On the other hand, an acre of tallgrass prairie contains as much as 250 tons of topsoil. The tough prairie sod is a great conserver of soil and water. Sod acts like a giant sponge catching and holding rainwater. As a result, the amount of water runoff from the prairie is small compared to land where there is no strong network of roots to absorb the rain. The roots also bind the soil tightly to the earth, protecting it against erosion.
Prairie sod is so dense that settlers once used it like bricks to build houses.
A square yard of soil just 4 inches deep may contain roots that would stretch for 20 miles if they were placed end to end. Grasses are the basic fabric of the prairie since they are uniquely adapted for life in the open. Most plants grow from their tips to add length. By contrast, grass grows from its base, emerging from a bud at, or just under the soil where the sensitive growth tissues remain protected from the elements and from grazing. Roots of some grass species extend 10-15 feet deep into the soil, and can extract moisture deep underground during dry periods.