Complete the sentence: Phosphate is _________________ in the soil except where P sorption capacity has been _________________?

Phosphate is considered immobile in the soil due to its tendency to bind with soil particles, making it less available for plant uptake. This immobility arises because phosphate ions often interact with calcium, aluminum, and iron in the soil, leading to their adsorption and reduced movement through the soil profile.

The phrase "except where P sorption capacity has been exceeded" indicates a specific condition under which the usual behavior of phosphate changes. When the sorption capacity of the soil has been exceeded, it implies that the binding sites for phosphate have become saturated. In such cases, phosphate can become more mobile in the soil solution, allowing for greater availability to plants and potential leaching.

Thus, the correct completion of the sentence reflects the general nature of phosphate's behavior in soil and conditions that alter that behavior, making the statement accurate in its agricultural context.