When is an element said to be deficient in plants regarding critical concentration?

Critical concentration:

1. The critical concentration is that concentration that an essential element below which the plant growth is retarded.
2. Critical concentration of an element is observed when the supply of essential elements is less.
3. The concentration that is below the critical concentration and the deficiency of nutrient elements can lead to a gradual fall in the plant growth.
4. Every essential element in plants has its own structural and morphological role.
5. The changes appear first in the older leaves or older parts of the plant.
6. For example, Necrosis or death of tissue occurs due to deficiency of calcium, copper, potassium, etc.