Stocker Horticulture and Hydroponic Supplies Ltd, Tirau, New Zealand

Frequently Asked Questions

Testing Nutrient Solutions

CF Test

One method of checking the strength of a nutrient solution is to measure how well it conducts electricity. Although absolutely pure water is not a conductor, the flow of electricity increases proportionally as salts are added. So measuring electrical conductivity (EC) is a direct indication of the nutrient level in a solution.

A CF (Conductivity Factor) meter determines conductivity by measuring the amount of electricity that passes between two electrodes placed in the solution. CF meters designed for use with hydroponic systems usually have a scale of 0-100 CF units. Nutrient level is also often expressed in Parts Per Million (ppm), so it is helpful to know that 65 PPM is equivalent to one CF unit. (Just for the record, one CF unit is equal to 10 milliMhos, also called milliSiemen.) 10 CF units = 1 EC unit.

Different crops have different nutrient needs, and so grow better at different CF values. Lettuce, for example, grows best between 6 and 12 CF units, while tomatoes are heavier feeders and are happiest at 28 to 40 CF units. Adding more nutrient (in the proper proportions, of course) will increase the CF level. To lower the CF reading of a solution, simply add more water. Plants for home growing generally grow best at levels of 7.5-20 CF, although readings of 20-30 CF are considered acceptable. Nutrient levels above 40 CF should be avoided, as there is the potential for soluble salt damage.

In order to optimize the nutrient solution for a specific crop, commercial growers generally segregate different crops, growing each in a separate system. This is not usually necessary for the home gardener, as satisfactory results may be obtained by using a nutrient level that represents a compromise of the needs of the various plants.


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