Finish the sentence: Sulfate is ________________ in ________________ soils?

The correct completion of the sentence is that sulfate is mobile in most soils. Sulfate, as a form of sulfur, tends to be quite soluble in water, allowing it to readily move through various soil types. This mobility is particularly important because it makes sulfate readily available to plants, which requires sulfur as an essential nutrient for growth and development.

In most soils, particularly those classified as well-drained or sandy soils, sulfate can easily leach, thereby moving through the soil profile with water. This characteristic ensures that plants can access these nutrients effectively, especially in systems where water movement is significant.

The other options suggest different combinations of sulfate's mobility and soil types. For example, clay soils can retain nutrients better due to their fine structure, which could lead to some immobilization of sulfates, but this is not a general characteristic of sulfate across most soil types. The same can be said for organic soils, where while some nutrients may become more fixed, sulfate's overall solubility still allows for its movement. However, “mobile, most” accurately captures the nature of sulfate across a variety of soil types.