define the function critical element

Some elements are essential for plant growth as these elements are required by plants at a certain concentration, below which plant growth is retarded. Every essential element has a special role in plant growth. Thus they are critical for their growth. Plants are not able to survive without them. For example magnesium is an important part of chlorophyll. If it's concentration is below critical level in any plant it would result in the chlorosis or yellowing of leaves.