Theory question 103
Soil Fertility and Plant NutritionWith the aid of a labelled diagram, describe briefly the nitrogen cycle.
Show the model answer
Model answer
Labelled diagram showing: atmospheric nitrogen → fixed by lightning and by bacteria (free-living Azotobacter, Clostridium; symbiotic Rhizobium in legume nodules) → nitrates/ammonium in soil
Plants absorb nitrates → plant protein → eaten by animals → animal protein
Death and excretion: dead plants/animals and wastes decomposed (ammonification) to ammonium
Nitrification: Nitrosomonas changes ammonium to nitrite, Nitrobacter changes nitrite to nitrate
Denitrification: bacteria (e.g. Pseudomonas) change nitrates back to nitrogen gas; losses also by leaching and volatilisation
Fertilizers add nitrogen to the soil
Explanation
The diagram needs arrows and each process named.