Which of the following is a free-living nitrogen-fixing bacterium?

$A$ 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.

For the limited nitrogen that is present in the soil,microbes compete with

Select the correct match for $X$ and $Y$.

Column $X$	Column $Y$
$(1)$ Muscle contraction	$(P)$ $Mg^{+2}$
$(2)$ Digestion of food	$(Q)$ $Ca^{+2}$
$(3)$ Energy exchange	$(R)$ $Mo$
$(4)$ Carbohydrate synthesis	$(S)$ $PO_4^{-3}$
$(5)$ Nitrogen fixation	$(T)$ $Cl^{-}$

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

Which of the following statements are true? If false,correct them:
$(a)$ Boron deficiency leads to stout axis.
$(b)$ Every mineral element that is present in a cell is needed by the cell.
$(c)$ Nitrogen as a nutrient element,is highly immobile in the plants.
$(d)$ It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.