Name the process where atmospheric nitrogen is converted into more usable forms like nitrites and nitrates.

Study the diagram given below and then answer the questions that follow :

(a) Briefly describe how nitrogen of the atmosphere is converted to nitrates by leguminous plants.

(b) Name the bacteria that converts (i) ammonium compounds to nitrites (ii) nitrites to nitrates.

(c) State how the nitrates in the soil get converted to nitrogen of the atmosphere.

(d) Role of plants and animal in the formation of ammonium compounds.