.

Thanks in advance!

Go to the search icon on the tool bar of this forum and type in the topic you want information on. There are many, many threads on these subjects. Just keep in mind, if you ask 10 different people, you will get 12 different answers.... at least!

Cym Ladye

[hagfish]

Fertilizers all list plant macronutrients according to an NPK scale. This is Nitrogen, Phosphorus and Potassium (K). I use "Miracle Grow" for everything, since it is water soluble. It also contains the trace elements plants need like Iron, Copper and Boron. One disadvantage of Miracle grow is that the Potassium comes as the chloride, which is technically salt. As long as water flushes the potting medium, salt accumulation should be no problem. Regarding pH, alkaline conditions inhibit the uptake of trace nutrients. This is why Miracle Grow adds a chelating agent to overcome the problems associated with hard water.

[MattWoelfsen]

Originally Posted by NYCorchidman

Can someone please clarify the relationship between potting mix type, pH, fertilizer type and its bioavailability?



If there are any old but good threads that already dealt with this topic, I would love to be directed to that as well.



Thanks in advance!

That is a good question and something I hope Ray Barlow would answer. I looked around the board to see if there have been any threads that discusses how a plant could be effectively nourished when planted in specific media. I could not find any threads.

I have Phalaenopsis planted in all spaghnum moss, some in bark mix, some in coir, some mixed with all of the above. Regardless of their potting media, I have always wondered which plants benefit the most. They all look healthy. In another forum dedicated to Phalaenopsis, I read about a study being done on the benefits of growing Phals in spaghnum moss from a volume perspective, I.e., how packed a plant should be in spaghnum moss.

[Orchid Whisperer]

Hi NYC

The following information is for plants in soils (so take the information with as large a grain of salt as you feel is necessary). Here are a couple links that include information on plant nutrient availability at different pH levels:NMSU: Interpreting Soil Tests for Efficient Plant Growth and Water Use
Nutrient Imbalance and How to Correct It - eXtension

I give two links because they provide slightly different versions of the same graph. Note that the graphs talk about nutrient availability - they are not linked to a particular kind of plant. Nutrients (e.g., phosphorus) can adsorb onto both organic particles (orchid media) and inorganic particles (soil minerals). Some nutrients are more available at high pH, some at low pH, and the 'happy medium' seems to be in the weakly acid range (ballpark pH = 6).

I water mostly with rain water, which is acid, but during the summer, I water with neutral pH tap water. I think low dissolved solids water is important. Here is a link to an article I refer to regarding water quality (thanks Houston Orchid Society!) http://www.houstonorchidsociety.org/...%20Quality.doc

Orchid media are really just materials that will keep the right water/humidity/moisture/drainage conditions around the roots. You select the orchid medium to provide the right moisture condition for the plant you are growing (what's good for a Cattleya might be terrible for a Restrepia). I think many, many orchid media can be used, so there is no one-size-fits-all. Some media start with high salt content (CHC or coconut husk chips, and coir fiber, are examples), these should be soaked and rinsed well before use.

I think there are many balanced water-soluble fertilizers that can be used successfully. All plants need more N and K than they need P, so fertilizers with a low middle number are typically OK. I like fertilizers that also are formulated to include micronutrients. Be sure you are providing nitrogen and other nutrients when plants are actively growing, dial back on the nutrients when they are not growing actively.

One last caveat: certain orchids may require special care with water, nutrients etc. at certain parts of their life cycle. If you are growing Catasetum or Nobile Dendrobiums for example, I would research specific requirements for growing those (and I can't claim to grow either of those!).

[Ray]

OW - the reason it is the same graph is because it was taken from the same, single study, not because it is "gospel".

While nobody can argue that proper pH plays a significant role in plant culture, that chart is something that might fit in the "urban legend" category. It may be a real representation of data collected in a study, but is actually of little value to orchid growers, as it was derived from a single study, using a single formula of fertilizer, using a single SOIL substrate.

The fact that it is from a single study makes it somewhat suspect to begin with.

Not all minerals dissociate the same in aqueous solutions, and that can also be affected differently by pH. Do we know what minerals were in the tested fertilizer, their relative ratios and concentrations, and how they compare to what we use?

The fact that it is pour-through results from a soil substrate really confounds its value to orchid growers, as soils can have significant cation exchange capacities (CEC), while most orchid media have orders-of-magnitude less, if at all.

The cation exchange capacity is a measure of how well soil components hold onto positively-charged ions - cations - and most of the CEC occurs at the edges of clay particles and fine organic matter. The development of positive and negative charges on a clay particle is affected strongly by pH, so we can understand how, at extreme pH values, the charges may strongly bind the cations in fertilizer solutions, making them unavailable to the plants. In orchid media, which tend to have essentially no CEC, the nutrient cations tend to stay in solution and remain available.

The bottom line is that, for the most part, in very dilute solutions, where ions dissociate with little-to-no interference from each other, a slightly acidic pH appears to be preferred. Beyond that, if it's in solution, it's available for the plants to take up.

What's also important, but has very meager scientific backup at this point, is the role the different ions play in supporting-, or interfering with the uptake of other ions. Excessive K, for example, can interfere with the uptake of Ca and Mg.

[Orchid Whisperer]

Originally Posted by Ray

OW - the reason it is the same graph is because it was taken from the same, single study, not because it is "gospel".

