Plants take in nutrients through their root systems and sometimes their leaves.
These nutrients come from a variety of sources, including soil, fertilizer or manure added to the soil, compost added to the soil, and residues left on the soil, says Dr. Charles Mitchell, Extension agronomist with the Alabama Cooperative Extension System. Plants need nitrogen, phosphorus and potassium in large quantities, and other nutrients in small quantities. Nitrogen is a critical part of proteins, which control the metabolic processes required for plant growth, says Mitchell. It also plays a key role in photosynthesis. An adequate supply of nitrogen is associated with vigorous vegetative growth and dark-green color of the plant.