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.