The secret to obtaining consistently good results from hydroponics is to provide the best possible nutrient solution. 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.
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.)
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. 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 generally grow best at levels of 7.5-20 CF, although readings of 20-30 CF are considered acceptable. 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.
Most people are familiar with pH ("potential hydrogen") as an indicator of how acidic or alkaline a solution is. The pH scale ranges from 0 pH (very acidic) to 14 pH (highly alkaline). Pure water, at 7 pH, is neutral--neither acidic nor alkaline. Maintaining proper pH is critical, whether growing in soil or hydroponically. Plants absorb each nutrient element only with within a certain pH range, and because this range is different for different nutrients, there is only a relatively small pH range in which all nutrients are available.
Plants can generally survive within a pH range of 5.0 - 7.5. Below 5.0 pH there is danger of burning and destroying the sensitive root tissues, while at pH levels of 7.0 and above some nutrients may precipitate out of solution and become unavailable to the plants. The optimum level is about 6.3 or 6.5 pH, with anything in the range of 6.0 - 7.0 being acceptable. Unless an automated controller is being used, the pH level should be manually tested and corrected daily.