Name a plant which accumulates silicon.

Which of the following plants is used as green manure in crop fields and in sandy soils?

Analogy type questions:
$(1)$ Manganese toxicity : Brown spots surrounded by chlorotic veins : Molybdenum deficiency : ........
$(2)$ Conversion of nitrogen to ammonia : Nitrogen fixation : Oxidation of nitrite to nitrate : ........

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

Column $X$	Column $Y$
$(1)$ Mycorrhiza	$(P)$ Free-living $N_2$-fixer
$(2)$ Nostoc	$(Q)$ Phosphorus absorption
$(3)$ Azospirillum	$(R)$ Leguminous plants
$(4)$ Rhizobium	$(S)$ Autotrophic $N_2$-fixer

Which of the following bacteria is associated with nitrogen fixation?

Nitrate present in the soil is reduced to nitrogen by the process of denitrification. Which bacteria are responsible for denitrification?