A hard crust on the bottom of the loaf can be caused by a dutch oven that is too close to the heating element of the oven. To remedy this, place a pizza stone or baking sheet on the oven rack that is one level below the rack that the dutch oven is resting on. (NOT on the same rack.) This will create a barrier between the dutch oven and the heating element of the oven to help keep the bottom of the dutch oven from getting too hot. An over-all hard crust can be caused by a high oven temperature or over-baking. Increase the covered baking time to 30 minutes and decrease the uncovered baking time to 20 minutes. This will help trap more moisture and produce a softer crust. If you bake often it's a good idea to use an oven thermometer once in a while to check the accuracy of your oven's temperature.