I just put in a cedar raised garden with 10.5 inch sides for the first time in May. I filled the space about 3/4 with Kellogg Organic soil. As the season as progressed the soil has sunken to maybe a few inches above the ground. My question: should I re-soil the garden at the end of this growing season (Sept/Oct) or wait until March/April 2018 to add new soil for the new plants? Thank you so much!
It is up to you when you add the soil. However, you do not want the leave the soil bare over the winter as it can be prone to erosion. You can add organic matter such as chopped up leaves to decompose over the winter and turn under in the spring.
Or you can plant a cover crop and till under in the spring which improves the soil and adds organic matter. Spring oats is recommended if you are sowing a cover crop for the first time. http://extension.umd.edu/growit/beyond-basics/cover-crops-protect-and-improve-your-soil
Here is more information on adding organic matter http://extension.umd.edu/growit/food-gardening-101/step-3-prepare-your-soil
You can also call around to topsoil/mulch suppliers for commercially available mixtures of topsoil and leaf compost. This makes an excellent media for raised beds - blended topsoil (70%) and leaf compost (30%). Can be purchased by the cubic yard.