Do Not Trust Your TDS Meter

Orchid growers are fairly unique in the horticulture world, as we often see recommendations for TDS or “Total Dissolved Solids” as a way to control our feeding regimens, while most professionals use nitrogen concentration (ppm N) and electrical conductivity, or EC. Why? Because the use of a TDS meter is inherently sloppy.

First, what does “TDS” or “total dissolved solids” mean? Simply, it is the sum of the minerals dissolved in a liquid. Looking at it more practically, if you allow the water to evaporate from a solution, the remainder is the dissolved solids content. It is often expressed a “parts-per-million” or just “ppm”, indicating a concentration equal to [so many] milligrams-per-kilogram.

TDS meter are really nothing more than inexpensive EC meters with a built-in conversion factor. Unfortunately, that is misleading because the TDS-EC relationship is not fixed at all, but is determined by which minerals are dissolved in the water, in what ratios and what concentrations. So, how about a “real life” example?

Many growers rely upon the original so-called “MSU Fertilizers” – “Orchid Special for Well Water” (MSUWW), “Orchid Special for RO Water” (MSURO), as well as “K-Lite Orchid/Epiphyte Fertilizer”, a derivative of the MSURO formula, all manufactured by Greencare Fertilizers and all made using the same raw materials. The table below contains some manufacturer-supplied data for solutions made with pure (distilled, EC=0) water:

Fertilizer Comparison
1.N (wt. %):1913.212.9
2.P (wt. %):431.3
3.K (wt. %):23151.3
4.Ca (wt. %):2810
5.Mg (wt. %):023
6.EC @ 100 ppm N (mS/cm):0.680.800.71
7.EC Ratio:11.1761.044
8.g/L for 100 ppm N:0.5260.7690.771
9.True TDS (ppm):526769771
10.Mass & TDS Ratio:11.4621.466

If we look at the electrical conductivities for the solutions @ 100 ppm N, a reasonable concentration for weekly feeding (row 6), then normalize them to that of the MSUWW value (row7), the EC of the MSURO and K-Lite solutions are only about 18% and 4% greater, respectively. If we used a “standardized” TDS meter to measure them, the TDS values would follow suit. For example, If you had the three formulas mixed to the same ppm N level, your meter might read 300-, 350-, and 313 ppm TDS, respectively

However, if we look at the mass of fertilizer needed per-liter to reach that 100 ppm N concentration (row8) – the true dissolved solids content (row 9, expressed as ppm) – and again normalizing for the MSUWW formula (row 10), we see that the TDS of the two other formulas are actually 46% greater.

Don’t throw that meter away! By using known fertilizer concentrations, you can create your own “calibration curve” for each formula, allowing you to use it to check the concentration of your fertilizer solutions.

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Using Science & Logic to Improve Orchid Growing