Help with TDS and PPM

[astrumes]

So I have been using Dynagro Orchid pro as the main fertilizer for my orchid collection. I have had good results so far since I started to use a nutrient solution on my plants (after repotting into s/h I only use a weak root solution on my orchids for 1 month) but according to Ray's website, I've been using too little of it in my mix and actually starving my orchids.

I've been looking at the TDS ppm reading, keeping it in the 80-100 range in my Reverse Osmosis water (12ppm). After looking through some posts about nutrition I realized my mistake when I saw the recommendation was for "100-125 ppm N" as opposed to total ppm.

I went back to Ray's site where there is a calculator for how much fertilizer to use based on the Nitrogen ratio of my Dynagro. That calculator suggested using 1.3 tsp per gallon to get the right N ppm.

Since I have been using such a weak solution until now, I decided to take things slowly and only use about half the suggested amt, mixing 1/2 tsp of Dynagro in a gallon of water. I was shocked when I measured the solution at 480-490 ppm. I read Ray's ppm is about 350-475 ppm TDS at every watering (using different fert and water most likely). Mine seems quite high for being at what is supposed to be half strength.

My question then is, is it safe for my orchids? I don't want to burn any roots but I also want to make sure my orchids are getting the nutrients they need.

Should I continue to use my Dynagro at this ppm or would it be safe to use the full dosage as recommended by the website? I would assume that at full strength, my nutrient solution TDS would be closer to or over 1000ppm. That seems high to me but I'm new to semi-hydro growing.

Any input or advice would be welcome. Does anyone use this fertilizer in their setup? If so at what TDS value?

[Ray]

How much to feed and how often are closely related. Think of the "ppm N" as being analogous to calories in our own diets. If you eat one meal a day, you probably want 2000-2500 calories in that meal. If you eat three decent meals a day, they should each average about 800.

When the MSU fertilizer was released, the developers recommended 125 ppm N, but frankly, didn't specify a frequency, and many - including me - started applying that much too frequently.

(Here's where things get a bit screwy, as I equate a concentration with a mass, but I'll explain.). With more research and experimenting, I have concluded that something more on the line of 50-100 ppm N applied over the course of a week is better for my plants. By that I mean that if I fed once per week, I'd use a concentration of about 75 ppm N. If I fed three times a week (i.e., at every watering), or the plant had a reservoir of fertilizer solution to draw from, I'd only use 25 ppm N.

An easy way to estimate ppm N from the formula is to divide 2 by th %N on the label - the result is teaspoons per gallon for 25 ppm N (2.3/%N = ml/L for that same concentration). For D-G Orchid Pro, 2/7= between 1/4 and 1/3 tsp/gal.

Mix up your solution, THEN measure the TDS if you want to use that to control it, but if you change fertilizer, you'll need to "recalibrate" that TDS reading.

[astrumes]

Originally Posted by Ray

...I have concluded that something more on the line of 50-100 ppm N applied over the course of a week is better for my plants. By that I mean that if I fed once per week, I'd use a concentration of about 75 ppm N. If I fed three times a week (i.e., at every watering), or the plant had a reservoir of fertilizer solution to draw from, I'd only use 25 ppm N...

...Mix up your solution, THEN measure the TDS if you want to use that to control it, but if you change fertilizer, you'll need to "recalibrate" that TDS reading.

Thank you for the updated information about the nutrition!

So according to your experience and applying to my situation, where I flush my orchids once every 7-10 days and leave the solution in the reservoir, I would be better served using a nutrient solution at 50 ppm N, and perhaps more like 75 ppm N during the growing months?

I appreciate the note at the end. I keep a journal since I rotate flushing days depending on their location in my home and its easier to keep track of all the info by using 'ppm' than by 'tsp' since I tend to make little mistakes here and there.

[Ray]

As each person's environments are different, you'll have to arrive at your own comfort level, but I think something around 50-75 ppm N is a good starting point.