Why are nitrogen fertilizers commonly needed for perennial fruit crops?

In the context of fruit farming, supplemental nitrogen is almost always required for perennial fruit crops because it is a fundamental nutrient essential for plant growth and development.1

Nitrogen is a critical component of chlorophyll, which enables plants to absorb energy from light, and it is necessary for the synthesis of proteins, DNA, and RNA. While nitrogen is abundant in the atmosphere, most plants cannot utilize it in its gaseous form and must instead absorb it as reactive compounds like nitrates or ammonia from the soil.23

Because perennial fruit plants have specific individual requirements and often rely on the natural fertility of the soil, supplemental nitrogen is frequently necessary to ensure healthy development and productivity. In some cases, such as during heavy crop years for apple, pear, or citrus trees, additional nitrogen applications are specifically recommended to support the plant's increased metabolic demands.1