The rate at which soil is drained varies throughout time. This is due to the fact that the texture of the soil varies as it gets deeper into the ground. This is due to the organic matter that makes up the topmost layer, which is highly porous and allows water to drain out extremely fast from the system. Layers of soil, known as soil horizons, will then be added on top of one other. Each one is likely to be distinctive in terms of texture or content of organic stuff. The rate at which each horizon drains will be unique. Your plants may be at risk of drying out if they don't grow roots deep enough into the soil to keep them hydrated. As a side note, if your soil drains rapidly in the first two inches, but then takes a long time to drain in the bottom two inches, you may be at risk for root rot.