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In soil got most of the minerals and enters into plants through roots. Some plant species accumulate selenium, some others gold, while some plants growing near nuclear test sites take up radioactive strontium.

An essential element has a specific structural or physiological role in the activities of plants and important for their life cycle.

In 1939, Arnon and Stout, proposed the criteria for determining the essential elements in plants as follows:

1. The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.

2. The requirement of the element must be specific and not replaceable by another element. means deficiency of any one element cannot be met by supplying some other element.

3. The element must be directly involved in the metabolism of the plant.

Based upon these criteria, there are 17 essential elements which are carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, sulfur, magnesium, calcium, iron, boron, manganese, zinc, copper, molybdenum, nickel, and chlorine.

Non-essential elements are also required by the plants in certain metabolic activities like cobalt, silicon, etc., but their deficiency does not cause disease to the plants.

Classification of essential mineral elements

1. Based on the source from which they are derived

These essential elements are of two typesas follows:

a. Mineral elements

Mineral elements are the essential elements that are derived from soil, such as potassium, magnesium, etc.

b. Non-mineral elements.

Non-mineral elements are those elements that are derived from air or water, such as carbon, hydrogen, oxygen, and nitrogen.

2. Based on the amount required by the plants

The essential nutrients are classified as macro-nutrients and micro-nutrients.

Macro-nutrientsMacronutrients are generally present in plant tissues in large amounts. The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium.

Of these, carbon, hydrogen and oxygen are mainly obtained from CO₂ and H₂O, while the others are absorbed from the soil as mineral nutrition.

Micro-nutrientsMicronutrients or trace elements are required in very small amounts. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

In addition to the 17 essential elements, there are some beneficial elements such as sodium, silicon, cobalt and selenium. They are required by higher plants. Essential elements can also be grouped into four broad categories on the basis of their diverse functions as follows:

1. Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).

2. Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and phosphorous in ATP).

3. Essential elements that activate or inhibit enzymes for example Mg²⁺ is an activator for both ribulose bisphosphate carboxylaseoxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn²⁺ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.

4. Some essential elements can alter the osmotic potential of a cell. The potassium plays an important role in the opening and closing of stomata. You may recall the role of minerals as solutes in determining the water potential of a cell.

Functions of essential elements

The essential elements are the constituents of biomolecules like nitrogen in amino acids, proteins, nucleic acid, etc.

The essential elements are the framework elements that constitute the cell wall like carbon, hydrogen, and oxygen.

The essential elements are the basic components of energy-related compounds like magnesium. Magnesium forms the component of chlorophyll, which helps in the photosynthesis process.

The some of essential elements are toxic to the plants, like aluminum, mercury, etc. Some elements minimize the toxic effect of heavy elements like calcium, magnesium, etc.

The essential elements regulate the osmotic potential in plants. They are required for water absorption and maintenance of cell turgidity like sodium, potassium, etc.

Metals with variable valency act as electron carriers which helps in the oxidation-reduction system in plants like iron, copper, etc.

Certain mineral elements act as cofactors like magnesium is an activator of respiratory enzymes hexokinase, etc.

Certain elements take part in the opening and closing of stomata and other turgor movements like potassium.

Deficiency symptoms of essential elements

In the case supply of an essential element becomes limited, plant growth is retarded. The concentration of the essential element below which plant growth is retarded is termed as critical concentration.

When present concentration less than critical concentration is said to be deficient. Every element has one or more specific structural or functional role in plants, in the absence of any particular element, plants show certain morphological changes.

These morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.

The deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is provided to the plant.

If deprivation continues than it may eventually lead to the death of the plant.

The parts of the plants that show the deficiency symptoms also depend on the mobility of the element in the plant. For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues.

For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves.

In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilising to younger leaves.

The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs.

For example, element like sulphur and calcium are a part of the structural component of the cell and hence are not easily released.

This aspect of mineral nutrition of plants is of a great significance and importance to agriculture and horticulture.

The kind of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds, and inhibition of cell division.

Chlorosis is the loss of chlorophyll leading to yellowing in leaves. This symptom is caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.

Similarly, necrosis, or death of tissue, particularly leaf tissue, is due to the deficiency of Ca, Mg, Cu, K.

Absence or low level of N, K, S, Mo causes an inhibition of cell division.

Some elements (N, S, Mo) delay flowering if their concentration in plants is low.