One simple way to track your fertilizer use / injector is to make a "hand mix" of your concentrate and your irrigation water at whatever ratio you always use and then stick whatever meter you plan on using in it. Regardless of whether it is a TDS or an EC meter you will have a benchmark to keep track of your mixing / injecting for all future feedings. If you want to know what is really in there you can send it off to a lab (A&L is a good one) and they will give you a precise breakdown in ppm.
I have a simple Excel spreadsheet that I use to calculate the resulting ppm for any fertilizer I might want to make that saves a sample to the lab. You have to know the ppm of any salts already in your water before you can use it though.
In essence the simple TDS/EC meters are only useful for tracking your data points to see if things are still on track. If you don't know where you are starting from (or what your goal is) they don't tell you much.
Paul LeBlanc
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1. The point brought up by Mark does have merit; different ions have different "mobilities" in an electric field, hence different conductivities. So what exactly does measuring EC tell you about the effect of a given solution on orchids?2. Technically, salts by definition include all substances, other than acids and bases, in which the charges of the particles [i.e., ions] balance each other. Magnesium carbonate is a perfectly good salt even if it appears to the eye to lack crystallinity.3. Urea is very stable in water. Solutions of it decompose when ubiquitous bacteria containing the enzyme urease get into them. Urea can cross cell membranes not only because it is an uncharged molecule, but also because it is small enough. Glucose, which is also neutral but about three times the size of urea, cannot.4. You orchidophiles have strayed into territory best left to those with technical backgrounds, but what does all this have to do with orchid culture?
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