Which one of the following statements can best explain the term critical concentration of an essential element?

1. essential element concentration below which plant growth is retarded
2. essential element concentration below which plant growth becomes enhanced
3. essential element concentration below which plant remains in the vegetative phase
4. None of the above

  

(a) The concentration of the essential element below which the plant growth is retarded is termed as critical concentration. Plants start showing deficiency symptoms if a particular elements present below the critical concentration.
Page 198, XI NCERT