Certified Crop Advisor Practice Exam

Which nutrient is most immobile in soil?

1. Nitrogen

2. Phosphate

3. Potassium

4. Calcium

The correct answer is: Phosphate

Phosphate is recognized as the most immobile nutrient in soil due to its strong affinity to soil particles and relatively low solubility in water. When phosphate is applied to soil, it tends to bind to particles, especially in soils with high clay content or iron and aluminum oxides. This binding limits its movement through the soil profile and makes it less readily available for plant uptake compared to other nutrients. In contrast, nitrogen is more mobile in many soil types as it can exist in various forms, including nitrate, which is soluble and can leach into groundwater. Potassium, while it can also be more mobile, is typically retained in the soil but can still move with water. Calcium, while less mobile than nitrogen and potassium, has a different behavior depending on soil pH and texture, but it is generally more available than phosphate due to its higher solubility. Thus, phosphate's strong adherence to soil particles and limited movement make it the nutrient that is most immobile in soils.