I was amazed when I looked into a recipe that is use since years and obviously very successfully with commercial Lettuce NFT growers here in Thailand. It's simple, uses very little Fe, P, Ca and Mg. The rates that farmers use here start from 1.2 mS/cm (EC) in summer, to 1.8 in winter (in the Northern regions). The absolute max they recommend is 2.0 and actual toxicity starts around 2.4 with their formula, as they report.

Don't take this personally scotty, but anyone reading this, please listen to this for God's sake: ALWAYS look first at, use and tell conversion rate with PPM recommendations, otherwise it could either be actual elemental ppm, 500, 700 conversion rate or else - thus it is not a factual or reliable info and creating confusion only (for yourself and for everyone else). If you don't know if it's actual ppm or can't tell the conversion rate that goes with it, best is to simply IGNORE it like royalty and leave it out of any equation, Thanks. Only mS/cm (commonly called EC) is the unit that is unmistakable.

Well, there always is the First Amendment to the United States Constitution that is sometimes thrown in, but isn't it often overruled by the old english principle, still in vigor: Thou shalt nÄht gather and distribute any confusing information LOL.

I feel like giving an example how the differences with "PPM" actually look in practice. To demonstrate this, I simply take the example from the lettuce formula I was talking about in the previous post:

I actually compose this formula by components, that is why and how I know how much actual ppm and even how much the EC of each component is (because I measured them one by one in advance). Hence I know the total elemental ppm and the total EC of the formula. Nevertheless and not wanting to brag around here or confusing anyone, here is what the formula and the "PPM" look like on paper:

The formula has a total of 488 elemental ppm (as a result of the total of the single elemental ppm of NPK, Ca, Mg, S, plus actually 3.5 ppm of trace elements, which I leave out of the equation here.

The electrical conductivity of these 488 elemental ppm containing nutrient solution is almost exactly 1.27 mS/cm. Why? Because the components that are used have different EC readings per gram/l and as I know them one by one, I only have to make the total.

But if I convert as it happens with any TDS meter that uses 500 conversion rate, I will get a 633.3 ppm reading. Thus, using any TDS-meter that uses a 500 conversion rate, my nutrient solution (of 1.27 EC) will read about 630 ppm. Why? Because this instrument actually reads the 1,27 EC and eventually multiplies it with 500. Hence displays ->1.27*500=633.3

But If I convert to 700 conversion rate, or in other terms: if using a TDS-meter that uses a 700 conversion rate, the same nutrient solution (of 1,27 EC or mS/cm) will approximatively read about 880 (886) ppm. Here it will be converted by this type of meter as follows: ->1.27*700=886

A HUGE difference between 488 elemental ppm and 886 converted ppm by one of two main standards, that you can say!

That's why you best stick with elemental ppm for nutrient making and formulas with SINGLE elemental data ONLY. Aware of the fact that these are elemental ppm and NOT those of any TDS-meter reading. And for concentrations you either stick with mS/cm (as in EC) that you get from converting "your converted" ppm back into electrical conductivity (where they actually belong). Or, - if you simply can't give up using "ppm" as your favorite unit, - at least and ALWAYS ADD the conversion rate that your meter uses or was used when measuring, -in brief: which PPM you are actually talking about in any context.

Thanks for the understanding!

And again, if you encounter any recommendations in PPM, by whomever WITHOUT any specifications if it's elemental ppm of a formula or without the specific conversion 500/700/640 rate in vigor, IGNORE them as they are not worth the trouble and the headache they may cause you and others!

Thanks lucas. I haven't taken any of this personally. Don't worry about that at all. The more I dig through the commercial/professional community, I hear more about what you're talking about. The guy I purchased my commercial nutes from (BTW - MUCH more resonably priced) almost scolded me for even asking about ppm:) That's fine with me. I am looking for solutions and these are the topics that lead to those solutions. I plan on picking up and EC meter today.

Any recommendations on the better EC meter to purchase?

Thanks again!