A material added to soil or applied to plants to supply nutrients — primarily nitrogen (N), phosphorus (P), and potassium (K). Fertilizers are labeled with three numbers representing the percentage of each (e.g., 10-10-10). Organic fertilizers (bone meal, blood meal, fish emulsion) release nutrients slowly as they decompose. Synthetic fertilizers release nutrients immediately but do not improve soil structure. Fertilizer is applied when soil nutrients are insufficient for the plant being grown, as determined by a soil test or by visible nutrient deficiency symptoms.