Bacteria in the soil converts this nutrient into a usable form. Plant take up the usable nutrient through the soil and assimilate it into proteins, making it part of the plant. An animal eats the plants, and the nutrient becomes part of the animal. When the animal dies, it decomposes, returning the nutrient back to the soil.

This describes the nitrogen cycle. Soil bacteria convert nitrogen into forms plants can absorb, and plants use nitrogen to make proteins. Animals get nitrogen by eating plants, and decomposition returns nitrogen compounds to the soil. The phosphorus cycle does not involve bacteria converting nitrogen into a usable form.

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