Choose a fertilizer designed for hydroponic growing. Make sure that it supplies adequate amounts of nitrogen during early growth stages. Typical hydroponic fertilizers have nutrient ratios of 9-5-10 or 18-6-16 (nitrogen-phosphorus-potassium, or N-P-K). Fertilizers used for later growth should have lower ratios of N. In addition to N-P-K, the fertilizer should supply secondary and micro-nutrients, which will be listed on the label of the fertilizer package. Some fertilizers seem to be deficient in magnesium (correct this by using Epsom salts) and iron-zinc-manganese (available in combination at large nurseries). The pH level can also affect solubility of nutrients, so try to keep the pH of the water between 6.3 and 6.8. (The easiest way to gauge your pH level is by using pH paper. You can pick some up at any garden shop.) Before mixing the nutrients into the water, adjust the pH using sulfuric, nitric, or citric acid if it is too high; lime or baking soda if it is too low.
Whatever system you decide to use, once the nutrient/water solution has been added, replacement water should be nutrient-free. If you notice a slowing of growth, or a nutrient deficiency, adjust the nutrient solution. For instance, if the plants show signs of a potassium deficiency (necrotic leaf tips and edges, yellowing of leaves), add potassium. Once the nutrient problem has been corrected, the plant will respond quickly and the improvement should be apparent within a few days.
About every month or two, replace the water/nutrient solution. The discarded water makes a good garden fertilizer. Every other time you change the water, rinse the medium with clear water to wash away any salts that have been left before adding new nutrient/water solution.
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