Quote:
Originally Posted by Dusty Ol' Man
I understand the conventional wisdom. What I'm looking for is a reaction to the information given in the video. As always, thanks for your response. Your experience is always taken into account.
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First a general observation: That is a 25-minute long video. Rather than asking people to dedicate a half an hour to give you an opinion, it would be smarter and more considerate to state what you learned from the video, giving people the info right off the top.
That said, I used the "transcript" feature to pluck out some of the info, so I hope I captured the stuff you were interested in.
The basis of the discussion was varying the amount of fertilizer applied based upon the size & growth rate of the plant and the cultural conditions. As all of those plants are in what was stated as "semi-hydroponics", but drying out was mentioned as one of the control factors, I first must question whether the technique is S/H or just traditional culture using LECA.
My second issue is the use of "ppm". It is never stated if that is ppm N, true ppm TDS, or measured ppm TDS. Considering the magnitudes presented, I'm going to give the author some slack and assume it's true TDS, as if it's ppm N, the regimen is called "plant poisoning", and if it's a measured TDS, the numbers have nothing more than relative value as we cannot trust the magnitudes at all.
The premise has three points to consider when deciding how much to feed:
- A plant with a big mass needs more fertilizer than does a plant with a small mass.
- A plant that is fast-growing needs more food than does a slow-growing one.
- A pot that dries out fast will need to be watered more, so the fertilizer concentration should be decreased to compensate.
My first reaction was "No sh*t, Sherlock", as all three are valid. If you have read my article on "
Support the Reserves", I describe about resource demand being needed in three primary areas - maintenance (staying alive), tissue addition (growth), and reproduction (flowering - I'll ignore this one for now, as it wasn't part of the video I saw). Summarizing that to bullet points:
- A plant with more mass has more tissue to keep alive, so needs more nutrition.
- A plant that is adding tissue faster needs more nutrition.
So... The video, on the surface, is acceptable and not greatly in conflict with "conventional wisdom". (If you think differently, Dusty, tell me what I missed.) It does, however, seem to miss a lot that could be useful.
The starting point is feeding "100 ppm". What was the fertilizer used? Using the MSU RO formula as an example, that suggests about 12 ppm N, which is quite low. If the stated levels are ppm N, it's far more appropriate for weekly feeding. Plus, a different fertilizer will have different N levels.
It was also stated that "300 ppm" is the upper limit for large, fast-growing plants. Applying the same factual criteria as my last paragraph. That's either still pretty low (36 ppm N) or too high.
Then there's the linear additive logic. It weas stated that if you water the plant with a 300 ppm solution twice in a week, you're giving 600 ppm. Not really - and this is an area where my "I need to quantify it" brain struggles.
Ppm is a concentration, not a mass. One liter of a 100 ppm N solution contains 100 mg of N. I will pour less volume into a small pot than I will into a large pot. OK, great - the large pot got more N than did the small one. Do we measure the volumes we apply? Probably not.
Even if we did, we know most of it pours right through, so what is the retained volume? That's what determines the mass of nutrition in the pot. A few years ago I measured the retention of several LECA brands, and it ranged from 16% to 25%, but what is it for sphagnum, bark (which one, which size, and what is it mixed with, and what do they retain?), or other potting materials?
Then...what percentage of the retained solution (and mass) is actually accessible by the plants' root systems? Unlike most terrestrial plants, whose hair-like root system can fully fill a container, orchid roots are few and thick, taking up a relatively small percentage of the container volume. Unless the solution is in direct contact with the root system, it is not absorbed so contributes nothing to the plant. Just think about THAT for a moment.
A bare root vanda has a certain volume of velamen on its roots. That determines the volume it can absorb per feeding. Period. That's it. If those roots are 3/8” in diameter and the velamen is 1/8” thick, each inch of root length has 0.01943 cubic inch of velamen (let’s call it 0.02 for convenience) which is about 0.32cc. Let’s also assume it can absorb 100% of that volume (it’s bound to be a bit less in reality). If the plant has 10 feet of total root length, is can absorb 120 x .32 = 38.4 ml of solution, or 0.0384 liter. That 0.0384 liter of a 100 ppm N (100 mg/kg) solution contains only 0.00384 g of N.
Contrast that to an oncidium in a pot - many more, much finer roots with thinner velamen. That volume of velamen (you do the calculations; I'm not bothering) determines what its instantaneous absorption volume is, as well, but the medium can wick nutrient to the roots over time. It's never going to be 100% of the medium retention, but it'll be
something.
Trying to quantify feeding to such detail is mind-exploding as there are just too many unknowns. That's why I have adopted a more semi-empirical approach:
I have learned over the years that targeting for 100 ppm N applied weekly is pretty good. I know that a smaller pot will retain less than an bigger pot and I know that a bigger, faster-growing plant will need more food than will a small, slow-growing one. I also know that temperature and light levels affect growth and plant metabolism in general. So, I just diligently observe my plants and make a judgement call as to whether they need to be watered or fed - and a lot of that is "automatically" controlled by the light and temperatures. If one dose, they all get it.
Increasing fertilizer?
Linear Mode
