Ray wrote:
(This one is probably semantic) Seems to me that an EC meter is measuring the conductivity of a solution, not of the components of it, per se. Maybe true salts affect the conductivity more than other minerals, but they are all involved in the solution conductivity to some degree. Magnesium carbonate is not a "salt" but does affect conductivity.
I agree with you so it must be semantics.
Ray wrote:
"Ingredients like urea do not conduct at all" ??? It hydrolyzes upon contact with water, and decomposes (with time and temperature) to release ammonia and carbon dioxide into solution. Also, if it does not conduct at all, that would suggest that no matter how much you put into solution, a TDS meter would never indicate its presence. Is that right? (Doesn't seem so to me, but I'm only guessing...)
Since I am just your average orchid grower, I go by what I read. In this case I will go to Hydroponic Food Production, Howard M Resh Ph.D, page 110. This book is considered the bible by hydroponic growers.
"However, the electrical conductivity (EC) varies not only to the concentration of salts present, but also the chemical composition of the nutrient solution. Some fertilizer salts conduct electric current better than others. For instance, ammonium sulfate conducts twice as much electricity as calcium nitrate and more than three times that of magnesium sulfate, whereas urea does not conduct electricity at all. Nitrate ions do not produce as close a relationship with electrical conductivity as do potassium ions (Alt, D 180). The higher the nitrogen to potassium, the lower will be the electrical conductivity values for the nutrient solution. Electrical conductivity measures total solutes, it does not differentiate among the various elements. For this reason, while a close theoretical relationship exists between TDS and EC, standard solutions of a nutrient formulation should be measured to determine their correlation in a given solution."
Reading a good section of this book you can see that there is a distinction between TDS and EC. The two are not the same and can vary widely depending on the solution tested. With most fertilizer though you can get a good approximation of TDS from EC, especially in using a "standard solutions of a nutrient formulation should be measured to determine their correlation in a given solution."
As to urea hydrolyzing upon contact with water and decomposing, I am sure some of this does happen and definitely increases over time, but you have to remember that urea does not conduct electricity. This implies in my mind something and I am fairly sure of it, but I am not going to state it without doing more reading in various books which I don't have time to do right now. But I can fast forward a bit here and say that urea compared to other nitrogen sources found in fertilizers passes the easiest through plant tissue because it is neutral. The urease enzyme is the primary means for the release of nitrogen from urea for use by plants.
Mark Sullivan
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