Certified Crop Advisor Practice Exam

Which nutrient is more available at a pH of 7 compared to a pH of 5?

1. Nitrogen

2. Phosphorus

3. Molybdenum

4. Iron

The correct answer is: Molybdenum

Molybdenum's availability significantly increases at a neutral pH of 7 compared to a more acidic pH of 5. This trend is due to the chemical behavior of molybdenum in the soil. At lower pH levels, molybdenum tends to form soluble complexes that are less available to plants, whereas at neutral or slightly alkaline pH levels, the forms of molybdenum become more soluble and accessible to plant uptake. In contrast, nitrogen and phosphorus dynamics do not favor a neutral pH over acidic conditions in the same way. For nitrogen, both ammonium and nitrate forms are available at various pH levels, but their overall uptake does not show the same sharp increase at a pH of 7. Phosphorus availability tends to decrease in acidic soils due to the formation of insoluble compounds, but it is more available at slightly acidic to neutral pH, making it less favorable in this comparison. Iron, on the other hand, can be more available at lower pH levels and binds more tightly in alkaline conditions, thus showing reduced availability at neutral pH. Overall, molybdenum's unique solubility characteristics at different pH levels, particularly the marked improvement in availability at