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The nutrient most commonly associated with a deficiency in younger leaves of a crop is nitrogen. This is because nitrogen is a mobile nutrient, meaning that plants can relocate it from older leaves to new growth when there is a deficiency. As a result, the symptoms of nitrogen deficiency, such as chlorosis (yellowing) and stunted growth, typically manifest in the younger leaves first.

In contrast, other nutrients like potassium, magnesium, and calcium tend to exhibit their deficiency symptoms in older leaves or depend on their specific roles and mobility within the plant. Potassium is also mobile but is more critical in older leaves for processes like photosynthesis and water regulation. Magnesium deficiency can lead to interveinal chlorosis primarily seen in older leaves due to its crucial role in chlorophyll production. Calcium is generally immobile within the plant, affecting primarily the growing tips and new tissues when deficient. Thus, the observed pattern of nitrogen deficiency aligns with its mobility and role in promoting growth in younger tissues.