Nutrient formulations are usually given in parts per million (ppm) concentration of each essential element. One ppm is one part of one item in one million parts of another. For example 250 ppm of calcium in a solution can be 250 microgram per gram or 250 milligram per litre.
Optimum nutrient requirement of plants depends on various factors - Plant species and variety, stage of plant growth, season of the year and the weather. Requirements, particularly of Nitrogen, Phosphorus and Potassium vary with plant type and stage of growth. Lettuce and other leafy vegetables need higher levels of nitrogen than tomatoes and cucumbers. The latter two requires higher rates of phosphorus, calcium and potassium.
Some of the complete hydroponic Nutrient Formulations ( Composition of elements in ppm) is shown in the table below.
In hydroponics all the essential elements are supplied to the plants by dissolving fertilizer salts in water to make the nutrient solution. The choice of salts to be used depends on a number of factors such as solubility in water, cost, availability and most importantly to satisfy the ppm requirement of all elements without excessive supply. For example one molecule of potassium nitrate (KNO3) will yield one ion of potassium (K+) and one ion of nitrate, whereas one molecule of calcium nitrate (Ca(NO3)2) will yield 1 ion of calcium (Ca++) and 2 ions of nitrate 2(NO3-). Therefore, if a minimum number of cations is wanted while supplying sufficient nitrate (anions) then calcium nitrate should be used.
The following table shows commonly avaliable fertilizer salts from which the elements needed (table above) can be supplied to plants in required quantities.