The Hydroponics Manual: Data Sheets: Conversion Charts

In science, to be totally correct, we would refer to the "molecular weight" (based upon the "atomic" weight) of an element in solution, expressed as "molecular" or "molar" volume (1 × Mole = 1000 MilliMoles and 1 × MilliMole = 1000 MicroMoles). Expressions such as PPM or mg/l [milligrams per litre] or µg/l [micrograms per litre] can be converted to mole equivalents. The table below shows the huge differences between the various elements:

The plant does not distinguish the different nutrient elements' strength (conductivity), therefore given advice to grow a particular crop at a specific PPM/temp, the best idea is to convert that figure to a conductivity value and continue to monitor the total soluble salts in the solution and dose additional nutrients upon the basis of any shortfall indicated by temperature compensated, conductivity readings. The book "Handbook To Nutrient Solutions" by Carl Barry gives a clear understanding of these and other matters pertaining to good hydroponic growing principles.