The two factors to monitor are nutrient strength, and how acid or alkaline the solution is. The CF Test and the pH Test will test these factors. 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. CF meters designed for use with hydroponic systems usually have a scale of 0-100 CF units.
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 22 to 28 CF units. To lower the CF reading of a solution, simply add more water. Nutrient levels above 30 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 nutrient level that represents a compromise of the needs of the various plants.