Which mineral element is essential regarding respiratory pigments in invertebrates and vertebrates?

$A$ farmer adds/supplies $Na, Ca, Mg$ and $Fe$ regularly to his field and yet he observes that the plants show deficiency of $Ca, Mg$ and $Fe$. Give a valid reason and suggest a way to help the farmer improve the growth of plants.

Match the columns:
$I$ $II$
$(A)$ Magnesium $(i)$ Found in middle lamella
$(B)$ Sulphur $(ii)$ $A$ structural component of chlorophyll
$(C)$ Calcium $(iii)$ Required for enzyme activity
$(D)$ Iodine $(iv)$ Found in some amino acids
$(E)$ Manganese $(v)$ $A$ component of sugars
$(vi)$ Not important for plants

Select the correct option:
$(1)$ During $transamination$ / $reductive$ $amination$,an amino group is transferred from one amino acid to the keto group of a keto acid.
$(2)$ $Azotobacter$ / $Clostridium$ are free-living bacteria.

$A$: Plants absorb nitrogen from the soil in the form of salts.
$R$: Nitrogen is essential for the synthesis of proteins and nucleic acids.

Assertion $A$: Leguminous plants increase soil fertility.
Reason $R$: Azotobacter fixes atmospheric $N_2$.