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The Ideal Fertilizer Concentration

Sorry to break it to you, but there isn’t one!

Sure, there are LOTS and lots of recommendations out there, based upon everything from personal experience to university studies. I recommend 75-100 ppm applied weekly, or 25 ppm N for plants fed daily and those in Semi-Hydroponics. The study done by folks at Michigan State University with their now-famous “MSU Fertilizer” recommended 125 ppm N, and another study on phalaenopsis production done at Texas A&M concluded 150-200 ppm N was best.

What do they all (even my own “optimal growth regimen“) have in common? “They tried it and it worked!

All of those recommendations are merely concentrations of applied solutions, not the mass of nutrients taken up, and we must not forget that the solution concentration plus the frequency of feeding plus the volume used, all combine to give us a mass of applied nutrition. Unfortunately, that still says nothing about the mass absorbed by the plant, and that’s what’s important.

If we do the calculations related to the chemical processes that lead to carbon fixation, and in order for a plant – any plant – to add a pound of mass, it must absorb and process about 200 pounds of water, but only 5 grams of fertilizer. In terms of what we find in tissue analyses, that would suggest only about 4.5g N and roughly 0.5g of everything else.

How long does it take a plant to gain a pound??? If we’re talking about corn, maybe a few weeks in summer. For a mature cattleya or vanda, maybe a year or two, while a phalaenopsis might take 3 or 4 years, and a tiny pleurothallis – a lifetime!

The ability of a plant to absorb nutrients is related to the time of exposure to the fertilizer solutions in direct contact with the roots. Let’s do a little bit of theoretical calculation to demonstrate that:

A vanda in a slat basket with no medium, with its roots wetted, can only absorb what hits the roots and is immediately absorbed by the velamen. If those roots are 3/8” in diameter and the velamen is 1/8” thick, each inch of root length has 0.01943 in3 of velamen (let’s call it 0.02 in3 for convenience) which is about 0.32cc (cubic centimeter or milliliter). Let’s also assume it can absorb 100% of that volume (it’s bound to be a bit less in reality).

A 25 ppm N solution of K-Lite requires 0.1925g fertilizer per liter, meaning there is 0.0001925g in a cc. That means that each inch of the vanda root can absorb 0.0001925 x 0.32 = 0.00006g of fertilizer. A mature vanda might have what? 20 feet of roots? If that’s correct, at each watering, it can absorb 240 x 0.00006 = 0.015g of fertilizer. (That assumes 100% of the liquid is absorbed with no evaporation.) If growing a pound requires 5g of fertilizer, that would mean that this vanda should do that in about 333 waterings. Having grown vandas and watered them every other day or so, that would suggest about 2 years of growth, which might not be such a bad estimate.

If we now shift our thinking to the opposite end of the spectrum – a phalaenopsis with 12″ of roots and similar velamen development, grown in Semi-Hydroponics – it only has 5% of the root volume for absorption, but staying moist, it can do so more-or-less continuously, limited by the dynamics of uptake and distribution – two more things we don’t know!

The bottom line is that there is no “ideal” fertilizer concentration. What the grower must do is seek the combination of factors that gives the plant at least “enough” of everything, without overdoing the nitrogen, as that can quash blooming.