Azotobacter provides nitrogen fixing bacteria which converts free nitrogen into nitrate and nitrites it increases soil fertility.

A) This is actively mobilized.
B) Highly mobile elements: Nitrogen and magnesium. Relatively immobile: Calcium and Boron.
C) The symptoms of deficiency of mobile elements are more pronounced in the older leaves and symptoms of deficiency of relatively immobile elements appear first in younger leaves. So this information can be utilized in horticulture as well as agriculture to get a broad idea of deficiency
elements in plants.

Biological nitrogen fixation (BNF)occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. The formula for BNF is:
N₂+6H⁺+6^e-⟶2NH₃

Potassium is involved in opening and closing of stomata. Stomatal opening is important for providing water for photosynthesis.

Manganese facilitates splitting of water into oxygen and hydrogen, which in turn are utilized to make glucose during photosynthesis.

Elements which are absolutely necessary, directly involved in metabolism and are irreplaceable are called essential elements.

Aeration is done to provide proper oxygen to plants through roots.

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.

Plants require various minerals for their normal growth, development and metabolism.It means that all essential elements must be present in soil or in the air, water etc., wherever the plant grows.
Now, if any particular element or a group of element is supposed to be missing in the soil or in the atmosphere, the element will be deficient to that plant and the plant will eventually die of some or the other deficiency.

The technique of solution culture or hydroponic is the better technique to avoid contamination because in solution culture, the plants are grown directly in pure salt solution which are taken in suitable vessels(preferably made up of natural polyethene or borosilicate contamination.

Nitrogen-fixation is the phenomenon that occurs in prokaryotes but not in eukaryotes. because the enzyme nitrogenase, which is capable of nitrogen reduction is present exclusively in prokaryotes and such microbes are often called fixers

Plants of leguminous family have root nodules in which the symbiotic bacteria Rhizobium fix atmospheric nitrogen into ammonia. The ammonia is subsequently available for many important biological molecules such as amino acids. proteins, vitamins and nucleic acids

Plants obtain sulphur in the form of sulphate  \(({ SO }_{ 4 }^{ 2- })\) and  is the main constituent of several coenzymes, vitamines(thiamine,biotin,coenzymeA) and ferredoxin.It plays role in the synthesis of chorophyll.It is essential for nodulation in legumes.It determines the structure of protein.Sulphur is present in two amino acids, i.e cysteine and methionine.

All elements that are present need not be essential to its survival because they are not directly involved in the composition of their body.
However, if the concentration of micronutrients such as Fe,Mn,Cu,Zn,Cl,etc rise above their critical values, they appear to be toxic for the plant.

Leguminous plants possess root nodules in which the symbiotic bacteria, Rhizobium, fix nitrogen. The fixed nitrogen makes the soil rich in nitrogen makes the soil rich in nitrogen when the leguminous crop is ploughed back in the field.