The tallgrass prairies were conserved, to a large extent, thanks to the root system of the grass plants on the prairie. The roots extended deep into the ground, providing an anchoring effect that protected the soil against being blown by the constant wind of the prairies. As the grass plants died, the decayed plants added to the topsoil of the prairie. Periodic fires set by lightning burned vast areas of grasses on the prairies, which could result in varied types of changes to the condition of the soil. Fire was a major factor in speeding the decomposition of plant matter, allowing the nutrients in the matter to reenter the soil. As bison and other game animals migrated over the plains while grazing on the grass, their wastes added to the soil.