At work I’m on a mission to better utilize our waste nutrient water. The latest tool acquired for that mission is the light green nitrate meter in this set. The purple potassium meter is also on order.
I took it out of the packaging yesterday, squinted at the instructions that come in seven different languages printed on crinkled light weight paper, calibrated it with the sleek little bottles of calibration solution, and checked the nitrate level in one of our recirculating fertigation tanks. 730ppm NO3-
I was confused. According to my Mega-Fancy Fertilizer Spreadsheet, I expected that nutrient solution to be at approximately 170ppm N, since the EC was 1.7 I checked it with the nitrate test strips–the reading was off the chart, above 500ppm. Had my spreadsheet been grossly wrong for all these years?!
I went and measured another familiar solution, a “high N” feed at EC 1.6. Again, the answer was way higher than I expected, 640ppm nitrate. I pulled out a text book and looked up that calcium nitrate mixed at 0.84g/L should make a solution with EC 0.8 and 100ppmN, then went to the fertilizer room to measure out the solution. The tester showed 440ppm nitrate. Eh??
Back at my desk I phoned the guy I bought the meter from. I must be doing something wrong, but I wasn’t sure what. We talked about calibration, checking the meter in clear water, using a standard solution….nothing was ringing a bell, until I looked at the container of test strips I was using as a comparison. There were two scales to read: nitrate-N ppm or NO3-ppm. I hadn’t registered that there was a difference, but the scales had different numbers–500ppm NO3- equals 113 ppm nitrate-N. That’s when I figured out my error. My fertilizer spreadsheet is calculated in N mg/L, which is different than NO3- mg/L by a factor of the molecular weight of 3 oxygen molecules. I glanced over at my nerdy wall poster of the periodic table of elements. A molecule of NO3- weights 62g/mole while N itself only weighs 14g/mole, so ppm calculations in NO3- are 4.42 times higher than those in solo nitrogen molecules.
Adjusting for the extra weight of the oxygen, suddenly all the numbers made sense. My Mega-Fancy Fertilizer spreadsheet was still telling the truth, albeit in a different language. SUCH a relief.
Delight of the Day: Chemistry WORKS. Math WORKS. And so does my nitrate meter.
Ah yes! The difference between elemental nitrogen contained, versus total nitrogen-containing compound. We get that in medicine too — pills of ferrous fumarate may be described as mg of elemental iron provided, or mg of total ferrous fumarate provided.
I’m very nerdily impressed that you had a wall chart of the periodic table on your wall at work already.