Acid soils are sandy with some peat, but generally low in organic material, and are usually underlain by an impervious hardpan or clay layer, 1-3 feet below the surface. Moisture levels fluctuate yearly from very dry to nearly saturated or inundated for long periods in the growing season. Under natural conditions, fires occur at 5-10 year intervals, preventing hardwood succession. Plants tolerate a wide range of moisture and soil fertility levels and are generally not tolerant of salt, shade or alkaline soils. Dry prairie is very similar to flatwoods but lacks the pine overstory. Wet prairies occasionally intermingle with pine flatwoods, forming a wetland mosaic.