want to add this,.. from a site.

Is an essential element for the chlorophyllian function, that is for the leaves to convert light and nutrients into carbohydrates.
Nitrogen is necessary for the plant to grow. But excess nitrogen will cause plants to grow excessively in size and may delay flowering ( think of those Vandas ! ).
On the other hand nitrogen deficiency will result in stunted plants.
A recent study by Dr. Yin-Tung Wand of Texas A & M University suggests that “under severe nitrogen deficiency, the proteins in the lower leaves are digested and the nitrogen is transferred out of the older leaves into the younger upper leaves (note : of Phalaenopsis). As a result some of the lower leaves start turning yellow and eventually fall off”.
Is believed to regulate many activities.
It is necessary for the formation of cells, it promotes root growth, it induces and stimulates flowering.
Deficiency is phosphorous will also result in stunted plants, with dark green leaves.
Is necessary for healthy growth. Deficiency may result in dwarfness.
Besides the essential elements (Nitrogen, Phosphorous and Potassium), plants need other minerals such as calcium, magnesium, iron, manganese,...usually referred to as “trace elements” or “micronutrients” because plants need them in much lower concentrations than Nitrogen, Phosphorous or Potassium.
An ideal fertilizer is one than contains the essential elements and all the micronutrients plants need.