Mineral elements play a paramount role in the performance and maintenance of life activities of a plant
(i)maintaining the structural constituents of the plant body.
(ii)influencing the osmotic pressure of cells 
(iii)helping in the permeability of cytoplasmic membranes.
(iv)reducing toxic and antagonistic effects.

The higher plants do not have the active form of nitrogenase which eventually converts atmospheric nitrogen into unable form through the agency of living organisms.

Plants need potassium because it helps in the maintenance of cell turgidity, opening and closing of stomata,cation-anion balance, increase hardness, essential for photosynthesis, respiration, proteins synthesis and in membrane permeability.Magnesium is required for chlorophyll formation, growth, metabolism and nodule formation in legumes.

The first essential condition for nitrogen-fixation is legume-bacteria relationship. Rhizobium bacteria cause nodule formation for this association. The enzyme nitrogenase is highly sensitive to the molecular oxygen.They are symbionts, which can fix atmospheric nitrogen for plants.

Role of K⁺ in plants
(i) Ppotassium ion helps in regulating stomatal opening and closing.
(ii) Helps in activating many growth related enzymes in plants

For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the order tissues.
e.g., 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 forst in the young tissues, whenever the elements are relatively immobile and are not transported out of the mature organs,e.g, elemnts like sulphur and calcium are a part of the  structural component of the cell hence, are not easily released.

(i)Phosphorous (P) It forms the constituent of cell membrane protein, nucleic acids, nucleotides, ATP, NADP and energy-rich compounds.
Deficiency symptoms It causes poor, leaves become dull green and chlorosis occurs.
(ii) Iron (Fe) It helps in chlorophyll synthesis, cytochrome synthesis, ferredoxin synthesis, activities enzyme and activates catalase.
(iii) Zinc(Zn) It activates enzyme synthesis. It is a constituent of carbonic anhydrase. This is also a constituent od dehydrogenase.

Plants absorb from the soil in the form of phosphate ions(either as \(​​{ H }_{ 2 }{ PO }_{ 4 }^{ - }\) or as \(​​{ H }_{ 2 }{ PO }_{ 4 }^{2 - }\) .Phosphate is a constituent of cell membrane.
It is constituent of nucleic acids, proteins and the nucleotides.
Two phosphorus deficiency symptoms are following 
(i) Delay in seed germination.
(ii) Purple spots on the leaves of the plants.

Nitrogenase enzyme is found in root nodules for N₂-fixation. Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen.
This enzyme requires a special pink coloured pigment for its functioning. Many nitrogen-fixing organisms exist only in anaerobic conditions, respire to bring down oxygen levels or bind the oxygen with protein such as leghaemoglobin.

(i) Nitrification is the process of oxidation of ammonia, first into nitrite and then into nitrate.
(ii) Ammonia is oxidised into nitrite, by bacteria like Nitrococcus and Nitrosomonas. \({ 2NH }_{ 3 }+3O_{ 2 }\longrightarrow { 2NO }_{ 2 }^{ - }+2H^{ + }+2H_{ 2 }O\)
(iii) Nitrite is further oxidised to nitrate with the help of bacteria Nitrobacter. \(2NO_{ 2 }+O_{ 2 }\longrightarrow 2NO_{ 3 }\)

Differences between passive and active salt absorption are

The technique of growing plants in a nutrient solution is known as hydroponics.
This method requires purified water and mineral nutrient salts. After a series of experiments, in which the roots of the plants were immersed in nutrient solutions and an element was added/ removed or given in varied concentrations, a mineral solution suitable for the plant growth was obtained.
By this method, essential elements were identified and their deficiency symptoms discovered. Hydroponics has been successfully employed as a technique for the commercial production of vegetables such as tomato, seedless cucumber and lettuce.
Yet, most of the crops are still grown on land because the nutrient solutions must be adequately aerated to obtain the optimum growth in hydroponis. Moreover, the minerals must be continuously added in the solution. No, such activity is required in the soil.

(i) Macronutrients These are generally present in plant tissues in large amount(in excess 10m mole Kg^-1 of dry matter).The macronutrients include carbon,hydrogen, oxygen, nitrogen,phosphorous,sulphur,potassium,calcium and magnesium.
(ii) Micronutrients Micronutrients or trace elements, are needed in very small amount (less than 10m mole g^-1 of dry matter).These include iron,manganese,copper,molybdenum,zinc,boron,chlorine and nickel.
(iii) Beneficial nutrients The elements which are not essential for plants, but their presence are beneficial for the growth and development. Such, elements are called beneficial elements.
Toxic elements Any mineral ion concentration in tissues, that reduces the dry weight of issues by about 10% is considered toxic. e.g., Mn inhibits the absorption of other elements.
(iv) Essential elements The macronutrients including carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium, which are required directly for the growth and metabolism of the plants and whose deficiency produces certain symptoms in the plants are known as essential elements.