Many people beleive that bagging your clipping helps to limit and reduce the thatch layer in your lawn.  This is not correct.  Thatch is not made up of grass clippings.  It is made up of decay resistant such as surface roots, runners and stems found close to the soil surface.  Grass Clippings are made up of soft tissue that is 85-90% water.  Therefore, when clippings are left to dry on the soil they shrink by 90%.  The advantages of letting clippings fall into the soil are 1) It takes less a third less time to mow 2) Up to 25% of the fertilizer nutrients are returned to the soil and 3) Reducing trash by 10% means less space is taken in landfills and fewer plastic bags are taken to the landfill.  Clippings contain approx. 4% nitrogen, .5% phosphorus and 2% potassium.