Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.

Macronutrients: They are the nutrients required by plants in large amounts. They are present in plant tissues in amounts more than 10 m mole kg–1 of dry matter.
Examples include hydrogen, oxygen, and nitrogen.
Micronutrients: They are also called trace elements and are present in plant bodies in very small amounts, i.e., amounts less than 10 m mole kg– 1 of dry matter. Examples include cobalt, manganese, zinc, etc.

Beneficial nutrients: They are plant nutrients that may not be essential, but are beneficial to plants. Sodium, silicon, cobalt and selenium are beneficial to higher plants.
Toxic elements: Micronutrients are required by plants in small quantities. An excess of these nutrients may induce toxicity in plants. For example, when manganese is present in large amounts, it induces deficiencies of iron, magnesium, and calcium by interfering with their metabolism.
Essential elements: These elements are absolutely necessary for plant growth and reproduction. The requirement of these elements is specific and non-replaceable.
They are further classified as macro and micro-nutrients.

Macronutrients: Elements which are present in large amounts in plant tissues are called macronutrients. They are in excess of 10 mmole per kg of dry matter. Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium are the macronutrients.

Micronutrients: Elements which are present in small amounts, i.e. less than 10 mmole per kg of dry matter are called micronutrients. Iron, manganese, copper, molybdenum, zinc, copper, boron, chlorine and nickel are the micronutrients.

Beneficial Elements: Apart from the 17 essential elements, many other elements are required by plants. For example; sodium, silicon, cobalt and selenium are required by higher plants.

Toxic Elements: If a mineral ion concentration in tissues reaches to a level that it reduces the dry weight of tissues by about 10%, the mineral then becomes toxic. This shows that any element can become toxic if it crosses a certain threshold in plants. 

Macronutrients:

Generally present in plant tissues in large amounts. The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, 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.

Micronutrients:

Also called Trace elements are needed in very small amounts. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Beneficial nutrients:

Include sodium, silicon, cobalt and selenium. They are required by higher plants.

Toxic elements:

Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 percent is considered toxic. The toxicity symptoms are difficult to identify. Micronutrients in excess causes toxicity.

Essential elements:

Four categories on the basis of their diverse functions.

(a) As components of biomolecules and hence structural elements of cells.

e.g. carbon, hydrogen, oxygen and nitrogen.

(b) As components of energy related chemical compounds in plants.

e.g. magnesium in chlorophyll

(c) Elements that activate or inhibit enzymes, e.g. Mg²⁺ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase.

(d) Elements that alter the osmotic potential of a cell.

e.g. Potassium in opening and closing of stomata.