I've wondered about this question too. With ordinary plants in potting soil, the risk of over-fertilizing is that as the water in the soil is taken up by the plant or evaporates, the concentration of fertilizer salts can increase greatly. At some point the osmotic pressure of the water and fertilizer surrounding the roots can interfere with water uptake by the roots and possibly damage the roots. With mounted orchids though, I haven't seen any information on what happens physiologically when watering with fertilizer. It seems three things are going on: (1) the roots are absorbing water into the velamen, (2) the roots are absorbing fertilizer ions, and (3) water is evaporating into the surrounding air. I would assume that you would want all of the fertilizer to be absorbed before all of the water on the surface of the roots is gone. Otherwise there would be a thin, dry coating of all the residual fertilizer on the roots. So the question is how fast roots can absorb fertilizer - anyone know? It would seem that for that reason, it would be bad to use any fertilizer at all when the air is warm and dry, since the water on the roots would evaporate too fast.

I bought this one. It was about $55 after s+h. It's quite cheap relative to others, and comes with batteries, but it's not clear how I'll be able to change them out when they die, since it seems to be a solid unit. Here's a picture. Oh well, I'm sure it will last a few years anyway.

from a well known hydroponic magazines website whose name is prohibited from being spoken here, I read "EC, measured in mS/cm, milliSiemens per centimeter" and from here i read "A new term for millimhos is millisiemens, which is the metric (SI) unit of expressing electrical conductance. There is no change in value, just terminology. Therefore you would shift the decimal place in your 2300 three places to convert it to milliimhos and the reading is 2.3 EC on the truncheon. Which is 23 on the CF scale. hope that answers your question. Just FYI, the google search words were "EC conversion" and "EC to CF conversion"

"weights are not percentages" Just checking up on what you meant. You meant: "fluid ounces are not weights" (or "fl. oz. are not mass" for nerds)? I'm sure you have it all figured out by now, but just making sure. Let's make this post the more important one to save for practical reasons of your 7-7-7, if you convert fluid ounces of water to avoirdupois (weight, always 28.3495 g) ounces, there are 1.04318 ounces (weight) in one fluid ounce of distilled water which weighs 29.5735 grams. However, being a denser material, one fluid ounce a bottle of Miracle Gro 7-7-7 African Violet fertilizer concentrate weighs 35.5 grams. Alternately, one standard teaspoon of it weighs 5.915 g. The dosage of 7-10 drops in a quart, say 8.5 drops, is 0.425 mL, or 0.51 grams the way I measure drops, although my 8.5 drops from a standard dropper only gives 0.0719 of a teaspoon, not 0.125 of a teaspoon (1/8 tsp) like yours does. For my drops, the ppm of nitrogen in a quart (0.95 L) for Miracle Gro 7-7-7 Nitrogen is 0.07*510 mg/0.95L, or 38 mg/L = 38 ppm. As you study this, you can write off the difference of 38 ppm N with my dropper, to actually 66 ppm Nitrogen if in fact you are putting 1/8 teaspoon of fertilizer in there. This is why using drops and teaspoons is hairy and grams are the way to go. 38 ppm vs. 66 ppm ... which is it? Ask the dropper? Reminds me of an old joke on why certain people think (hold index fingers 6 inches apart) is more inches... The eye is pretty good to reproduce the quantity, but if the calibration is off, manufacturers, machos, and cheating businessmen can use the confusion to abuse, exaggerate and lie, which assists them in the bottom line of their endeavors... BTW, the US was primed to switch to the metric system in 1927 and would have and this would all be a big joke today. It was not a revolt among the population at that time that prevented the switch. It was industrial lobbyists for big business in part saying it would cost them too much, and secretly of course, not wanting the public to have transparency in measurement since universal transparency in measurements allows you to easily swap one product for another. This is one big reason we see "trade secret", "proprietary", "art", inflated, misrepresented, abusively truncated, and other deceitful practices on fertilizers. It is so interesting that the 7-7-7 are percents by weight you are being sold, but, no where on the bottle will you find the weight you are buying. If an alien came to Earth to understand our customs, they would go away scratching their butts if that were their custom to express being perplexed. On the bright side of things, the bottle does have a good conversion into "onzas fluidas", which nobody on the planet has ever used except at gunpoint. This post was edited by PupillaCharites on Thu, Apr 17, 14 at 15:45