Mechanism of Absorption of Elements
(i) The process of absorption can be demarcated into two main phases.
(ii) In the first phase, an initial rapid uptake of ions into the 'free space' or 'outer space' of cells - the apoplast, is passive.
(iii) In the second phase of uptake, the ions are taken in slowly into the 'inner space' - the symplast of the cells.
(iv) The passive movement of ions into the apoplast usually occurs through ionchannels, the transmembrane proteins that function as selective pores.
(v) On the other hand, the entry or exit of ions to and from the symplast requires the expenditure of metabolic energy.
(vi) The inward movement of ions into the cells is influx and the outward movement is efflux. Translocation of Solutes. Mineral salts are translocated through xylem along with the ascending stream of water, which is pulled up through the plant by transpirational pull. Analysis of xylem sap shows the presence of mineral salts in it. Use of radioisotopes of mineral elements also substantiates the view that they are transported through the xylem.

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, and necrosis is the death of tissue, particularly leaf tissue.

Nitrogenase is found in root nodules of leguminous plants. The enzyme binds with nitrogen and helps it in getting transported to the plant.

Oxygen and carbon are not obtained through soil as mineral nutrition. Oxygen is necessary for respiration and carbon dioxide is necessary for photosynthesis.

It is important to know that manganese completes with iron and mahnesium for the uptake and with magnesium for binding with enzymes.Manganese also inhibites calcium translocation in shoot apex therefore,excess of mangganese may , infact, induces deficiency of iron, magnesium and calcium.
Thus, what appears as symptoms of manganes toxicity may actually be the deficiency symptoms of iron, magnesium and calcium.
The farmer should not add excess manganese in the soil to prevent manganese toxicity.

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.

The concentration of the essential element below which plant growth is retarded is termed as critical concentration.The element is said to be deficient when present below the critical concentration.
Yes , we can find the values of critical concentration and deficient for minerals Fe and Zn by absorbing the morphological changes.If the concentrations of these elements are present below the critical values, then it may eventually lead to the death of the plant.Morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.

(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.

Impure water contains a large number of soluble minerals dissolved in it.Likewise, the salts also contain impurities.If such impure water and minerals salts are used as solution culture for growing plants in hydroponics, these impurities hinder with the experiments used for detection of essentially of an element.
As a result use of purified water and known amount of purified nutrients salts are used in studies involving minerals using hydroponics.

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. \(2NO_{ 2 }+O_{ 2 }\longrightarrow 2NO_{ 3 }\)

On observing the section of root nodule
(i) The pink colour of root nodule is due to the presence of pigment leghaemoglobin.
(ii) The pigment protects enzyme nitrogenase from oxygen as this enzyme is highly sensitive to O₂ and functions only in anaerobic conditions
(iii) During the process of biological nitrogen-fixation, the molecular nitrogen present in the air is converted into inorganic nitrogenous compound through bacteria, cyanobacteria, etc.
The organism reduces nitrogen by the help of an enzyme nitrogenase

(i) Nostoc, Anabaena
(ii) Azotobacter, Beijerinckia
(iii) Frankia, Rhizobium

Differences between passive and active salt absorption are