Fertilizer Concentration, Formula & Frequency

[Ray]

OK, I'm on a soapbox here about an acknowldeged pet peeve, but I think this is work stating so folks know what they're doing.

When reading through the threads here, you'll often see comments like "feed weakly" or "feed at 1/4 strength". The first is totally meaningless and the second only has meaning if you know the formula. "1/4 strength" of a 20-20-20 formula is identical to "full strength" of a 5-5-5.

Then there's the "I use {this brand}" folks, who either think the readers are clairvoyant, or don't understand that many fertilizer brands have different formulas.

Feeding frequency is also important. At the same teaspoons-per-gallon mixing ratio, feeding a 20-20-20 once provides the same nutritional level as feeding four times with a 5-5-5.

Fertilizing our plants is analogous to meal planning for us. The formula (particularly the nitrogen content) combined with the mixing concentration is the "calories in a meal", while the feeding frequency is the "number of meals to be eaten". Without considering all three, a discussion about nutrient needs is worthless.

[hypostatic]

I disregard any fertilizer recommendations that aren't in PPM.

Everything else is hocus pocus to me. Just like "bright light" lol.

[SouthPark]

It's true. People have to make sure that they're not over-dosing the plants. For some cases, if there is some product that a company sells that's recommended for orchids, then one could probably trust them that they did their homework with recommended concentrations - or how much water to mix for a certain amount of liquid fertiliser concentration (and when to apply - frequency etc).

But definitely - having an understanding of precise quantities for particular plants is certainly very beneficial - for both the plant and the grower.

[gwyn]

Originally Posted by Ray

Feeding frequency is also important. At the same teaspoons-per-gallon mixing ratio, feeding a 20-20-20 once provides the same nutritional level as feeding four times with a 5-5-5.

I agree with everything except for this. No, feeding 20-20-20 once is not the same as feeding 4 times at 5-5-5.

Plant uptake is a little more complex then that.

So lets say you give one dose at 100% 20-20-20 strength.

You calculations are only correct if everything is sucked up by the roots at time/s of feeding.

Plants don't work like that. They have an uptake rate. So if you feed 20-20-20 to a plant that can only take up 1-1-1, then you are only feeding the plant 1-1-1.

When you feed 5-5-5, you will only get an uptake if 1-1-1 x 4 feedings. Which means over the same period, you fed the plant 4 times more uptake food.

If someone wants to do the reading, there is an article called.

Nitrogen uptake by tropical orchids
Author links open overlay panelC.S.HewL.Y.LimC.M.Low

Effects of Nitrogen and the Various Forms of Nitrogen on Phalaenopsis Orchid—A Review in: HortTechnology Volume 27 Issue 2 (2017)

Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use | UGA Cooperative Extension

The math says that anything over 200mg-l will not be taken up by the plant or just build in the media.

200mg-l is 200ppm.

So. Math.

Most 20-20-20 consider 1 Tablespoon which is about 15 grams of fertilizer per liter. That is about 3000 ppm.

1 teaspoon of 20-20-20 about 5 grams, which means 200ppm.

Most plants can not take up more the 200ppm in any give time.

So feeding once at 3000pm means you only fed 200ppm of food.

Feeding 200ppm 4 times means you fed 800 ppm of food.

[SouthPark]

gwyn ...... that makes sense. It also depends on other things too, like the media. If fertilising a particular media, and the fertiliser hangs around in that media ...... then next time we add fertiliser or water etc, then there's these extra effects, like nutrients already in the media, plus the extra being added by more fertilising.

[hypostatic]

Hmmm according to my calcs, I'm dosing at ~162 PPM nitrogen using Ray's fertilizer haha. That should be 1/2 a teaspoon, into 60 oz water.

For most orchids I'll hand spray a couple of ounces once per week. For the lycastes and catasetums, once they're in full growth they pretty much get watered with the fertilizer haha

[Ray]

Gwyn - While I agree that nutrient uptake is not necessarily linear, I think that for the orchid-grower’s purposes it’s fine to think of it that way.

