Fertilizers made for soil amendments focus strongly on the top three nutrient needs of plants--nitrogen, phosphorus and potassium--because the soil will provide the other 10 nutrients needed for proper growth. When using a hydroponic solution as a growing medium, the gardener must provide all 13 needs of the plant. One of the advantages of hydroponic growing is the careful attention given to monitoring the nutrition needs of each individual plant variety and being able to adjust those nutrients as needed.
The most abundantly used nutrient is nitrogen and it will have to be replenished the most. In soil gardening, nitrogen is used for a variety of things, including the decay of mulches and dropped plant matter. This makes monitoring the nitrogen use of the growing plants more difficult. With hydroponics, the plant is competing with nothing for the nitrogen use. Nitrogen turns your plants that healthy green that is needed to produce chlorophyll. A deficit of nitrogen is obvious when your plant starts to yellow around the tips of the leaves. Be aware of your particular vegetable plant needs. For example, tomatoes, if given too much nitrogen, will look gorgeous but not develop fruit.
Phosphorus and Potassium
Phosphorus and potassium are the other macronutrients. They are the building blocks of healthy plant stalks and roots. Both are used in a slower pace than nitrogen but are equally as important as nitrogen. Both nutrients are vital to the development of seeds and fruit. Signs of older leaves wilting and dying off is a good indicator of a phosphorus or potassium deficiency.
Other nutrients, known as micronutrients, are readily available in soil but must be duplicated in hydroponic solutions. These nutrients include carbon, oxygen, calcium, magnesium, zinc, iron, copper, boron, manganese and molybdenum. Commercial hydroponic solutions should contain the proper mix needed for healthy growth.