A macronutrient essential for vegetative growth, chlorophyll production, and protein synthesis. It's the first number in the NPK ratio and most commonly deficient in soils.
Nitrogen (N) drives shoot and leaf growth. Plants need it most during spring when growth is vigorous. It's the most common nutrient deficiency because nitrogen leaches away with rain and is consumed by plants. Legumes (beans, clover) fix atmospheric nitrogen; most plants rely on soil or fertilizer nitrogen.
Nitrogen deficiency is the most common nutrient problem. Understanding when to apply nitrogen prevents wasted fertilizer and improves yields.