Which nutrient is least mobile in soil due to strong binding interactions?

Phosphorus is considered the least mobile nutrient in soil primarily due to its strong binding interactions with soil particles, particularly with calcium, iron, and aluminum oxides. This adsorption process significantly reduces its availability to plants, as phosphorus tends to form stable complexes that are less soluble and thus less accessible for uptake.

In contrast, nutrients like potassium and nitrogen exhibit higher mobility in the soil. Potassium, while it can bind to soil particles, is more readily available due to its solubility and the cation exchange capacity of soils. Nitrogen, especially in its inorganic forms, tends to move more easily through the soil profile as it is subject to various processes such as leaching and mineralization.

Calcium, while it interacts strongly with soils as well, is often more mobile than phosphorus due to its role in soil structure and its presence in various forms within the soil solution. Therefore, phosphorus's strong tendencies to bind and form complexes in soil accounts for its classification as the least mobile nutrient compared to the others listed.