Many commerical fertilizers are pretty generic, offering a one-size-fits-all approach to gardening. You'll find a few specialty types for tomatoes, for flowering plants, or bulbs, for example. This is often acceptable because soil-grown plants have access to a certain amount of nutrients that are commonly present in the soil, and they can make up the difference. In hydroponics, however, there is no soil buffer, and the nutrients supplied to your plants must be complete, including micronutrients and trace elements, so that your crops are healthy and plentiful. In fact, fertilizers intended for broadcast across your garden or incorporated into pots often depend on microbes present in the soil to break compounds down to a state that plants can absorb.
In contrast, fertilizers designed for hydroponics will be easy to dissolve and will provide a complete spectrum of nutrients in forms that are immediately available to the plants. The source of nitrogen, in particular, is quite different between soil fertilizer and hydroponics. Soil fertilizers present nitrates that are only gradually available through the action of soil bacteria, like urea and ammonium. Use of these fertilizers will provide insufficient nitrogen to your hydroponic crops, and may acidify your nutrient solution to the point of toxicity.
