To improve drainage in a heavy clay soil, add about 25 percent peat moss by volume. For example, if you dig your soil to a depth of eight inches, add two inches of organic matter. The addition of about 30 percent sand and 30 percent peat moss to heavy clay also works well. This means that five inches of sand needs to be added to the eight-inch depth. If less than 60 percent is added, a mixture similar to cement may result.
Sand can also be beneficial in an artificial situation such as in a raised bed or in a pot.
Unfortunately, peat will not hold up well over time. Another alternative is incorporating a soil-conditioning grade of pine bark. This will last much longer than peat moss, and it will greatly improve the condition of a heavy clay soil. Before planting herbs, make a planting sketch that shows where the herbs will be planted. When you are ready to plant, place the potted herbs on the bed surface as they appear on the planting sketch.