While nobody can argue that proper pH plays a significant role in plant culture, that chart is something that might fit in the "urban legend" category. It may be a real representation of data collected in a study, but is actually of little value to orchid growers, as it was derived from a single study, using a single formula of fertilizer, using a single SOIL substrate.

The fact that it is from a single study makes it somewhat suspect to begin with.

Not all minerals dissociate the same in aqueous solutions, and that can also be affected differently by pH. Do we know what minerals were in the tested fertilizer, their relative ratios and concentrations, and how they compare to what we use?

The fact that it is pour-through results from a soil substrate really confounds its value to orchid growers, as soils can have significant cation exchange capacities (CEC), while most orchid media have orders-of-magnitude less, if at all.

The cation exchange capacity is a measure of how well soil components hold onto positively-charged ions - cations - and most of the CEC occurs at the edges of clay particles and fine organic matter. The development of positive and negative charges on a clay particle is affected strongly by pH, so we can understand how, at extreme pH values, the charges may strongly bind the cations in fertilizer solutions, making them unavailable to the plants. In orchid media, which tend to have essentially no CEC, the nutrient cations tend to stay in solution and remain available.

The bottom line is that, for the most part, in very dilute solutions, where ions dissociate with little-to-no interference from each other, a slightly acidic pH appears to be preferred. Beyond that, if it's in solution, it's available for the plants to take up.

What's also important, but has very meager scientific backup at this point, is the role the different ions play in supporting-, or interfering with the uptake of other ions. Excessive K, for example, can interfere with the uptake of Ca and Mg.

Ray, I never said it was gospel. Please carefully re-read my first paragraph. The graph was published in Brady, N.C. 1974. The nature and properties of soils, 8th ed. MacMillan Pub. Co., Inc., NY. It is widely quoted because the relationships (in soils) are widely confirmed by subsequent, peer-reviewed scientific investigations (scientific backup, as you say). I'm not as familiar with NMSU, but I can assure you that a venerated agricultural institution like Clemson does not pass along "urban legends"; they verify what they publish.

From your second-to-last paragraph, it appears that you agree with what I said in my second paragraph - that a weakly acid pH is best. Nutrients are most available to plants in that pH range. That is essentially what the graph is showing; avoid the extreme pH ranges, and you should be OK with respect to nutrient availability.

I understand the importance of CEC with respect to plant nutrients; I've routinely run soil tests (including CEC) as a part of my work for the past 22+ years, although I have not analyzed orchid media for CEC. I would imagine that the CEC of orchid media would depend on a lot of things. For example, partially decomposed fir bark might potentially have a higher CEC than fresh fir bark. I have read that certain orchid media (especially coconut husk chips-well rinsed) have moderate CEC, but this not from peer-reviewed research. If some orchid media do have moderate CEC, it would still make sense to keep pH in that weakly acid range.

Regarding potassium and potential interference with uptake of calcium and magnesium, I have not seen any peer-reviewed, published research on the matter (scientific backup, as you say). If you know of any articles on the matter (even if it's only one or two), can you post a citation? I can only say what I've observed in my own plants - I typically use fertilizers that provide NPK and micronutrients, as well as supplemental Ca and Mg (from gypsum and Epsom salts), and haven't observed signs of Ca and Mg deficiency.

[Ray]

Originally Posted by Orchid Whisperer

From your second-to-last paragraph, it appears that you agree with what I said in my second paragraph - that a weakly acid pH is best. Nutrients are most available to plants in that pH range. That is essentially what the graph is showing; avoid the extreme pH ranges, and you should be OK with respect to nutrient availability.

Yep. I agree completely. What concerns me about that graph is the fact that in solution, even at realtively "extreme" pH levels, some of those cations are still available to a plant, while they would be sequestered by the CEC of a soil at that pour-through pH.

I have not analyzed orchid media for CEC. I would imagine that the CEC of orchid media would depend on a lot of things. For example, partially decomposed fir bark might potentially have a higher CEC than fresh fir bark. I have read that certain orchid media (especially coconut husk chips-well rinsed) have moderate CEC, but this not from peer-reviewed research. If some orchid media do have moderate CEC, it would still make sense to keep pH in that weakly acid range.

I don't think there has been any published research in that area. My "gut feel" however, is that the CEC in orchid media is relatively insignificant, compared to that in soils.

Regarding potassium and potential interference with uptake of calcium and magnesium, I have not seen any peer-reviewed, published research on the matter (scientific backup, as you say). If you know of any articles on the matter (even if it's only one or two), can you post a citation? I can only say what I've observed in my own plants - I typically use fertilizers that provide NPK and micronutrients, as well as supplemental Ca and Mg (from gypsum and Epsom salts), and haven't observed signs of Ca and Mg deficiency.

I have found Marschner's book on Mineral Nutrition in Higher Plants to be thorough, if not overwhelming, although again, it relates primarily to terrestrial plants, not orchids. Benzing's book on epiphytes (for which I can't recall the title) has been a great help in making that informational translation.

As to fertilizers, I thing Bill Argo and the group at MSU probably did the most for making the importance of calcium and magnedium known to the orchid world.

[MattWoelfsen]

THANK YOU Ray BARKALOW (sorry I mispelled your name).