Right off the top, I believe your statement that plants absorb only 1-1-1 is incorrect. I’m quite sure you were trying to convey that the uptake is but a fraction of that applied - and I agree 100% - but when we talk about fertilizer ratios, we are not talking about uptake. That will vary, plant to plant and likely among nutrient ions, as well.

Secondly, your explanation about maximum uptake levels might be inapplicable, or at best, poorly applicable to orchids, and you misunderstood my meaning.

I was referring to what is applied to the plants, that is, the mass of contained nutrients, so a 5-5-5 applied 4 times is identical to a 20-20-20 applied once. Again, that says absolutely nothing about the rate of uptake.

Because of the network or fine roots and root hairs, most terrestrial plants are quite thorough in their uptake of nutrients contained in their potting media. As long as they are still in solution, the plant can take them up. With orchids, it’s a “whole different animal”, as the roots are not so extensive, root hairs are missing-to-minimal, and the potting media used have degrees of retention ranging from great to just about zero. Then there’s the “twist” in uptake dynamics added by the presence of velamen.

It is well established in the poinsettia trade that it takes one-half gram of nitrogen to take a plant from rooted cutting to salable for Christmas. By knowing the pot volume, media retention, fertilizer formula, fertigation frequency and time to harvest, the grower calculates the ppm N needed for the solution to be applied. With orchids, I believe it’s better to think in terms of probability. Keep in mind that we’re talking part-per-million in solution, not the weight percentages of the concentrated fertilizer.

Take the simple case of a vanda, suspended in air with no potting medium or container. When watered, pretty much all the plant gets is what’s immediately absorbed by the velamen. As the velamen has a fixed volume, my 5-5-5/20-20-20 comparison holds true, with the latter containing four times the nutrient level, so the velamen will absorb four times the mass of nutrients. How quickly the plant absorbs that is not relevant to the conversation. It will take up 100% of what it absorbed. If that wasn’t the case, there would be mineral buildup in the velamen that would eventually poison the root.

If we now take into account the variability in the solution-holding capacity of potting media and how quickly they dry, we have added more variables that prevent us from knowing what the true nutrient uptake will be.

That loops me back to my original points that when applying the same mix (g/L, tsp/gal, etc.) of a 20-20-20 or a 5-5-5 fertilizer, it is probable that the plant will absorb four times the nutrient mass with the former.

When we irrigate a potted plant with a gallon of 25 ppm N solution, do we have any idea how much of that is actually absorbed? No!

Your referenced ASHS article doesn’t address uptake dynamics at all. It compares fertilizer concentration effects and those of nitrogen form. It isn’t specifically mentioned, but I assume that if the studies are valid, the variations were all applied at the same frequency.

[hypostatic]

Originally Posted by Ray

When we irrigate a potted plant with a gallon of 25 ppm N solution, do we have any idea how much of that is actually absorbed? No!

Heh, I guess if anyone wanted to test this, they could take a leaf, grind it up, and measure the NO2/NO3 levels with a test strip; comparing pre and post fertilizing levels. Optimally you would want a time-curve of tests, to know when you'd want to re-fertilize.

I also guess that for any of this to be useful, you'd want to know what the optimal nitrogen levels in the orchid tissue should be...

[SouthPark]

hypo ..... that's true.

Tests could certainly be carried out. The results could become very useful - or of particular interest for nurseries with lots of orchids, aiming to produce consistently super healthy orchids in the shortest time they can. Or maybe even for some kind of competition growing.

I don't even know what's optimal for my own nutrition for my own body, let alone for my orchids hahaha.

I think that for a lot of the orchids we grow ----- as long as we give them something - nutrients, elements every once in a while, they can do just great.

Ray's comments about the various different fertiliser mixes, and the need for people to be aware of mixing - for the sake of getting their ppm levels correct (if that's what they're aiming for) ...... is absolutely right.

And definitely - having some idea about what is happening when we apply fertiliser to a media and state of roots (and location of roots) and other aspects ------ can be beneficial for orchid and grower.

[hypostatic]

Yeah, I'll look for some publications on the subject. In order for that information to be actually useful, we'd also want to know how the N levels affect growth and flowering.