Plants obtain nitrogen through the Nitrogen cycle. Air consists of 78 percent nitrogen, which is in the form of nitrous oxides. Rain water dissolves these oxides and nitrogen enters the soil, which plants then absorb by drawing in water through the roots.
Nitrogen is also added to the soil trough nitrogen-fixing bacteria present in the soil and in the root nodules of leguminous plants. Most fertilizers also add nitrogen to the soil. Animals introduce nitrogen into the soil through the discharge of urine and feces. Dead and decaying matter in the compost cycle also allows nitrogen to mix into the soil.