Essential Elements

There are seventeen elements known to be necessary for plants to complete their life cycle.  They are called essential elements.

What Makes An Element Essential?
Arnon and Stout (1939) proposed the following criteria:
1. A plant must be unable to complete its life cycle in the absence of the mineral element.
2. The function of the element must not be replaceable by another mineral element.
3. The element must be directly involved in plant metabolism.

Of the seventeen essential elements, hydrogen (H), oxygen (O), and carbon (C) come from the air and water and by nature, readily available.  Although nitrogen can also be found in the air, its gaseous form is not useable by plants.  Along with nitrogen, the rest of the elements are found in the soil.  Depending on the soil properties, however, some of these elements may be present but not in forms that are useable for plant use.   Still some soils may lack one or more of these elements. 

When one or more of the essential elements is deficient, plants cannot complete their life cycle.  Such deficiency will be expressed in abnormal behavior in plant growth.  For this reason, fertilizers are used to supply the elements that are deficient within the growing area. 

Macronutrients 
Macronutrients are essential elements that are required by the plants in huge quantities (parts/100 of dry plant matter).  Macro nutrients are not in any way more important than the rest of the 17 essential elements. 
1. Nitrogen (N)
2. Phosphorous (P)
3. Potassium (K)
4. Calcium (Ca)
5. Magnesium (Mg)
6. Sulfur (S)

Micronutrients
Micronutrients also known as trace elements are elements that are required by plants in small quantities (ppm of dry plant matter).  Micronutrients should not be mistaken as less important than their macro counterparts. 
1. Boron (B)
2. Chlorine (Cl)
3. Copper (Cu)
4. Iron (Fe)
5. Manganese (Mn)
6. Molybdenum (Mo)
7. Nickel (Ni)
8. Zinc (Zn)

Scientists continue to evaluate the essentiality of more elements that are not included in the list.  Nickel for example, recently won the status of an essential trace or micro element.