Which process is primarily used to improve soil fertility?

The process primarily used to improve soil fertility is crop rotation. This agricultural practice involves alternating the types of crops grown on a particular piece of land in sequential seasons. By rotating crops, farmers can enhance soil fertility and reduce soil erosion. Different crops have varying nutrient requirements and contributions, which helps to maintain an optimal balance of nutrients in the soil. For example, legumes can fix nitrogen in the soil, enriching it for subsequent crops.

In contrast, overgrazing can lead to soil degradation, reducing its fertility by disturbing the soil structure and depleting the necessary nutrients. Desertification refers to the degradation of land in arid and semi-arid regions, leading to a loss of fertility and productivity. Pollination, while crucial for plant reproduction, does not directly impact soil fertility but rather assists in the propagation of plant species. Thus, among the provided options, crop rotation stands out as the most effective method for enhancing soil fertility.