Quote:
Originally Posted by bpolky
Have I made any blatant errors in that calculation?
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In the calculation, no. You have, however, probably grossly overestimated the nutrient concentration by assuming that all of the fertilizer TDS is N, P, & K.
As an example, the MSU fertilizer for RO water is 13-5-19-8Ca-2Mg, plus trace elements. At 125 ppm N, the solution has a TDS around 1000 ppm.
All cations and anions contribute to the net electrical conductivity of a solution, and a TDS meter is just a cheap EC meter with a built-in conversion to TDS (making the meters less than reliable for actual levels).
You must "calibrate" your meter with solutions of KNOWN nitrogen concentrations if you want to be reasonably correct in your estimates.
It's easy - get three, 1L containers of your plain water @ 50 ppm TDS (as you stated). Add 0.5 gram of your fertilizer to one and 1g to another. If the fertilizer is 20% N, then you've added 0.1 and 0.2 g N, respectively, or stated another way, 100 mg/kg and 200 mg/kg of solution, which is the same as ppm. {Note: my fertilizer additions were chosen arbitrarily, and I have rounded the "per kilogram", as technically it should be 100/(1kg water + 0.5g fertilizer)}
So you now have three solutions - 0, 100, & 200 ppm N. measure the TDS of each, plot it on a graph versus the N concentration, and you're all set.
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