For hydroponic growing, knowing the amount of nutrients (or dissolved salts) in the nutrient feed solution is critical to success.  The amount of nutrient (or dissolved salts) in water or Nutrient Strength of a solution can be measured and expressed as Electro-conductivity (EC) usually expressed in millisiemens per centimetre (mS/cm), parts per million (ppm) or in conductivity factor (cF). 

                    1mS/cm = 10cF = 700ppm (or500ppm)*

The easiest way to do this is with the use of an EC or Nutrient Meter.  In order to get the most from your meter it is advisable to check its accuracy and recalibrate it often (fortnightly or when required).

For ease of growing multiple plant types together, plants can be categorised and grouped as low, medium or heavy feeders. In order to maintain the optimum growth rates of your plants and to maximise both the quality (size, taste, colour etc) and quantity of produce, it is advisable to maintain your hydroponic system in the correct nutrient range for the plants that you are growing.

Generally young, tender plants and seedlings should be established in a hydroponic system at low nutrient strengths (i.e. ½ strength) and the nutrient gradually increased over time to optimal levels (full strength) as the plant establishes itself in the hydroponic system and grows.

 

* ppm is only an approximation and should only be used in relation to the measurment/expression of single a element.  As a nutreint solution is a complex mixture of elements/nutrient a correction factor of 700 or 500is used.  As a result it is generally better to use the international standard unit (SI) of Electricl Conductivity (EC).