Dr Estelle Kempen, Department of Agronomy, Faculty of Agrisciences, Stellenbosch University Photos supplied T he quality of the water used to fertigate plants in soilless systems is vitally important. Not only should the presence of potentially harmful micro-organisms be tested for but, the chemical composition should also be evaluated. It is critically important to determine the water quality as a first step when contemplating growing crops hydroponi- cally. This will define what crops can be grown successfully and also determine the composition of the nutrient solution to be used. The total concentration of the ions in the feeding water, measured as the electrical conductivity (EC, mS cm-1) can serve as initial indication of the quality of the water. High EC feeding water can only be used to grow saline-tolerant crops such as amaranthus, swiss chard, melon and cherry tomatoes. When the EC or total salt content of the water becomes too high, water uptake by the plants will become limited. As a general indication figure 1 indicates the average optimum EC for a range of crops that can be grown hydroponically. These values can be affected by the climate and may differ between cultivars. Figure 1: The ideal nutrient solution concentration (EC, mS cm -1) for some hydroponically grown crops. Figure 1: Tomatoes grown in a soilless systems with a well-balanced nutrient solution. Tomatoes are relatively tolerant to high EC feeding water. Figure 2: Lettuce plants are sensitive to saline water and good quality water is essential to produce lettuce in such a hydroponic system.