There are 13 mineral nutrients that are essential for completion of the plant's life cycle. Macro-elements are required in large quantities: nitrogen, potassium, phosphor, calcium, magnesium, sulfur. Micro-nutrients are required in very low concentration: iron, manganese, zinc, copper, molybdenum, boron, chlorine.
All of these nutrients should be provided in the hydroponic nutrient solution, in the right concentrations, and in adequate ratios.
According to the law of "limiting factor" , if one nutrient is deficient, other nutrients cannot compensate for the deficiency, and the crop may suffer, resulting in decreased quality and/or yield.
Most water sources contain only very small amount of these nutrients, if at all, therefore they must be provided in the hydroponic nutrient solution using fertilizers.
Commonly used soluble fertilizers are MAP, potassium sulfate, ammonium nitrate, potassium nitrate.
These nutrients are usually found in source water, sometimes in adequate concentration for plant needs, especially in well-water. If the concentration is higher than required, the source water should be pre-treated.
Calcium nitrate is the only fertilizer appropriate for calcium addition to hydroponic nutrient solution. Magnesium nitrate and magnesium sulfate are both appropriate sources for magnesium addition. Note that calcium nitrate and magnesium nitrate also contribute nitrogen to the hydroponic nutrient solution.
Sulfur is present in a wide range of concentrations in various water sources, and plants grown in hydroponics can tolerate relatively high concentration. But sulfur excess might have untoward effects and even limit nitrate uptake.
Iron, manganese, zinc and copper can be provided in the sulfuric form, but their availability is greatly decrease in pH greater than 6.5. The chelated forms ,may also be used, because they are available for uptake in a wider range of pH (learn more about chelate fertilizers). Some growers regard EDTA as harmful for plants, and avoid its use.
Molybdenum is usually provided using sodium molybdate. The presence of sodium in this fertilizer should not be a cause for alarm. Because molybdenum is needed in minute quantities, very small amounts of this fertilizer are usually used, and the sodium addition is negligible.
Boron can be provided through Boric Acid or Solubor. Solubor also contains sodium, but again, quantities are small enough as to not have a significant effect on sodium concentration in the solution. Range for adequate boron levels is very narrow (0.2-0.5 ppm) and can easily be missed, resulting in either deficiency or toxicity. Therefore boron supplements should be carefully added. Well water often contain sufficient boron levels, so no boron addition is needed.
Chloride is required by plants in minute quantities and most water sources contain chloride concentration well above and beyond plants require, so chloride deficiency is extremely rare. Chloride related problems are more commonly those of toxicities rather than of deficiencies. Therefore, using fertilizers which contain chloride is uncommon in hydroponics.
Sodium can be very harmful in recirculating systems, since it builds up with time in the hydroponic solution. Threshold concentration of sodium and chloride for most hydroponics grown plants is 75 ppm.
Several nutrients compete with each other over uptake by the plant, so keeping adequate ratios is important for avoiding deficiency. For example excess of potassium compete with calcium and magnesium absorption. A high iron/manganese ratio can result in manganese deficiency, and high sulfur concentration might decrease the uptake of nitrate.