[Campbell Biology P.869] A peanut farmer observes that the older leaves of their plants are ... | Practice Question

A peanut farmer observes that the older leaves of their plants are turning yellow after a prolonged period of heavy rainfall. Based on the information provided regarding nutrient deficiencies and the nitrogen cycle, what is the most likely reason for this symptom?

  • A: Iron deficiency, as iron is an immobile micronutrient and heavy rain washes it away.
  • B: Phosphorus deficiency, because phosphorus is mobile and leaches easily in wet soil.
  • C: Nitrogen deficiency, due to leaching of nitrate and/or reduced nitrogen fixation in waterlogged soil.
  • D: Potassium deficiency, as potassium is immobile and its uptake is inhibited by excess water.

Explanation

Nitrogen is a mobile nutrient, meaning plants can reallocate it from older leaves to younger, growing parts, causing older leaves to show deficiency symptoms first, typically yellowing chlorosis . Heavy rainfall can lead to waterlogging, which can reduce oxygen in the soil. This condition inhibits aerobic nitrogen-fixing bacteria and promotes denitrification by anaerobic bacteria, converting nitrates back to gaseous N₂, thus removing usable nitrogen from the soil. Additionally, nitrates NO₃⁻ are negatively charged and are easily leached from the soil with excess water.