Agronomy Degree Practice Exam

Where will a deficiency of any primary macronutrient first appear?

1. Lower leaves

2. Roots

3. Petioles

4. Upper leaves

The correct answer is: Lower leaves

A deficiency of any primary macronutrient—such as nitrogen, phosphorus, or potassium—will typically first manifest in the lower leaves of the plant. This is because macronutrients are mobile within plants, meaning that if the roots are unable to take up sufficient amounts of a nutrient, the plant will prioritize the distribution of available nutrients to the new, growing tissues, which are usually located at the top of the plant. Consequently, the older tissues, which include the lower leaves, will show deficiency symptoms first. This physiological behavior is particularly evident with nitrogen, as it is highly mobile within the plant. In cases of nitrogen deficiency, the lower leaves may turn yellow and eventually die as the plant reallocates nitrogen to newer leaves and growing points. Similarly, phosphorus and potassium deficiencies also manifest first in the lower leaves for similar reasons, as the plant adjusts its nutrient allocation based on current availability and growth needs.