If the essential elements are supplied in limited qunatities, the growth of plant will be reduced. The concentration of essential elements below which the growth of plant is reduced is known as critical concentration of the element. If the element is present in lower concentration than its critical concentration then it is said to be deficient.
If every element has a specific structural and functional role in plants, the plants show morphological changes in the absence of that particular element. These morphological changes identify certain deficiencies of elements and are called as deficiency symptoms. The deficiency symptoms differ for one element to another and they vanish when the mineral element that is deficient is supplied to the plant. If the deficiency of that element continues then it would lead to the death of the plant. The plant parts that show the deficiency symptoms depends on the mobility of the element in the plant. If the element is actively mobilized inside the plant and is exported to the younger parts of the plant tissue, the deficiency symptoms are seen earlier in older tissues.
For example, the deficiency symptoms of potassium, nitrogen, and magnesium are seen initially in senescent leaves. Older leaves get separated making these elements mobilized to younger leaves. The young tissues are initially made to experience the deficiency symptoms when the element is immobile and not transported out of the mature organs.
Sulphur and calcium are the elements that are part of the structural component of the cell and so are not released from the organs. This aspect of the mineral nutrition of the plants is of higher significance and importance to agriculture and horticulture.
The deficiency symptoms that are generally appearing in plants include necrosis, chlorosis, premature fall of buds and leaves, and ceasing of cell division. Chlorosis is the chlorophyll loss leading to yellowing of leaves, which is caused from the deficiency of elements like Fe, S, Mn, Mg, Zn, Mo, N, and K. Necrosis is another symptom of the leaf that occurs due to the deficiency in Mg, Ca, K and Cu. Reduction in the levels of N, Mo, S and K stops the division of cells. Some elements such as Mo, N and S delay the flowering process when their levels in plants are low. Therefore, deficiency of elements can lead to several symptoms caused by the deficiency of various different elements. Same symptoms might be caused due to the deficiency of a single element out of many. To learn about the deficient element, the symptoms developed in all parts of the plant have to be studied and compared with the standard symptoms that are aleady given in the standard tables. It is also vital to know that different parts of the plant will show different symptoms in response to the same element.