The two factors to monitor are nutrient strength, and how acid or alkaline the solution is. Fortunately, there is a simple test for each of these. 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 28 to 40 CF units. To lower the CF reading of a solution, simply add more water. 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.