New Grower What do the ppm's in runoff water mean?

[turtle360]

Is it valid to measure runoff with a meter like the HM Digital TDS EZ to determine how much charge is left in the soil? I measured runoff from a 3 gallon container of Happy Frog soil. The soil was cut with perlite so it wasn't pure soil. The plant was in it for a month with only one dose of SensiBloom at half strength after 3 weeks. Fox Farm claims HF feeds for about 3 weeks. So, after a month in the soil with one 1/2 strength feeding I watered with 130ppm water (Calmag) and the runoff came out 1200ppm, 1070ppm over what went in. This strikes me as high. Questions one might ask:

Is it a valid measurement to determine nutes in soil?
If Happy Frog sample out of bag just measured 1040ppm how can it be essentially the same after a month? Is it substances still breaking down or something?
Did I get a hot bag?

What does all this mean?

 

[Dr Who]

The measurement is based on conductivity, and the calibration to ppm is usually based on CaCl2. If the dissolved components of the in going fertiliser solution differ from the leachate then the conductivity of the solutions will differ, and you'll get different ppm values.

I'm not sure that the measurement of the soil tells you anything useful though, as you are only measuring the soil solution which depends a lot on the ratio of soil to solution. Whatever you think you have measured in the soil it certainly isn't the nutrient concentration.

 

[turtle360]

Whatever you think you have measured in the soil it certainly isn't the nutrient concentration.

That would be the bottom line. Could be dolomite lime and other stuff breaking down I guess.
Anyway, thanks for the reply. I know it's not a fascinating subject if you know what I mean.