Desertification occurs when soil loses its ability to sustain life and growth. When water in the soil dries up and the nutrient levels drop, the soil is said to lose its productivity potential; desertification by definition affects drylands more than wetlands, since water brings a certain amount of respiration and biological life with it.
Generally, land that has lost more than 25-50% of its productivity potential can be said to have undergone desertification. The process usually involves soil becoming almost entirely sand, with no structure or nutrients for life and roots to take hold. When the land is so dry, root structures must range very deeply to find water, and with no soil structure to hold the plant up, many cannot support themselves. The 25-50% range covers "slight" to "moderate" desertification; when the productivity level drops further, land can be almost impossible to reclaim, especially in very dry and arid regions.