Raised Bed Soil Preparation

I'm planning to grow my herbs in a raised bed. How do I prepare the soil? What type of soil do I need. I understand they require good drainage. I have no idea where to start.

How much preparation you have to do will depend on the type of soil you are starting with. If you have sandy soil - it will trickle through your fingers when dry and only stick when quite wet- you should add about half the volume in well-rotted manure or well rotted garden compost. If you have clay soil, it will stick together when moist and turn into a hard brick-like consistency when dry. This type of soil needs about a quarter per volume of well-rotted manure/compost and another quarter sharp builder's sand. If you are lucky enough to have a sandy loam mix on your property, then all you will have to do is add some well-rotted manure/ compost each year to keep up the nutrient value and water retention properties of the soil. How much you add depends on how fertile your soil is to start with. But organic matter will soon rot away and eventually you will have to add about one quarter volume of new well-rotted manure/compost.
If your raised bed is going to be constructed on top of heavy clay, it might be a good idea to have a 15 centimeter (6 inches) layer of gravel at the base of the bed to improve drainage.
If you use more than one layer of retaining material, such as logs or blocks, make sure that the layers are indented towards the center of the bed, to make sure the bed does not collapse outward -such as when you water it! Most books recommend 15 centimeter (6 inches) set back of each layer. The steps created in this way can be used to plant with cascading plants such as silver thyme and the whole thing can look fabulous.