TDS, pH, and Fertilizer

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I know there are many threads that deal with these subjects and I have read all of them that I could find. I think I have collected all the data that will allow for informed judgements about my situation.

My TDS tester, an Oakton, allows you to set the EC to TDS conversion factor from 0.4 to 1.0. I set it to 1.0 as it seemed to me to be the logical choice. (Only after I had finished fertilizing everything did I pulp up this link again and see that they recommended a conversion factor between 0.50 and 0.72. http://

My tap water: ppm 70 and a pH of 8.89.
¼ tsp of Dyna-grow 7-9-5 = 160 ppm & pH of 6.73,
½ tsp = 230 ppm & pH of 6.45
1 tsp = 450 ppm and a pH of 5.91

Thus, by my math (and I could be wrong) ¼ tsp = 6.3 ppm of N, ½ tsp = 11.2 ppm, and 1 tsp = 26.6 N. The recommended rate on the bottle is ¼ to ½ tsp per gallon of water.

I used 1 tsp. Because I have read several post where when using MSU fert people were shooting for 125 ppm of N. Based on this, and I realize that ferts are different, I thought all of the values for N were way to low. Though even at 1 tsp per gallon the N is way below the 125 ppm of N that seems to be what people are shooting for when using the MSU fert. Are my plants sitting there burning up? But if N is one of the main three ingredients that plants use in making food, do you not need the same amount regardless of which fert you are using? There is obviously no way you can get to 125 ppm N using this fert and stay under the 500 ppm which many of you seem to think is the max ppm that should be used when fertilizing orchids.

Your thoughts please.

Most of you seem to be saying that a pH of 6.5 is ideal for non-calcereous phaps and for those that are calcereous the ideal would be between 7 and 7.2. Can baking soda and vinegar be used to modify the pH of a fert solution when watering orchids? I see many that are using “Pro Tekt” to move the pH up. What do you use if the pH needs to be lowered? Do any of you know what kind of pH Phals. like? What about Masdevallia?

Now even this small bottle of Dyna-grow is almost new, if I need to be fertilizing at a nitrogen ppm of 125, I will order some MSU. Your thoughts. By the way does anybody know the recommended N feeding level for Phals?


Well-Known Member
Jun 9, 2006
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Elk Grove, CA
You've asked many questions and It's still early and I'm kind of brain dead...

So, I'll pick one topic.

I fertilize all my orchids the same. The all get the MSU at 125 PPM N. I do flush my dendrochilums and phrags more because they seem to be my most salt sensitive plants. But phals, vandas, catts, paphs, phrags all get the same amount of fertilizer. If you've got a small collection you could probably tailor your fertilizer regime a little more, but when you've got a somewhat large assortmant, as I do, I'd go insane if I had to treat them all differently.


Well-Known Member
Sep 14, 2006
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New York City Apartment
Yep too much info. I mix my fertilizer as specified than add a couple of drops of Superthrive and a teaspoon of Protekt silicon formula/gallon of R.O. water. When I water my plants I mix roughly 50:50 fertilizer water:R.O. water, to further dilute the solution as I water so much. I adjust the ratio depending on which plants I'm watering. Paphs - more dilute, Phals - more fertilizer water, etc. BTW, I don't worry about pH, and I have a TDS meter but got so confused about readings that I just compare the reading in a container of solution to what I get in my growing trays once and a while so see if they're close. :)

John D.

I'll try to explain this the way I think of it for my simple mind. Your meter is measuring resistance to electrical current flow through the liquid. The more disolved salts the easier current flows. Each cehmical is a little different so a mixture is not the same as a single salt at the same concentration. That said it does not have to be too exact. your 7-9-5 is one third nitrogen so the 450 ppm at 1 teaspoon per gallon is about 150 ppm N. I would stay in the one half to three quarter teaspoon per gallon range and then adjust if you are not happy with the results. A pinch of ground dolomitic limestone can help buffer the low pH and add a little Ca and MG as well.

Bob Wellenstein

your 7-9-5 is one third nitrogen so the 450 ppm at 1 teaspoon per gallon is about 150 ppm N

No, no, no. This is wrong. 7-9-5 will contrain on a weight/weight basis 7% N, 3.9% P and 4.3% K.


No, no, no. This is wrong. 7-9-5 will contrain on a weight/weight basis 7% N, 3.9% P and 4.3% K.

So Bob what part of the 380 ppm (450 ppm water & fert - 70 ppm water) is nitrogen?:confused:
Just read your article again, so if I knew the approximate weight of a teaspoon of water I could get close though to get accurate I would need to know the weight of a teaspoon of the fertilizer liquid.

OR from Bill Argo's article 125 ppm desired x 1 gal= 125, 7% x 75 =525, 125 /525=.238 oz. and given 1 tsp is about .2 oz the ppm of nitrogen in my 1 tsp mix was a little more than 125 ppm N.

I'm :rollhappy: at least until you tell me I'm wrong.

Here is the full analysis:


Thanks for your time.
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