There are several ways of measuring the concentration of nutrient solution. In the past, growers measured every primary nutrient individually. Now we feel that, in most cases, simply knowing the total nutrient level is sufficient. Our experience shows that if you start out with the right fertilizer mix, all nutrients will be used at the same rate. This permits the use of either of the following simplified measuring methods. These methods will not be accurate if your nutrient solution is to old, as evaporation will "distill" pure water leaving concentrated salts which may not be in the proper proportions. For this, and other reasons, it is suggested that the nutrient solution be changed every two weeks. It is critical not to exceed the concentration required, as too much fertilizer can be worse than not enough.

Electrical Conductivity (EC) measures the rate at which a small electrical current flows through a solution. usually measured in micromhos per centimeter (umhos/cm) or millisiemens per centimeter (uS/cm), 1.0 mS/cm.

Total Dissolved Solids (TDS) measures the dissolved solids in water using conductivity. Measured in milligrams per liter (mg/l) or parts per million (ppm), 1.0 mg/l = 1.0ppm

General Guidelines for EC and TDS

Stage 

Fruiting Plants 

Leafy Plants

EC umhos/cm

TDS (ppm)

EC umhos/cm

 TDS(ppm)

Seedling 1600 -1800

1120 - 1260

1400 - 1600

980 - 1120

Average

2500

1750

1800 

1260

Fruiting  2400 - 2600 

1680 - 1820

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